Stale-seedbed can be followed by drill seeding to shorten the delay between burndown treatment and rice planting. Drill seeding rice typically involves sowing seed to 1.25-2 cm and flush irrigating fields for the first few weeks as the rice stand develops and herbicides are applied, before flooding for the remainder of the season . This method discourages aquatic weeds and algae, but tends to favor grasses . Furthermore, as the crop is typically sown fairly shallowly, it emerges synchronously with competitive grasses reducing the stand’s ability to compete and further limiting available herbicides. However, if rice is drilled to depths greater than 2 cm, the stand should emerge later than the majority of grasses. This may allow novel weed management practices to be used without stand injury . We hypothesized that planting California rice cultivars below the zone of active weed germination and emergence would delay rice emergence, yet not result in reduced rice stand. This would allow us to combine drill seeding with a stale seedbed as an integrated approach to weed management. This “stale-drill” method would permit the use of a novel mode of action in a post plant-burndown treatment, which would safely manage weeds prior to stand emergence, without injury or delayed planting. CHAPTER ONE details field trials conducted in 2016-2017. We explored the feasibility of staledrill planting, by planting cv. M-206 to depths up to 5.1 cm and evaluating stand establishment and herbicide programs. Aquatic broadleaf weeds and algae were suppressed by water management, and were not present in either study year. We applied glyphosate at 870 g a.e. ha as a PPB treatment just prior to rice emergence, either alone or in conjunction with other herbicides. Treatment delays had mixed effects on weed control. Glyphosate PPB was more effective at controlling Echinochloa spp. in 2017, vertical farming supplies reducing density by 30%, 48%, and 73% at 1.3 cm, 2.5 cm, and 5.1 cm depths, respectively. The greatest overall weed control either year was found with glyphosate + pendimethalin followed by penoxsulam + cyhalofop with rice seeded to 1.3 cm depth.

Planting rice deeper than 1.3 cm delayed emergence by 3 to 4 days in both study years. We found that rice stand and yield components were more strongly affected by planting depth in 2017 than in 2016, possibly owing to cool weather immediately after seeding. Yields in 2017 were reduced in deeper plantings by up to 72%. CHAPTER TWO outlines research conducted in glasshouses, with the aim of elucidating relative vigor of four California rice cultivars. Two experiments were conducted to evaluate rice cultivars for traits that would facilitate the stale-drill cropping methodology. Cultivars M-105, M-205, ‘M- 206’, and M-209 were evaluated for differences in germination, elongation, emergence, and early season morphology. M-205 and M-209 were found to have greater rates of total below-soil elongation, and greater rates of mesocotyl and coleoptile elongation overall, across depths. M-205 and M-209 were also found to have higher rates of emergence across depths. Differences between cultivars in above ground growth parameters of emerged seedlings were only found for rice planted at 0 cm, 6.4 cm, and 7.6 cm planting depths. Based on observed below- and above-soil growth and development, M-205 and M-209 exhibited greater vigor overall, as well as high levels of emergence from depths greater than 2 cm. CHAPTER THREE describes research conducted in the field in 2018-2019. Rice cultivars M-206 and M-209 were drill seeded to 3 cm and 6 cm depths. A PPB application of glyphosate at 870 g a.e. ha-1 was applied 6-7 days after planting at rice emergence, which controlled >50% of grass and sedge weeds. Aquatic broadleaf weeds and algae were suppressed by water management, and were not present in either study year. Glyphosate PPB caused rice first-leaf dieback, but no other symptoms developed. Planting depth and cultivar did not affect date of emergence either year. Deeper seeding reduced M-206 and M-209 stands by 15.4% and 5.2%, respectively, in 2018, but not in 2019. Increased tillering compensated for stand reductions in 2018.

Panicle yield components were largely unaffected by planting depth in 2018, however florets panicle-1 and filled grains panicle-1 were slightly greater for both cultivars seeded at 6 cm, compared to 3 cm planting depth. In 2019, M-209 suffered reductions in florets panicle-1 and grain filling when planted to 6 cm depth. Grain yields were not affected by planting depth in either study year. M-206 and M-209 grain yields were 10.2 T ha-1 and 12.2 T ha-1 respectively, in 2018, and 9.4 T ha-1 and 9.1 T ha-1 respectively, in 2019. This body of research has identified high-vigor California rice cultivars that are suitable for planting to depths up to 6 cm. In doing so, we identified critical rice vigor traits that may aid breeders in selection for lines that can rapidly escape deep seeding. It also serves as a successful proof-of-concept for the stale-drill method as an alternative stand establishment method in mechanized rice production. Finally, it establishes that using stale-drill permits the safe use of a PPB treatment with a non-selective herbicide at stand emergence. Proper water management and scouting are essential to ensure that PPB treatments do not injure emerging rice to the extent that weak or reduced stands result. However, by way of permitting the use of novel herbicidal modes of action, this method may be a useful rotational strategy in fields with difficult-to-control weeds, including herbicide-resistant populations or weedy rice.The California rice [Oryza sativa L.] growing region comprises approximately 200 000 ha in the Sacramento Valley. The rice cropping system is almost exclusively water-seeded , wherein pre-germinated seed is sown by aircraft into flooded fields. Seeds sink to the soil surface and peg down roots, emerging from the water after several days. Floodwaters are generally kept to 10 to 20 cm depth for the entire season. Water seeding was widely adopted in the region in the 1920s as a means to suppress competitive grass weeds , and has been the predominant method of rice cultivation in California ever since . Continuous use of water seeding has resulted in a small spectrum of weed species that are well-adapted to the system, and are very competitive with rice .

Water seeding conditions encourage aquatic broadleaf weeds such as arrowheads , ducksalad [Heteranthera limosa Willd.], resdstems , and Monochoria spp., and the sedges rice field bulrush [Schoenoplectus mucronatus Palla], tall flats edge and small flower umbrella sedge . In addition, grass ecotypes that are able to escape flooding depths of up to 20 cm, such as barnyard grass [Echinochloa crus-galli P. Beauv.] , early watergrass [E. oryzoides Fritsch], late watergrass [E. oryzicola Vasinger] , and bearded sprangletop [Leptochloa fusca Kunth ssp. fascicularis N. Snow] have become an important weed management issue in California rice. As the permanently-flooded cropping system effectively precludes the use of most other cultural weed management practices, for most growers herbicides are the sole means of weed control outside of water management .Although effective herbicides have been available for California rice since the 1960’s, the nearly exclusive use of the water-seeded system has meant that the number of registered active ingredients remains small, amid water contamination concerns and California’s stringent regulatory structure . To date, there are 13 registered active ingredients for water seeded rice in California, weed rack across nine modes of action . Most MOAs have only one registered A.I. . This limited palette restricts herbicide rotation. Since California’s rice acreage is largely planted back to rice each year, the combined effects of water seeded monoculture, limited available herbicides, and extensive use of individual MOAs on a small weed spectrum has resulted in widespread cases of herbicide resistance in the region . Herbicide resistance has been a major biologic and economic issue in rice for decades . The lack of diversity of registered herbicide A.I.’s and modes of action means that once resistance to a particular MOA arises, it can spread rapidly within and among fields as there may be few alternative herbicides to control the resistant populations. For example, L. fusca populations resistant to clomazone have only three other effective herbicides available , and two of those three, cyhalofop and thiobencarb, are subject to long water-holding restrictions after application which may reduce their utility for some growers. Efforts to combat herbicide resistance in California are also hampered by the fact that rice herbicides are more costly in California than in much of the world. Therefore, many growers are burdened with potentially unsustainable herbicide costs in order to control resistant weeds in their fields. Most cultural methods for weed and resistance management in California are modifications of the dominant water seeded system . One such method used by some growers is a stale seedbed. In this method, rice seedbeds are prepared as usual and flushed with water to promote weed germination.

Non-selective herbicides are used as a burn down treatment , and afterward the fields are flooded and seeded as usual. This method can be a useful strategy to manage weeds that are resistant to current rice herbicides, as well as reducing weed seed banks. However, due to the time needed to reflood and seed fields afterwards, stale seedbed use can delay rice planting, shortening the growing season and potentially depressing yields . Another common rice cropping system in mechanized rice is drill seeding . Drill seeding rice typically involves drill seeding dry seed to 1.25-2 cm and flush-irrigating fields for the first few weeks as the rice stand develops and herbicides are applied, before flooding for the remainder of the season . This method discourages aquatic weeds and algae, but tends to favor grasses . Furthermore, as the seed is typically sown to fairly shallow depths, it emerges synchronously with competitive grasses . However, if rice is drilled to depths greater than 2 cm, the rice should emerge later than the majority of grasses and sedges. This may allow novel weed management practices to be used without causing injury to the emerging rice . Although older semidwarf rice cultivars tended to have lower emergence rates from deep plantings , higher vigor semidwarf cultivars have been produced in recent years . For example, California rice cultivars are bred for water-seeding, and thus have suitable vigor to emerge through water depths of up to 20 cm . This high vigor may make California rice varieties suitable for drill seeding to depths greater than 2 cm.If rice cultivars can emerge quickly and evenly from deeper plantings, it may be possible to combine a stale seedbed with drill seeding. This “stale-drill” method could permit the use of modes of herbicidal action not registered for use in water-seeded rice. This would allow growers to safely manage herbicide resistant weeds and reduce seed banks prior to rice stand emergence, without injuring rice, delaying planting, or shortening the season. If used in rotation with water seeding, stale-drill can also vary the weed spectrum year over year, reducing the tendency of a small number of species to dominate. In this way, the stale-drill method might be a useful tool for herbicide resistance management in mechanized rice production worldwide. The purpose of this study is to test the hypothesis that drilling rice below the zone of active weed germination will delay rice stand emergence sufficiently to allow a safe application of a non-selective post plant-burndown herbicide treatment.Echinochloa spp. were the dominant weeds present in both years, followed by Leptochloa fusca ssp. fascicularis and sedges. Cyperus difformis and C. eragrostis were the only sedges present in 2016; no sedges were present in 2017. No broadleaf species were present in either year. Treatment timing differences due to rice planting depth had mixed effects on weed control, however, overall weed control was greatest with treatment 5 either year, regardless of rice planting depth. In both years, UTC, T1, and T2 plots were very weedy at all planting depths. Weed population density varied between years. Echinochloa pressure was greater in 2017 than in 2016, with 2017 UTC plots roughly 3.75-fold weedier than 2016 UTC plots. Echinochloa plant density generally decreased with more comprehensive herbicide treatments in both years , although decreases were more consistent in 2016. In 2016 glyphosate alone reduced Echinochloa spp. density from UTC by 40%, 19%, and 6% in 1.3 cm, 2.5 cm, and 5.1 cm riceplanting depths, respectively, whereas glyphosate f.b. pendimethalin reduced Echinochloa spp. density by 72%, 36%, and 17% over the same depths.

It was hypothesized that, as the water treatment pH increased, there would be greater removal of herbicide from the soil in the rinsate. As pH of the solution increased, the equilibrium of the weak acid herbicide would be pushed towards the anionic herbicide form resulting in lower sorption to the soil and greater removal in the rinsate. However, the opposite trend occurred for two of the herbicides . As water treatment pH increased, the concentration of saflufenacil and penoxsulam decreased in the aqueous solution extracted from the soil. Saflufenacil removal in the rinsate was greatest at pH 5 with about 78% and lowest at pH 8 with about 64%. At pH 5, about 35% of the penoxsulam was removal from soil and this decreased to about 22% removal at pH 8 . Indaziflam was below the detection limit of the instrumentation in all rinsate samples, regardless of pH.The results of the EC water treatments show that, as EC increased, herbicide removal decreased slightly . The effect of ionic strength on ionizable pesticide adsorption to soil has been well documented12; the common trend is that as ionic strength increases, the pesticide adsorption also increases . These data support that trend as well. The greatest amount of saflufenacil and penoxsulam was removed from soil in the 0.5 dS m-1 water solution rinse; about 70% of saflufenacil was removed from soil and about 25% of penoxsulam was removed. Meanwhile, in the 1.5 dS m-1 and 3.5 dS m-1 solutions, approximately 65% of saflufenacil and 22% of penoxsulam was removed . Indaziflam was below the detection limit of the instrumentation in all rinsate samples regardless of ionic strength of the rinse solution.Indaziflam was below the detection limit in all samples; however, it is not clear if this is due to strong sorption to soil or to degradation processes.

While indaziflam is considered moderately mobile to mobile in soil14, vertical farming racks it does have a higher Koc range than saflufenacil or penoxsulam23 meaning indaziflam would be more strongly sorbed to soil than the other herbicides in this study. Indaziflam has been reported to undergo photolysis in aqueous solutions rather quickly 14; samples were stored in the dark for much of the duration of the experiment. A brief follow-up experiment confirmed the laboratory lights did not cause photolysis of the chemical in aqueous solution under the conditions of the experiments . Saflufenacil dissipates relatively quickly in the environment23. The herbicide has biotic and abiotic degradation pathways but the most relevant pathway to this study would be hydrolysis in alkaline water13. The data set shows a significant decrease in herbicide removal from soil from pH 5 to pH 6 and 8 . The pH 7 data point was not statistically different from the other pH water treatments. There have been differing reports on penoxsulam hydrolysis. The Environmental Protection Agency states that penoxsulam is stable under hydrolysis conditions15 while Jabusch and Tjeerdema report triazolopyrimidine sulfonamide herbicides do undergo hydrolysis and the rate is dependent on pH24. There have been studies completed on two other herbicides in the TSA class which support pH dependent hydrolysis rates25-26. Given that the experimental samples were held at field capacity for seven days in this study, pH dependent hydrolysis could explain why penoxsulam concentrations were decreasing from 34% removal from soil at pH 5 to 22% removal at pH 8 .Adsorption mechanisms of pesticides are difficult to define because of the complex interactions between the soil surface, soil solution, and pesticide. Additionally, it is likely more than one adsorption mechanism occurs.

There are several mechanisms by which weak acid pesticide adsorption could be positively influenced by ionic strength – cations could displace hydrogen atoms from the soil surface resulting in a slight pH decrease that would favor a neutral pesticide form, more cations could be available to bridge the anionic form of the pesticide to the negatively charged soil surface, or cations could bond with the anionic pesticide resulting in a neutral form. A recent study on the adsorption-desorption properties of penoxsulam narrowed down the possible sorption mechanisms to H-bonding, cation bridging, and surface complexation with transition metals. The data set presented here supports the cation bridging mechanism. As ionic strength of the water treatment was increased, cation concentration increased resulting in the greater likelihood to bridge the anionic form of penoxsulam to the negatively charged soil surface. Figure 1.3 shows no statistical significance between ECw 1.5 dS m-1 and ECw 3.5 dS m-1 , this likely indicates most of the available binding sites of the soil were occupied close to ECw value 1.5 dS m-1 . Due to the similarity in size and ionizable functional group to penoxsulam, it is likely that saflufenacil is undergoing the same phenomena. The water treatments representing different irrigation water quality parameters did have a slight effect on saflufenacil and penoxsulam sorption to soil. The pH treatments indicated that both herbicides likely experience pH-dependent hydrolysis; saflufenacil and penoxsulam showed a decreasing trend in herbicide removal with increasing pH, the opposite of what the hypothesized pH effect would be. This indicates that even if irrigation water has relatively high pH, it is unlikely to substantially change the availability or movement of saflufenacil orpenoxsulam in California orchard soils. Results from the ECw treatments showed that flushing soil with a solution with moderate ionic strength could help saflufenacil and penoxsulam bind to soil versus low ionic strength. While there were statistically significant differences between water treatments, the overall effect on herbicide dissipation was minimal; the observed difference between the highest and lowest ECw treatment was only about 10% for each herbicide.

In the United States almonds are a $6 billion commodity grown solely in California making almonds the second highest grossing commodity in the state behind only dairy products . As of 2020 there were more than 500,000 bearing hectares of almond trees planted in California which produced 1.3 billion kilograms of almonds . Almonds are harvested by mechanically shaking the trees, sweeping the almonds into windrows, and picking the nuts up from the orchard floor. Preharvest herbicide programs and mowing are used to control vegetation that would otherwise reduce harvest efficiency . Glyphosate has been registered in almonds since the early 1990s and glufosinate has been registered since the early 2000s ; these are commonly used herbicides for preharvest orchard preparations because of their broad spectrum weed control and relatively short preharvest interval , three and 14 days, respectively. In 2018, over one million kilograms of glyphosate and nearly 300,000 kilograms of glufosinate-ammonium were applied in almond orchards . Because of the harvesting process, there is ample opportunity for the almond hulls, shells, and kernels to be in close contact with herbicide-treated soil. The majority of California’s almond crop, about two-thirds, is exported and generated more than $4.9 billion in 2019 . Of the exports, 22% were shipped in shell and 78% were shipped shelled . Asia is the largest aggregate market for in shell almonds while the majority of shelled almond shipments go to European markets . Exported shipments of almonds are subject to pesticide residue testing and must be at or below a maximum concentration set by the region’s food safety authority.The maximum residue limit for glyphosate and glufosinate in almonds differ by definition as well as concentration between the European Union and the US. In the United States, both glyphosate and glufosinate MRLs, which are commonly called tolerances, are defined to include the parent compound as well as its primary metabolites . For clarity these MRLs will be referred to as “total glyphosate” or “total glufosinate” if the concentrations of the metabolites are to be summed with the concentration of the parent compound. The US MRL for glyphosate in almond hulls is 25 mg kg-1 and 1 mg kg-1 for kernels. There is not a separate US MRL for in shell almonds because the residue in inshell almonds is determined by shelling the almonds and measuring the residue in only the kernels. The US MRL for total glufosinate in almond hulls and kernels is 0.5 mg kg-1 . In the European Union, the MRL for glyphosate is 0.1 mg kg-1 in almond kernels . The EU MRL for glufosinate includes its metabolites; the MRL for total glufosinate is 0.1 mg kg-1 . Glyphosate is registered in the EU until 2022 . A recent review completed by the European Food Safety Authority recommended that the MRL for glyphosate be reduced to 0.05 mg kg-1 and an optional total glyphosate MRL for the summation of glyphosate and its primary metabolites, AMPA and N-acetyl-glyphosate, set to 0.2 mg kg-1 . Hence, it is anticipated that in upcoming years glyphosate MRLs will be reduced, vertical grow rack and it is a possibility that the chemical may not be re-registered. According to statute, if at any time thesafety of a current MRL is reconsidered, the MRL can be reduced to the lowest limit of analytical detection which is 0.01 mg kg-1 .

Because of the importance of the European markets to the California almond industry and the importance of glyphosate and glufosinate to preharvest preparations, lab and field studies were conducted to evaluate the herbicide transfer from soil to almonds during harvest. The objectives were to determine if glyphosate and glufosinate residues can transfer to almonds from soil particles or directly sprayed almonds, whether increasing the PHI could substantially reduce the risk of herbicide in or on almond fractions and quantify the concentration of soil-bound herbicide in almond samples.This experiment was conducted to determine glyphosate transfer from directly-treated almonds to non-treated almonds. This was intended to mimic a situation where a small number of almonds fall to the ground very early and could conceivably be directly sprayed with preharvest treatments and then contaminate the later-harvested crop during harvest and handling steps. Two almonds were directly treated with 0.8325 MBq [14C]-glyphosate by using a microsyringe to dot the stock solution over the entire almond including the inside of the split hull and exposed shell. The two treated almonds were tumbled with nine non-treated almonds using the apparatus and methods described earlier. The almonds were tumbled using a rock tumbler for 15 minutes and let rest for 15 minutes. Before analysis the treated almonds were removed from the bottle, and the untreated almonds were dissected and analyzed for [14C]- glyphosate. This experiment was replicated four times.The whole almonds from each replicate from both soil transfer experiments and the almond-to-almond transfer experiment were separated for three different analyses: whole almond rinse, herbicide adsorption to almond fractions, and a surface swipe after a post-harvest mimicking process. All samples were analyzed using a liquid scintillation counter . The data were corrected for the background levels of radiation in the scintillation counter. The rinsate of whole almonds was used to determine how much [14C]-herbicide was loosely associated with the surface of the almonds. Three whole almonds were rinsed with water using gentle inverted shaking. The rinsate was collected into glass scintillation vials and evaporated using a vacuum evaporation system at 30°C . Once the samples were evaporated to near dryness, 10 mL of Ultima Gold™ was added to each vial. The samples were analyzed using the liquid scintillation counter. To determine how much herbicide was adsorbed to the almond fractions, three almonds were dissected into their hull, shell, and kernel components. Each component was homogenized using a mortar and pestle and liquid nitrogen. Approximately 500 mg of each homogenized almond fraction was collected into a combustion cone and combusted using a sample oxidizer . The combustion product, [14CO2], was collected in 20 mL of scintillation cocktail composed of 10 mL CarboSorb E® and 10 mL Permafluor® . Glass scintillation vials containing the [14C]-samples were analyzed using the liquid scintillation counter. The remaining three almonds went towards a post-harvest mimicking process. The almond hulls were discarded, and the shells were opened by hand cracking through a plastic barrier then discarded. The plastic was swiped using a filter paper and the swipe was added to a glass scintillation vial with 10 mL Ultima Gold™. The swipes were analyzed using the scintillation counter. The kernels were collected, homogenized and combusted, and the combustion product was mixed with scintillant and analyzed using the scintillation counter as described above.

This one application will provide 4 to 6 years of adequate sulfur nutrition to the new pasture plants. Visible symptoms of sulfur deficiency include stunting and yellow color, although these symptoms also commonly indicate nitrogen deficiency. If you suspect sulfur deficiency after pasture establishment, cut the top 4 to 6 inches off of leaves at early bloom and submit a plant tissue sample to a lab for analysis. If grass tissue results indicate a sulfur level of less than 0.10 to 0.15 percent, you may have a sulfur deficiency. Common solutions are broadcast applications of soluble sulfur fertilizers such as ammonium sulfate or gypsum.The overriding characteristic common to newly established, dryland plant species is their low seedling vigor and slow growth and maturation. Typically, dryland perennial seedlings are so slow to get started that even several months after seedling their slim, vertical stalks are difficult to see. Since perennial seedlings mature slowly and are particularly susceptible to stress, a grower may not consider the stand to be “established” until it is 3 years old. Potential causes of pasture damage or loss may include lack of moisture and consumption by any number of animals, including cattle, deer, rabbits, ground squirrels, mice, and insects such as grasshoppers. Proper timing of cultural practices, seedling methods, and weed control help improve soil moisture conditions, but grazing must be light and controlled during pasture establishment to avoid excessive seedling losses. Try not to allow grazing in the year of establishment, indoor weed growing accessories at least until the seedlings have completed their growth for the first growing season. Timingwill vary depending on elevation and site conditions, but this usually means no grazing before July 1.

Under favorable growing conditions, the seeded plants will have developed numerous leaves and produced a seed head. After the plants produce a seedhead and go dormant , livestock can graze them down to 3 to 4 inches in height. If growing conditions are poor and the plants do not produce a seedhead, do not allow grazing at all. Regardless of growing conditions, many people merely plan not to graze a newly established pasture in its first year, just to be on the safe side. Proper grazing management is hard to determine during that first year. Often, the greatest benefit to the pasture from grazing is that grazing animals remove weeds such as annual mustards and grasses. Consider any grazing during the first year to be a prescribed clean-up operation rather than an extensive feeding on perennial seedlings. The key is to avoid grazing either too early in the summer, too close to the ground, or in muddy conditions. Grazing in late fall or winter should be avoided if muddy conditions exist, since the pasture will have developed very little sod by then to stabilize the soil and prevent soil compaction or erosion. Although livestock grazing can be controlled during seedling establishment, it is often impossible to control grazing by wildlife. Small plantings usually are the most susceptible to wildlife damage since it only takes a few animals to completely defoliate a small stand. In areas with large numbers of deer, rabbits, ground squirrels, or elk, you can seed areas of five or more acres at a time to reduce the chance of a complete loss of stand from wildlife grazing, although some areas of your planting may still be severely damaged. Along with wildlife, insects such as grasshoppers, Mormon crickets, and black grass bugs can cause significant damage to new seedlings. During a year with high insect populations, you may have to apply some sort of control. In most cases, insecticides provide the best control for insects feeding on new seedlings, since a large insect population can destroy a stand in a matter of a few days.

You will have to choose the most appropriate insecticide to use based on the particular insect pest and local site conditions, so you will do best to consult an agricultural specialist in the area before you apply a treatment. The second season after seedling, your management efforts should still focus on pasture stand establishment and you should continue to follow similar grazing guidelines to those you used the first year. Try to delay grazing until the plants have had the opportunity to complete their full growth for the season. Once drought conditions and cool temperatures have forced the plants into dormancy, graze them to a height of 3 to 4 inches and avoid grazing in muddy conditions.If growing conditions are favorable during the first two years, plants generally are well established by the third spring and you can proceed to manage the field as an established perennial grass pasture. If the first two years are marked by drought conditions, allow a third year of restricted grazing. This timeline is most applicable to introduced species such as crested or intermediate wheat grasses that have good seedling vigor and grazing tolerance. Native species that grow slowly and are sensitive to grazing may need three to five years to become completely established.Be cautious and conservative when you manage livestock grazing on dryland pasture, since desirable, dryland plants are quick to be degraded and slow to recover. Do not start a problem that may persist for years, just in order to get a little extra grazing in one year. Be especially careful when you graze livestock during drought cycles that persist beyond a single growing season. The combined stress of drought and heavy grazing will significantly diminish plant vigor. Under severe conditions, improper grazing can lead to the loss of desired plant species, which will then be replaced by weeds. Weeds make pasture rejuvenation difficult and often lead to permanent changes in vegetation. Historical practice indicates that the best approach is to vary the season of use and to leave half of the forage growth ungrazed. For example, forage used in the spring this year should be grazed later in the growing season next year.

When grazing in late spring and early summer, leaving half of the current year’s growth is a conservative practice that preserves the plants’ energy reserves in their roots and stems. If grazing is delayed until late summer or fall, pastures can be grazed down to a 3- to 4-inch stubble height without causing harm. In practice, these recommendations are best suited to producers who have enough pastures to permit a rotation that grazes some fields early one year and late the next year.But if you only have a single pasture to work with, it is best to defer grazing until early summer in order to prevent weed invasion and allow the plants to recover their energy reserves. If the area receives ample precipitation, a single-unit pasture can occasionally be grazed in early spring, but always make sure to leave half of the current year’s growth and stop the grazing before soil moisture is depleted. When wheat grass pastures are grazed in early summer, it may be possible to graze them again lightly in the fall or early winter. If you do allow fall or early winter grazing, begin after the onset of winter dormancy and stop before the initiation of spring growth . Depending on what type of livestock are grazing, you may need to supplement their feed with hay in order to provide adequate nutrition during fall and winter grazing periods. Early spring grazing usually is not appropriate for degraded ranges or areas that are susceptible to invasion by annual weeds. In field trials conducted on Siskiyou County pastures that had high annual weed pressure, early spring grazing resulted in a more extensive invasion of undesirable annuals than in pastures subjected to late spring or summer grazing . This research suggest that canopy removal in early spring allows greater exposure to sunlight and seed-to-soil contact to encourage the establishment of annual weeds that out-compete later maturing perennials. For this reason, it is probably best to avoid early spring grazing.Even if you do not graze your pastures, vertical grow rack system you sometimes have to remove plant cover using non-grazing management practices to prevent insect and disease problems and fire hazard concerns. The decision whether to mow or burn depends on how you want the pasture to look as well as several site characteristics. If unmanaged, perennial grasses create large amounts of dry, dead grass , posing a fire hazard. You can leave some plant material intact to help prevent invasion by non-native annuals and weeds, but if you leave too much it can lead to insect and fire problems. When practical, try to mow and bale excess forage and so reduce the accumulation of thatch and accelerate its decomposition. The best time to mow is after perennial grasses produce seed. You may also want to use controlled burning to remove accumulated thatch. Burning is a risky endeavor, though, and must be coordinated with the local fire department or the California Department of Forestry and Fire Protection . The best time to burn is after completion of perennial grass growth, but the exact timing of the burn depends on permit restrictions as well as site characteristics and other vegetation growing on the site. The influence of soil properties on herbicide efficacy has been widely studied as well as the influence of spray water quality on herbicide performance. However, limited studies on the effects of irrigation water quality on herbicide dissipation have been completed. This study was conducted to evaluate the effects of water pH and salinity on the dissipation of saflufenacil, indaziflam, and penoxsulam in two representative California orchard soils.The experimental protocol is a modified version of the method published by Sheppard et al.. The experiment was a completely randomized design with each herbicide being tested in both soil types at every water treatment and replicated three times. The amount of soil used in this experiment was determined by the bulk density of the soil and the assumption of a 25 cm2 spray area and a 2 mm soil depth. Each loam experimental unit contained 5.4 g of soil and each sand replicate contained 6.1 g of soil. Soil was first treated with herbicide by weighing appropriate amounts of each soil into a weigh boat, pipetting 1 mL of herbicide solution onto the soil, homogenizing by vigorousmixing, then letting the mixture sit for 24 hours until completely dry. The treated soil was then transferred into a 50 mL centrifuge tube equipped with a 0.22 µm Nylon filter . Soil was brought to field capacity by adding 1.220 mL of water treatment to loam soil or 0.780 mL of water treatment to sand soil, covered with parafilm, and left in the dark, at room temperature for seven days. After the resting period, the parafilm was removed and the samples were centrifuged at 6000 m s-1 for 15 minutes using a Sorvall Legend XTR centrifuge . After the initial centrifugation, an additional 1 mL of the respective water treatment was added to each sample then samples were centrifuged again at 6000 m s-1 for 15 minutes. This process was repeated once more for a total of two 1 mL water treatment aliquots washed over every sample after the field capacity water was removed. A separate pilot study completed to establish the number of water treatment washes needed to remove the unbound herbicide from the sample indicated that two 1 mL washes was adequate for the purpose of the experiment . The centrifuge filter was removed and discarded while all water from initial incubation plus the two 1 mL aliquots were collected from the centrifuge tube and filtered using a 0.22 µm Nylon syringe filter. The filtered solution was collected in an HPLC vial and analyzed using high performance liquid chromatography .Analyses were performed with an Agilent C-18 Poroshell 120 column in an HPLC system equipped with a diode array detector. Mobile phase A consisted of ultrapure water and mobile phase B consisted of acetonitrile with 0.1% formic acid. Chromatography was accomplished using an isocratic elution of 60% mobile phase A and 40% mobile phase B. The method run time was 9 minutes. All samples were observed at 270, 268, and 205 nm which corresponded to the absorbance of saflufenacil, indaziflam, and penoxsulam, respectively. The approximate retention time of saflufenacil, indaziflam, and penoxsulam were 4.9, 2.5, and 2.6 minutes, respectively . Samples were background corrected and converted into units of percent removal from soil using 5-point calibration curves .

Expansion of intermodal and inland port capabilities can significantly lower transportation costs for commodity import and export flows, helping to make global markets more cost competitive, accelerating regional economic development, and attracting business. By extending the gates of container ports inland, inland port systems enable shippers to efficiently serve new logistics pathways supporting online business divisions and e-commerce. Being able to serve customers with next-day, same-day, or even one-hour parcel deliveries is highly valuable to businesses, and many have reorganized their supply chains into multichannel configurations by replacing regional distribution centers with smaller, forward distribution centers in urban areas. Many of these warehouses need to be replenished with multiple incoming truckloads each day, in addition to generating many outgoing trips for local deliveries. Such fulfillment centers are increasingly co-located with manufacturing centers and intermodal ports, leading to more numerous and larger freight clusters around intermodal rail heads. Three commodities, “mixed freight,” “plastics and rubber,” and “other foodstuffs,” appear in the top ten commodities in Georgia for both ton-miles and value. Mixed freight is a commodity group suggesting the cargo consists of a variety of different types of products. It is the most common commodity arriving at distribution centers as well as many retail businesses and restaurants because the commodity can include certain food items, hardware, office supplies, clothing, and much more. Because distribution centers often handle a variety of goods to serve their customers, the generalized nature of the mixed freight commodity make it a useful classification and reduce administrative burden, compared with using multiple specific product classifications. While not all mixed freight movements can be unequivocally associated with distribution centers, vertical cannabis freight flows for the commodity are more likely to be observed along intermodal freight systems in drayage movements between intermodal terminals and fulfillment centers, and beyond in delivery movements to customers and points of sale.

Because of its expected on-road behavior, the commodity is more likely to be one that could technically achieve high penetration of BE truck technology.The frequent proximity of forward distribution centers, intermodal ports, and population centers improve electrification prospects for vehicles on freight vocations connecting these locations by shortening typical trip distances and encouraging a “out-and-back” tour cycle, where trucks begin and end their routes at the same location, making charging equipment siting more straightforward. It is hypothesized that trips to and from intermodal ports could have a relatively high number of operational characteristics that make these flows high value opportunities for investment in BE deployments, especially as intermodal flows continue to grow. High utilization and miles traveled typically improve the economics of BEVs. Increasing trip frequencies could represent increasing electrification benefits, so long as BE technology can adequately fulfill service demands within the constraints of battery capacity and charging requirements. Exploring these parametric relationships and testing these hypotheses was central in the design of the use-case study.Container drayage is typically conducted using Class 8 combination tractor trailer day cab units. For this use-case, we assume that the fleet is a small privately owned and operated third-party logistics operation consisting of three MY 2008 Class 8 combination trucks, all performing similar drive cycles on the same route. The trucks are assumed to travel from ARP to the distribution center with a full 20-foot shipping container of payload. Upon arrival, the full container is unloaded on-chassis at a staging area or delivery bay. The truck then picks up an empty container chassis and returns to ARP. The total distance of the tour on public roads is 122.2 miles. The vehicles have been operating on this vocation since their acquisition.

The owner-operator is planning to evolve their fleet and is seeking to understand how the energy use and economic implications of fleet management decisions will affect their business. They have decided to replace their vehicles with new MY 2023 trucks and want to understand electrification potential for their operation. To quantify the pros and cons of fleet electrification compared to the purchase of new traditional diesel trucks, we analyze both onroad and upstream energy consumption and emissions for each technology and fuel type for this use-case. We also quantify fuel, maintenance, and capital costs of both purchasing scenarios. MOVES is the federal regulatory model for quantifying on-road emissions and energy consumption for any use-case. MOVES essentially calculates the second-by-second power demand for a vehicle in units of vehicle specific power , or in the case of heavy-duty vehicles, scaled tractive power .For this analysis, road grade impacts on energy consumption are not considered. This area of northern Georgia does possess considerable elevation changes and the presence of grade on truck routes could significantly improve or hinder electrification process. Route segments with high percentages of downgrade are opportunities for energy savings and charge regeneration via regenerative braking systems. Routes with steep upslopes increase energy demand on the driving cycle. Given that downgrades never recover all of the energy lost moving uphill, routes with significant grace can limit the feasibility of some routes for BE MHDV applications. Grade effects impact outcomes on a case-by-case basis and warrant inclusion in subsequent studies but are out of scope here. In the TCOST tool, which will be discussed in the next section, energy demand effects of road grade are captured by the fuel consumption user input which informs the model’s energy use assumptions.

To calculate VSP for every second of the vehicle’s driving cycle, a driving cycle with 1-hz speed and acceleration data is required. To capture the energy consumption effects of evolving driver behavior across different road types on this tour, a composite driving cycle was created using various standard regulatory cycles for heavy duty trucks with some modifications. In this manner, the generated driving cycle was crafted to be as realistic as possible until telematic device deployment can provide second-by-second data. The driving cycle is one-way between ARP and the distribution center. Its profile is depicted in Figure 6, which is color-coded to show how each segment of the trip was combined. The composite driving cycle was manufactured using various heavy heavy-duty truck cycles made available by Georgia Tech through the U.S. EPA and state regulatory agencies. The beginning of Segment 5, which is the leg of the trip on GA 140 between the I-75 exit ramp and the distribution center access road , is a modified version of a U.S. EPA HHD truck creep cycle used for characterization for truck emissions in California. The total travel time for this one-way driving cycle is 104.3 minutes. For simplification, we assumed a truck on this use-case would execute this driving cycle with a full container load before executing it again in reverse with an empty container as the return trip.Energy consumption for the driving cycle was 461.58 kWh for the MY 2023 truck in 2022. Assuming a ten metric ton payload, this equates to 1.324 ton-miles per kWh for the MY 2023 truck. Fuel consumption was 11.34 gallons. BEVs have higher curb weights than vehicles with traditional power trains because of the added weight of their battery packs. For weight-constrained shipments, studies have found electrification of freight trucks will require a maximum payload reduction anywhere from 1.25 to 2.0 or more tons to accommodate the increased weight of the electric power train.Reducing the tonnage of cargo per truckload can negatively affect profit margins and potentially disrupt supply chains and the effects of electrification on the ton-mile capabilities of a fleet must be considered for comprehensive analysis. In this example, we assume the payload is constrained by volume rather than tonnage such that an increased curb weight will not necessitate a decreased payload and should not have any effect on the quantity of goods delivered. More refined analyses can be performed if actual payload data can be collected for individual use cases. Electrification of this use-case is technically achievable. Many new electric Class 8 combination tractors on the market have battery capacities of 500 kWh or more. However, grow racks nameplate capacity is not representative of available charge, as BEV systems will typically prevent batteries from depleting below a certain threshold of total capacity to preserve battery health and longevity. Assuming a 550-kWh battery, the driving cycle could be completed so long as at least 84% of capacity was actually available.

The primary implementation challenge is operational. Designing charging schedules that are symbiotic with delivery schedules and do not cause unacceptable amounts of downtime is critical to the success of BE technology on this vocation. To be able to transition to BE trucks entirely, the vehicles would need to have an opportunity to charge after each one-way trip on the route, which might necessitate the acquisition of multiple chargers and garage locations near each terminus. To evaluate the financial aspect of BE truck purchases as compared to traditional ICE truck purchases, a series of assumptions guided by real-world conditions observed today and projected into the future were constructed. Purchase prices for Class 8 BE MHDVs were collected from PG&E’s vehicle catalogue. Diesel truck purchase prices were collected from OEM specification sheets and websites, as well as from California HVIP. For this use-case example, a new MY 2023 Class 8 diesel truck was estimated to cost $107,433 and a comparable BE option was estimated to cost $300,000. In Georgia, new vehicle purchases are subject to a 6.6% title ad-valorem tax . Additionally, all new Class 8 vehicles are subject to a 12% federal excise tax at the time of purchase. Diesel and electricity price projections were collected from EIA and are shown in Figure 9 and Figure 10. Maintenance costs per mile were gathered from AFLEET and California HVIP. All financial assumptions are displayed in Table 6.Using the parameter values in Table 6, the total cost of ownership of purchasing new BE trucks is compared to that of new diesel trucks. It was assumed the fleet manager would opt to pay a 10% down payment at the point of sale for the new vehicles and that they would finance the capital cost of the vehicles over a 72-month period at a 5% interest rate. Based on the composite driving cycle constructed for this use case, we assumed that a typical day’s operation would be about 140 miles round trip for 250 workdays per year, and that the average fuel economy was 5.38 miles per gallon. The new vehicles were assumed to have a 20- year lifespan, and retired vehicles were assumed to have no real resale or salvage value. With these assumptions, the total cost of ownership was modeled, including purchase price and financing, operation cost, and maintenance cost. All future cash flows were discounted using a 5% discount rate. BE powertrains are more efficient than ICE powertrains. CARB has found that heavy duty electric trucks have energy efficiency ratios ranging from 3.5 to more than 7 when compared to diesel trucks, depending on operational speed. The efficiency ratio curve produced by CARB is reproduced in Figure 11. The average speed on the composite driving cycle is 32.8 mph. By using the regression equation provided by CARB, the average speed equates to an efficiency ratio of 3.73. Based on the ICE efficiency calculated at 5.38 mpg, the BE efficiency is found to be 20.05 mpge, or 0.52 miles per kWh.Electrifying this use-case would save the fleet $3,732.41 per vehicle, if operations could be designed to accommodate the technology. This includes the cost of two Level 2 charging systems, which could in theory be deployed near either end point of the route to allow for charging as needed after each one-way trip. Of course, other real-world considerations, like acquiring property for a second depot to install the charger on, may also increase costs for the BE truck pathway. Overall, the break even point for the BE truck would not be until its 20th year of operation. Performing more than one round trip per day, leading to a greater number of miles travelled, would improve the economics of electrification on this route because the bulk of the savings are in per-mile operating cost and maintenance cost. If charging schedules can be designed to accommodate delivery needs, and the demand for deliveries is adequate,increasing the freight activity in this freight operation would make electrification much more attractive. Finally, BEVs do not have tailpipe emissions.

Transgene flow from GE rice to weedy rice can result in diverse fitness effects, depending on the type of transgenes and the selective pressure to which the GE crop-weed hybrid descendants are exposed. In addition, other factors such as the genetic background of weedy rice populations that have obtained the transgenes may also influence the fitness effects of a particular transgene . Many studies have indicated that insect-resistance transgenes confer a fitness benefit for crop-weed rice hybrid progeny under high insect pressure . However, the same transgenes do not confer such a benefit to the hybrid progeny under low insect pressure. The fitness studies based on multiple generation descendants of GE crop-weed hybrids provide similar results . Therefore, we conclude that fitness change and evolutionary potential for transgene flow from GE insect-resistant rice to weedy rice populations are quite limited because of the low ambient insect pressure expected in extensively planted transgenic commercial rice production fields . In contrast, the movement of herbicide-resistance transgene to weedy rice populations appears to considerably change the fitness of the crop-weed hybrid progeny, both with and without the application of glyphosate herbicide sprays, and possibly the evolutionary potential of the hybrid progeny by altering their rate of biosynthesis and photosynthesis . This indicates that the movement of this specific herbicide-resistance transgene to weedy rice populations may result in increased weed problems.Our understanding of the fitness effects and expected evolutionary dynamics brought by transgenes, including those conferring herbicide resistance, drought and cold tolerance, hydroponic trays and stacked traits with diverse functions are still limited. It is clearly shown from the results of studies already done on the cultivated rice and weedy rice system that simple expectations from the transgene’s intended phenotype are not sufficient to predict what will occur under experimental conditions.

Are these results general? We sought to compare the results discussed above with a sample of results from similar field experiments involving different species and/ or different transgenes. We review a collection of such studies that are included in Table 2. Our sample involves twelve studies representing six crop donor species and eight recipient weedy/wild species. Four different transgenic phenotype classes are represented. The only generality that emerges is variability. Introgressed transgenes may or may not confer a fitness advantage under selective pressure associated with the intended transgenic phenotype. Without that selective pressure, the presence of the transgene may correlate with increased fitness, decreased fitness, or no significant fitness change. Taken collectively, the cultivated rice – weedy rice system case study reviewed above and the additional studies featured in Table 2 make it clear that the fitness changes associated with transgenic presence in unmanaged populations cannot be predicted a priori. While increased fitness in itself may not be sufficient to predict an environmental hazard, it does provide support for the conclusion that the transgene will persist and spread . Obviously, with regard to introgression-based transgene risk assessment, the current regulatory policy of case-by-case analyses informed by field-based research is sound and superior to predicting the fitness correlates of introgressed transgenes without such data.Glyphosate kills plants by inhibiting a particular enzyme, 5-enolpyruvyl shikimate-3-phosphate synthase. This enzyme is one of several in the shikimic acid pathway, which is how plants produce the aromatic amino acids phenylalanine, tyrosine, and tryptophan. Amino acids are building blocks for the plant, so a plant not able to manufacture all amino acids is unable to grow and develop normally.

Plants also use these three specific amino acids to synthesize more complex structural compounds and a host of plant defense molecules , which together can make up 60 percent of a plant’s dry weight. Consequently, inhibition of this pathway causes serious consequences for a plant, and it helps to explain why glyphosate is such an effective herbicide. It is also an herbicide with a very low mammalian toxicity, as mammals do not have the EPSP synthase enzyme.Glyphosate is normally formulated as a salt, which is a compound that can split into positively and negatively charged portions when mixed with water. Glyphosate salts include potassium, diammonium, isopropylamine, trimethylsulfonium, and sesquisodium. Formulations differ in how much glyphosate ends up in the final product, due to the chemistry of the salt and the different adjuvants used by the various manufacturers. The amount of the glyphosate salt in the formulation is listed on the herbicide label as the active ingredient . In the case of glyphosate, however, only the glyphosate portion of the salt is actually herbicidal; the other portion of the salt is nonherbicidal. Why would a manufacturer formulate glyphosate as a salt? Glyphosate salts are better able to enter into plant tissues than is the free glyphosate acid, so these formulations provide better weed control. Since different salts have different molecular weights, it would be difficult to determine how much actual glyphosate is contained in different formulated products if we just look at the a.i. content, usually listed as pounds of a.i. per gallon or grams of a.i. per liter . When comparing different formulations of glyphosate, it is better to look at the acid equivalent , which is the amount of glyphosate in the negatively charged or acid portion of the salt, the part of the a.i. that binds with EPSP synthase. Therefore, using the a.e. is also the best way to select the appropriate application rate for various formulations, since the a.e.represents the amount of glyphosate needed to control certain weed species .

Surfactants are the most commonly used adjuvants; they modify the surface tension of water and, when in mixture with an herbicide, cause applied droplets to spread out on leavesand improve herbicide uptake. Most agricultural surfactants are nonionic, although crop oils are also widely used; other surfactants are organosilicon based. Most glyphosate formulations contain an adequate concentration of surfactant for general use, so additional surfactant is usually not necessary. Exceptions occur when applying glyphosate to weeds with dense hairs or thick cuticles on their leaves or when using a formulation that does not contain added surfactant, such as aquatic formulations of glyphosate. Read the label to determine whether adding a surfactant to a particular glyphosate formulation, or for a particular weed species, is necessary. Water-conditioning agents are another major type of adjuvant. Because glyphosate can exist as a negatively charged molecule after the herbicide is mixed with water, it can react with positively charged ions or molecules in the water. Water containing a high concentration of cations is commonly called hard water. Some common cations in hard water include sodium , potassium , calcium , magnesium , and iron . Cations with more than one positive charge bind strongly to glyphosate and reduce its ability to be absorbed into plant leaves. Water conditioners, such as ammonium sulfate or other proprietary adjuvants, help to soften hard water. When AMS is added to water, the compound splits into two ammonium ions and one sulfate ion . This ionized AMS helps improve glyphosate performance in two ways. First, if glyphosate binds to ammonium, the resultant molecule is much more easily absorbed through the leaf cuticle, through the cell wall, or across the plasma membrane of certain weed species than when glyphosate is bound to other cations, resulting in more herbicide penetrating the weed. Second, sulfate preferentially binds to calcium, magnesium, and iron cations in the water, thus removing them from the solution and leaving more glyphosate free to move into the weed. Studies show that translocation of glyphosate is increased when AMS is added, pipp mobile systems due to improved phloem mobility, probably because more glyphosate in plant cells increases phloem loading and translocation of the herbicide in the weed. The general recommendation is to add 1 to 2 percent of AMS by weight to glyphosate mixtures, which is equivalent to 8.5 to 17 pounds dry AMS or 2.5 to 5 gallons of liquid AMS per 100 gallons of spray solution. Buffering agents are another type of adjuvant. The pH of water is a measure of the hydrogen ion and hydroxide ion concentration. As the number of H+ increases relative to OH− , water becomes more acidic and pH decreases. As noted above, when glyphosate is unbound, it has a net negative charge and is absorbed more slowly across cuticles and cellular membranes than when it is bound to certain cations as a salt. At a lower pH, more glyphosate exists as a salt than as a free acid, so plant uptake of the sprayed solution is improved. Consequently, slightly acidic water is most suitable for mixing with glyphosate. When water pH exceeds 7, consider adding buffers or acidifiers to lower the pH.Since glyphosate binds tightly to soil particles, its application to dusty plants results in inactivation of much of the herbicide before uptake can occur.

Glyphosate activity is usually poorer on weeds growing in wheel tracks, probably due to dust or mud on the surface of the plant foliage. Also, weeds that have been run over by sprayers or other vehicles may not be healthy enough to translocate absorbed glyphosate to their growing points, resulting in poor control. For optimal weed control with glyphosate, weeds should be relatively dust free at the time of application. Applications are therefore best made prior to the onset of dusty conditions in the summer. If weeds are already dusty, irrigation may be an option to wash dust off the foliage, followed by glyphosate application after the foliage has dried.Weed control with glyphosate has sometimes been observed to be better when applied at low volumes than at high volumes. This may occur if the low volumes are achieved by using nozzles with small orifices, resulting in the production of smaller droplets and increased foliar coverage. Perhaps, too, lower volumes of hard water contain fewer cations to bind with glyphosate in the mixture. Also, smaller droplets are more likely to drift, reducing coverage of weed foliage and increasing the chance of crop injury, particularly when glyphosate is applied when the crop is bearing leaves and is actively growing.When other pesticides or additives such as fertilizers are mixed with glyphosate solutions, an opportunity exists for the chemicals to bind with otherwise-inactive glyphosate. Sometimes the mode of action of certain herbicides may also slow or prevent translocation of glyphosate. Metribuzin , carfentrazone , and sulfentrazone are herbicides that antagonize glyphosate activity on certain weed species, while certain anti-drift agents have also antagonized glyphosate. The best way to avoid antagonism is to mix glyphosate formulations only with other products listed on the glyphosate label. Applying herbicides in separate applications rather than in a tank mixture may also reduce antagonism between herbicides. Since tank mixtures may offer improved control of other weed species, however, antagonism observed in certain weed species may be an acceptable trade-off.Glyphosate absorption through treated foliage is affected by environmental conditions shortly before, during, and after glyphosate application. Glyphosate must translocate from foliage to the site in plant cells where shoots or roots are being actively produced. Therefore, weeds under stress due to cold, heat, or improper amounts of soil moisture or weeds displaying symptoms from plant disease or previous herbicide application are usually not actively growing and may not respond as quickly or as completely to glyphosate application. Excess leaf moisture from dew or rainfall too close to the time of application can also reduce glyphosate performance. Conversely, glyphosate activity is usually improved with higher relative humidity. Leaf cuticles are usually more hydrated under humid conditions, resulting in better herbicide uptake, provided that leaf surfaces are dry during and after the application.The stage of growth and the life cycle of targeted weed species are important to consider if maximal control with glyphosate is to be achieved. Annual weeds are best controlled when they are small, when less glyphosate is necessary for a lethal dose. If killed prior to flowering, seed production will also be prevented. Glyphosate is strictly a foliar herbicide and does not exhibit residual soil activity. Weeds that have not emerged at the time of application are not controlled, so multiple applications are usually necessary to fully control both early- and late-emerging seedlings. Tank mixtures or sequential applications with soil-residual herbicides may improve weed control while reducing the number of herbicide applications necessary to fully control weeds. Perennial weed species frequently become more problematic the longer a perennial crop is kept in production. Directed sprays or spot applications of glyphosate are usually necessary to gain adequate control while preventing crop injury.

The odds ratio analysis showed that the likelihood of occurrence of feral sorghum along a gravel or dirt road is 1.5 or 2.4 times greater than that of a paved road, respectively . Dirt and gravel roads are prevalent in rural areas surrounding farmlands and high likelihood for the presence of feral sorghum populations along these road types suggests that movement of farm equipment and production activities greatly contribute to sorghum seed dispersal into roadside habitats. Further, wash boarding, corrugation and any potholes on the surface of unpaved roads can increase vehicle bouncing and thus increase the chances of seed spill from seed transport trucks and farm equipment. Contrary to our findings, the higher frequency of feral oilseed rape along the paved compared to the gravel and dirt roads in France was attributed to the higher traffic intensity with commodity transport on paved roads. Although the Chi-square test suggested significant differences between the road types , the odds ratio analysis failed to detect such differences . This may suggest that all road types are equally likely to accommodate feral sorghum, following a seed immigration event. Although previous studies have found a strong relationship between the presence of feral crops and road type, this was not the case for feral sorghum in South Texas. One notable exception in the present survey was that feral sorghum populations were common along U.S. Highway 77 between Kingsville and Raymondville, TX where sorghum and crop production in generalis very sparse. The high intensity of grain transport via truck movement is likely contributing to sorghum seed dispersal along this highway. Feral sorghum was more likely to be present at the road shoulders as shown by the odds ratio values , but they were also present in the field shoulders and at field edges . However, cannabis drying system as the road and field shoulders are only separated by few meters in many cases, feral sorghum found along the road verges could also be sourced by sorghum production activities in the adjacent fields.

This postulation is supported by a lack of significant difference between the odds of feral sorghum presence at field shoulders relative to that of the road shoulders, as shown by the odds ratio estimates . However, a logical question could be why feral sorghum is less abundant at field shoulders or field edges if the adjacent fields could contribute to propagule immigration. Field edges are typically disturbed , disrupting the establishment and persistence of feral sorghum in these sites. Nevertheless, the microhabitats at the roadside could provide more moisture for the establishment of plants compared to field edges. The nearby land use had large effects on the presence of feral sorghum . Results showed that the odds for the occurrence of feral sorghum in sites adjacent to sorghum cultivation was larger than that of all other land uses; the likelihood of finding feral sorghum in a location closer to a sorghum field was almost twice as high as a location contiguous to corn, hay, pasture, shrubland, urban or fallow lands . Sorghum is one of the major crops grown in South Texas and results suggest that sorghum cultivation and seed transport activities in the region contribute to seed immigration and establishment of feral sorghum on roadside habitats. Further, the sorghum seed dispersed following harvest might germinate instantly due to the lack of seed dormancy and the warm environmental conditions in South Texas may allow feral sorghum to produce viable seed prior to killing frost and establish self-perpetuating populations. There is also a possibility for spring establishment of feral sorghum from the seeds entered into the soil post-harvest should they be able to survive during the fall and winter. Data on seed survival rate of sorghum coupled with early spring monitoring are needed to address this question.The population size of the feral sorghum at each site was scored based on visual estimations.

The results from the analysis of variance showed no significant relationship between all the measured factors and the population size of the feral sorghum at sampled sites expect for the vegetation cover . The largest feral sorghum population sizes were associated with the highest vegetation cover , a finding that is unexpected given that vegetation with higher canopy cover should be more resistant to invasion than those with low canopy cover. One possible explanation is that the roadsides are regularly treated with herbicides by the Department of Transportation and in some cases by county weed control specialists for controlling tall vegetation such as johnsongrass. It is likely that herbicides might have been recently applied at sites with low vegetation cover, thus reducing the chance of observing feral sorghum individuals.The co-occurrence of feral sorghum and johnsongrass was rare and both species were found together only in 48 of the 2,077 survey sites visited . Fig 4 shows co-occurrence of feral sorghum and johnsongrass in a roadside site near Corpus Christi, TX. Results from the logistic model showed a negative relationship between the occurrence of johnsongrass and feral sorghum . The likelihood of detecting feral sorghum at locations without johnsongrass was 4.3 times greater than the locations where johnsongrass was present. Three possible scenarios might explain this finding: the presence of johnsongrass in the site may have a negative influence on the germination and establishment of feral sorghum ; however, the data collected in this study was not sufficient to establish any causal relationship or there is no anecdotal evidence to support such a scenario in production fields, the dispersal of sorghum seed on roadsides as a function of intensive sorghum cultivation and seed transport occurs primarily in the much Southern parts of Texas from Victoria towards Brownsville, an environmental gradient increasingly less suited for johnsongrass as well as in the habitat suitability map for johnsongrass; thus, the cooccurrence of both species was perhaps naturally limited, and/or roadside herbicide applications that target johnsongrass may eliminate any feral sorghum plants present within these sites, while johnsongrass could regrow from rhizomes. It is very likely that the second and third scenarios have substantial influence on the co-occurrence of these two species. A followup observation conducted in summer 2017 has revealed supporting evidence for the third scenario in that several of the feral sorghum-johnsongrass complex sites we identified in the 2014 survey were severely impacted by roadside herbicide applications that typically target johnsongrass. In these sites, several johnsongrass plants survived, but almost all feral sorghum plants were eliminated. Given that feral sorghum and johnsongrass can hybridize, the co-occurrence of these two species may facilitate the persistence of feral sorghum through gene flow and introgression of adaptive traits from johnsongrass. In addition, cultivation of diverse sorghum lines, including sudangrass and sorghum-sudangrass hybrids, in the vicinity may enrich the diversity within the feral sorghum populations and thereby increase the adaptive ability of feral sorghum. Such an outcome has been reported for feral populations of oilseed rape and alfalfa. Since this is the first record of the presence of feral sorghum innature, no information is available on the diversity, population genetic structure and longterm persistence of feral sorghum populations.Although most of the feral sorghum populations were observed along the roadsides, they may have the potential for spread to their contagious natural and unmanaged areas. To investigate the potential for broader distribution of feral sorghum in South Texas, we calibrated a model using nearby land use type and regional habitat suitability for johnsongrass as reliable predictors.

These two variables were chosen because they were statistically significant and the georeferenced data for the entire region was available. The combination of these two variables effectively predicted the distribution of feral sorghum, with an increasing trend in abundance from the Upper Gulf Coast towards the Rio Grande Valley, which corresponded to an increasing intensity of sorghum cultivation and seed transport activities in the landscape. Further, in the more Southern areas of Texas, sorghum seeds germinating after the harvest season will have a high chance to produce mature seed prior to killing frost, if any. The projected map for feral sorghum distribution is shown in Fig 5, which corroborates with the overall trend observed in the survey. Conversely, growing tray the distribution of johnsongrass showed an opposite trend, with more abundance in the Upper Gulf Coast region than in the Rio Grande Valley, attributable to its habitat suitability as evident in the habitat suitability map .The current survey showed that roadsides and field margins are the initial niches for feral sorghum to establish outside of cultivated fields. We found that the occurrence of feral sorghum in South Texas is highly associated with sorghum cultivation in the nearby area, providing propagules for the establishment of feral populations in field edges and roadsides during planting and grain transport operations. We did not find any relationship between the frequency of feral sorghum and road characteristics . Although johnsongrass can be found commonly along the roadsides in South Texas, the co-occurrence of feral sorghum and johnsongrass was infrequent. Yet, there are significant opportunities for outcrossing to occur between the two species outside of cultivated fields. More research is necessary to understand the frequency of outcrossing between the two species and fitness of the progenies. Experiments are on the way to characterize, using phenotypic and molecular markers, the progeny of seed harvested from feral sorghum plants during this survey in sites where both species co-existed. Further, field surveys and monitoring are being carried out to confirm and characterize potential hybrid progenies in nature in these feral sorghum-johnsongrass complex sites.When transgenic plants were initially developed, most plant evolutionary biologists and geneticists considered spontaneous hybridization between species to be rare and of little importance in terms of evolution. This view extended to both crops and their wild or weedy relatives, but has now radically changed. More than twenty years of gene-flow research has shown that interspecific hybridization is very common in some groups of vascular plants and may be of considerable evolutionary significance. Hybridization may occasionally result in the extinction of a population, may trigger the evolution of plant invasiveness, or initiate speciation. A substantial body of evidence has now accumulated, demonstrating the high potential for interspecific hybridization between agricultural crops and their wild or weedy relatives. Transgenic crops are no exception, and empirical studies have provided evidence of transgene dispersal from GM crops to their weedy relatives. Many factors have been shown to influence the rate of hybrid formation between crops and their wild or weedy relatives. Population effects such as the local densities of the parental types and their relative frequencies, have been demonstrated in several cases. Mating system differences at the individual level due to, for example, selfing rates and apomixis, have also been found to affect hybridization rates. Moreover, several studies have shown that overlap in the flowering periods of crop and weed plants affect opportunities for hybridization. The aim of this study is to gain insight into the impact of hybridization with transgenic crops on the evolution of the weedy relatives by verifying that hybridization opportunities for weedy plants depend on their phenotypic traits , measuring the relative fitness of hybridizing weeds, and searching for associations between the transgenic trait and the phenotypic traits increasing hybridization opportunities in the offspring of weedy plants. We studied hybridization opportunities, phenotypic traits and offspring phenotype of weedy individuals in experimental plant populations cultivated under glasshouse conditions. Experimental populations were composed of weeds and transgenic plants in a 1:1 ratio. Transgenic plants were crop plants of the Brassica genus , F1 hybrids between B. rapa and B. napus, or first-generation backcrosses. Crop plants were all homozygous for the Btcry1Ac transgene from Bacillus thuringiensis, F1 hybrids were all hemizygous and first-generation backcrosses and consisted of an equal mixture of hemizygotes and null homozygotes. Hybridization opportunities for each weedy individual was calculated as the expected proportion of pollen received from transgenic plants based on the observed flowering schedules. This experimental system was ideal for addressing the question of interest in this study, for three reasons. First, despite barriers to interspecific mating such as apomixis or preferential exclusion of hybrid zygotes, numerous studies have shown that B. napus and B. rapa readily hybridize under controlled conditions, but also in the field.

The introduction of TYLCV into CR in 2012 was the second invasion event: a highly invasive OW monopartite begomovirus that can cause devastating losses to tomato production worldwide . Indeed, TYLCV has added a new challenge for tomato production in CR, as growers have reported increased losses due to begomoviruses disease since 2012 . However, what was less clear was how TYLCV interacts with the already existing bipartite begomoviruses ToYMoV and ToLCSiV. In Florida, the introduction of TYLCV led to a reduced incidence of the indigenous bipartite begomovirus ToMoV . Therefore, this provides an opportunity to investigate the invasion biology of these very different begomoviruses with the long-term objective of making predictions and management suggestions. In the infectivity experiments, interactions were revealed based on symptom severity and viral DNA accumulation. A general synergistic interaction was observed in which mixed infections resulted in more severe symptoms, with plants infected by all three viruses showing the most severe symptoms. A similar situations has been observed for the interaction of three indigenous begomoviruses in Brazil, as measured based on symptom severity . Furthermore, the interaction among begomoviruses detected in the present study is a type of neutral synergism, which has also been previously described, including for begomoviruses . For example, N. benthamiana and tomato plants co-inoculated with TYLCV and tomato yellow leaf curl Sardinia virus developed more severe symptoms than plants inoculated with either virus alone, curing cannabis and accumulation of each virus in co-infected plants was similar to that in single infections . Interestingly, we did detect an initial antagonistic or negative interference effect on ToYMoV and ToLCSiV accumulation at 7 dpi in all mixed infections, indicating it was not virus specific.

These results are consistent with a previous study showing a transient negative effect on viral accumulation early in mixed infections of tomato rugose mosaic virus and tomato yellow spot virus in N. benthamiana and tomato . The overall mechanism of antagonism in mixed infections remains to be elucidated, but may involve competition for host factors or stimulation of a more efficient defense response of the host . For example, potato spindle tuber viroid can interfere with TYLCSV accumulation in tomato by activation of the host DNA methylation pathways . The fact that TYLCV accumulation was not affectedduring mixed infections with NW bipartite begomoviruses in the present study could be due to the combine activities of multiple viral suppressor of gene silencing, e.g., C2, C4 and V2 . Taken together, our results with these three begomoviruses revealed the existence of temporary antagonism followed by a more sustained neutral synergism. We further showed that in mixed infections with TYLCV, TYLCD became dominant at 14 dpi and beyond, even in the presence of the two co-infecting NW tomato bipartite begomoviruses. Indeed, this dominant TYLCD phenotype has been observed in the tomato fields in Costa Rica since the introduction of TYLCV . Therefore, in terms of the invasion biology of these viruses, it appears they can effectively co-exist in tomato plants, which leads to more severe disease and the persistence of all three viruses in tomato production in CR. Another observation in mixed infections with TYLCV was that, although TYLCD symptoms eventually became dominant, symptoms induced by co-infecting bipartite begomoviruses appeared earlier , with TYLCD symptoms appearing at ~10 dpi. This observation can be explained in terms of the tissue tropism of these viruses. The more rapid appearance of mosaic/mottle symptoms in leaves may reflect the capacity to infect cells outside of the phloem, and more rapidly colonize and accumulate in plants.

The sap transmission of ToYMoV is evidence this virus is not phloem limited. In summary, we used the tomato begomovirus situation in CR to examine the invasion biology of three viruses, two of which were introduced. We first confirmed ToYMoV caused ToYMoD in CR and is a locally evolved NW bipartite begomovirus. We then used infectious clones to investigate interactions in mixed infections. We found that these viruses exhibited a neutral synergism, in which the viruses co-exist and induced more severe symptoms. In mixed infections with TYLCV, TYLCD became predominant. These results indicate that all three viruses are likely persisting in CR and causing more severe symptoms and losses, particularly in the presence of TYLCV. Thus, an effective management of these complexes will require an integrate approach, including the identification of varieties with resistance to all three viruses.The genus Begomovirus is comprised of a large and diverse group of plant viruses that possess a circular, single-stranded DNA genome encapsidated into twin quasi-icosahedral virions . These viruses infect dicotyledonous plants and cause numerous economically important diseases of fiber, fruit, ornamental and vegetable crops, mostly in tropical and subtropical regions of the world . Begomoviruses are transmitted, plant-to-plant, by whiteflies of the Bemisia tabaci cryptic species complex . The genome of begomoviruses is composed of either a single genomic DNA of ~ 2.8 kb or two ~2.6 kb DNA components , designated as DNA-A and DNA-B . The genomic DNA of monopartite begomoviruses is homologous to the DNA-A component of bipartite begomoviruses, and both are organized with overlapping virion -sense and complementary -sense genes transcribed in a bidirectional manner from an intergenic region , which contains the cis-acting elements involved in replication and gene expression .

In bipartite begomoviruses, an ~200 nucleotide noncoding sequence is shared between cognate DNA-A and DNA-B components, and this common region maintains the specificity of replication for these components. Otherwise, the sequences of the DNA-A and DNA-B components are different, and bothcomponents are needed for induction of typical disease symptoms . In terms of begomovirus evolution, continental drift is believed to have separated ancestral monopartite and bipartite begomoviruses, resulting in the predominance of monopartite begomoviruses in the Old World and bipartite ones in the New World . The subsequent independent diversification and evolution of OW and NW begomoviruses involved different combinations of mutation, recombination and acquisition and modification of foreign DNAs . For OW monopartite begomoviruses, acquisition of satellite DNAs has played a major role in evolution, whereas acquisition and modification of the DNA-B component was essential for bipartite begomoviruses, and allowed for pseudorecombination to act as an additional mechanism of evolution . Furthermore, the emergence of new begomoviruses has been facilitated by the global spread of the highly polyphagous B. tabaci species MEAM1, which can introduce mixtures of viral components/genomic DNAs into a diversity of plant species . Finally, human activities have led to the long distance intercontinental movement of numerous begomoviruses, blurring the geographic separation of OW and NW begomoviruses . The remarkable diversification of begomoviruses has been reflected in the appearance of diseases of crop and non-cultivated plants in tropical and subtropical regions worldwide. In these agroecosystems, it is common to observe non-cultivated plants showing striking golden/yellow mosaic symptoms, which are commonly associated with begomovirus infection. Inthe Caribbean Basin and other parts of Latin America, non-cultivated plants with these symptoms have been reported from species in the families Asteraceae, Capparaceae, Convolvulaceae, Euphorbiaceae, Fabaceae, Malvaceae, Nyctaginaceae and Solanaceae . Importantly, characterization of begomoviruses associated with these diseases has revealed substantial genetic divergence from viruses that cause economically important crop diseases, although there are some exceptions such as the golden/yellow mosaic symptoms of Malachra alceifolia associated with tobacco leaf curl Cuba virus infection in Jamaica , and mosaic and crumpling symptoms of Nicandra physaloides infected with tomato severe rugose virus in Brazil . This suggests that begomoviruses infecting crops and weeds have co-evolved independently with their hosts, with the practical implication that most of these symptomatic weeds are not major sources of inoculum for crop-infecting begomoviruses. However, these begomovirus-infected weeds can serve as a mixing vessels for evolution of viruses with the potential to infect crops . The family Malvaceae, commonly referred to as mallows, is comprised of >4225 species of annual and perennial plants .

Members of this family are distributed worldwide, and occur in temperate, tropical and subtropical regions . Some species are important crops, such as cotton and okra ; others are grown as ornamentals or for medicinal purposes; and others are considered invasive weeds, e.g., Abutilon spp., Sida spp. and Malachra spp. . Moreover, weed dryer these malvaceous weeds are commonly infected by begomoviruses and develop striking golden/yellow mosaic symptoms . As part of a long-term study to characterize begomoviruses causing golden/yellow mosaic symptoms in weeds and assesses the potential of these viruses to cause diseases of crop plants in the Dominican Republic , we describe here the molecular and biological properties of two bipartite begomoviruses associated with these symptoms in Malachra sp. and Abutilon sp. plants on Hispaniola. Sequence and phylogenetic analyses together with infectivity studies with infectious clones were used to establish that the symptoms in Malachra sp. were caused by the crop-infecting bipartite begomovirus TbLCuCV, whereas those in Abutilon sp. were caused by a new species of weed-infecting begomovirus for which the name Abutilon golden yellow mosaic virus is proposed. Host range experiments showed that TbLCuCV also induced moderate to severe disease symptoms in Nicotiana benthamiana, tobacco and common bean plants plants. In contrast, AbGYMV induced mild or no symptoms in these plants, indicating a high degree of adaptation to Abutilon sp. from the DO and low potential to cause crop diseases. TbLCuCV and AbGYMV are closely related species in the Abutilon mosaic virus lineage of NW begomoviruses and we present evidence that recombination and pseudore combination play a role in the evolution of these viruses.To detect begomovirus DNA-A and DNA-B components, PCR tests were performed with the degenerate primer pairs PAL1v1978/PAR1c496 and PCRc1/PLB1v2040, respectively . PCR-amplified fragments were purified with the QIAquick gel extraction kit and directly sequenced with the PAL1v1978/PAR1c496 and PCRc1/PLB1v2040 primers. To estimate the number and genetic diversity of begomovirus DNAs present in the samples and to identify single-cutting restriction enzymes for obtaining full-length clones, restriction fragment length polymorphism analyses of circular DNAs generated by rolling circle amplification with Φ-29 DNA polymerase were performed . The RCA products were first digested with the fourbase-cutting enzyme MspI to generate RFLPs for estimating the number of begomovirus DNA components infecting the samples. Next, RCA products were digested with selected six-basecutting enzymes to identify sites in each DNA component for obtaining full-length clones. The linearized DNA components were ligated into pGEM11Z  or pSL1180 digested with the appropriate enzyme. Recombinant plasmids having the full-length DNA-A and DNA-B components were identified by restriction enzyme digestion and DNA sequence analyses. Based upon sequencing and RCA results, the begomovirus isolates from the M1 and M4 samples were selected for further studies. Thus, full-length DNA-A and DNA-B clones were obtained from sample M1 , sample M2 , sample M3 and sample M4 .The complete sequences of the cloned full-length DNA-A and DNA-B components of the bipartite begomoviruses from samples M1-M4 were determined and analyzed with Vector NTI advance software . Pairwise nt sequence alignments were performed with MUSCLE within the Species Demarcation Tool v.1.2, and with full-length DNA-A and DNA-B sequences of the ten begomoviruses with the highest identities revealed by the BLASTn search . The Vector NTI advance software was used to make more extensive comparisons, including individual open reading frames and non-translated regions from both components. The cis-acting elements involved in begomovirus replication were identified as described in Argüello-Astorga and Ruiz-Medrano .For the phylogenetic analyses, we used the complete nt sequences of the DNA-A and DNAB components of: the bipartite begomoviruses from the M1-M4 samples; TbLCuCV isolates from CU ; the ten most identical viruses revealed by the BLASTn search; and selected viruses representing the AbMV, Brazil, squash leaf curl virus , bean golden yellow mosaic virus and Boerhavia golden mosaic virus lineages of NW begomoviruses. Multiple sequence alignments for the DNA-A and DNA-B component sequences were generated with the MAFFT algorithm implemented in the Guidance2 Server . The alignment quality was analyzed, and unreliable regions were removed with the GUIDANCE algorithm . The resulting alignments were then exported as Nexus files. Phylogenetic trees were constructed with a Bayesian inference and Markov chain Monte Carlo simulation implemented in MrBayes V3.2 . The best-fit model of nt substitution for each data set was determined with the program MrModeltest V2.2 . The analyses were carried out by running 2,000,000 generations and sampling at every 100 generations, resulting in 20,000 trees. The first 10% of samples were discarded as a burn-in. Trees were visualized with Archaeopteryx tree viewer and exported in Newick format .

Weeds are the greatest biological constraint to rice yields, and farmer inputs towards weed management are expected to increase as herbicide resistance spreads worldwide . The potential yield lost to weed infestation is species dependent, and the practice of continuous rice monoculture in California has resulted in an abundance of highly competitive weeds that negatively impact rice yields . In California rice fields, weedy grasses are the largest predictors of overall yield loss. Late watergrass [Echinochloa phyllopogon . Koss] competition has caused rice yield losses as high as 59% . Studies in Arkansas have shown rice yield losses to be 79% from competition with barnyard grass [Echinochloa crus-galli Beauv.] and 36% from bearded sprangletop competition . Weedy rice densities of 30 to 40 plants m-2 can reduce rice yields by 60-90%, depending on the cultivar . In the United States Midsouth region, yield losses due to ducksalad infestations can reach 30% . Most California rice herbicides are limited in the spectrum of weeds controlled and the length of residual activity, requiring herbicide treatment plans to consist of multiple herbicides to enact weed control over a range of weeds . Continuous use of herbicides with the same mode of action aids in the development of herbicide resistance in a crop . Confirmed herbicide resistance from various populations of watergrass species and bearded sprangletop have been documented. California arrowhead and small flower umbrellas edge were the first confirmed instances of herbicide resistance in rice to bensulfuron-methyl, dry racking an ALS-inhibitor, in 1993 . Eight other rice weed species have since been identified with resistance to commonly used herbicides, some with resistance to more than one mode of action .

A direct result of herbicide resistance development to more than one mode of action is the necessity of using combinations of different modes of action to combat weeds in rice systems. Permanently-flooded rice agroecosystems are limited to few available herbicides in California, largely due to ecotoxicity and strict regulatory structure . As of 2019, there are 13 registered active ingredients for water-seeded rice in California and 9 modes of action registered for use . The rise in herbicide resistance has increased the cost and difficulty of weed management, necessitating demand for novel herbicide development to delay resistance expansion and assist the management of current herbicide-resistant weed biotypes . The following studies examined the crop response to chemicals not currently in use in California water-seeded rice. CHAPTER ONE describes field studies performed in 2019 and 2021 at the Rice Experiment Station in Biggs, CA. The efficacy of pyraclonil, a protox inhibitor, was explored alone and in combination with several currently available rice herbicides against common grass, sedge, and broadleaf weeds in California rice field. Combination treatments included pyraclonil at 0.3 kg ai ha-1 applied the day of seeding, in combination with or followed by recommended rates of propanil, clomazone, benzobicyclon plus halosulfuron, thiobencarb, bispyribac-sodium, penoxsulam, or florpyrauxifen-benzyl at their respective recommended application timings. Rice phytotoxicity and yield in response to pyraclonil and these registered herbicides was evaluated. Pyraclonil applied alone had mixed effects on weed control, but all pyraclonil herbicide combination treatments controlled watergrass species, bearded sprangletop, ricefield bulrush, smallflower umbrellas edge, ducksalad, and redstem consistently better than pyraclonil applied alone.

Pyraclonil applied alone caused minor visible rice injury that varied by year but did not reduce yields. This study determined that pyraclonil was effective as a base treatment herbicide and may prove to be a new useful tool for rice growers to incorporate into their weed management programs.CHAPTER TWO details greenhouse studies undertaken in 2021-2022 to evaluate the response of several rice genotypes to five different rates of foliar-applied metribuzin, a Photosystem II inhibitor herbicide not currently used in California rice systems. Short-grain rice cultivars as a group were found to be more susceptible to crop phytotoxicity than the long-grain or medium-grain rice lines. Crop injury from metribuzin was correlated with biomass reductions and plant height reductions . The results indicate that further research is needed to establish metribuzin’s candidacy for development as a POST emergence product in rice. This exploration of novel herbicides has characterized the activity of pyraclonil in California rice, both alone and in combination with other water-seeded rice herbicides. The efficacy of the herbicide, as well as the response of the target crop, has been identified and establishes pyraclonil as an herbicide with great potential for integration into existing rice weed management programs. The differential responses of various rice cultivars to increasing doses of foliar metribuzin has described heretofore unknown rice responses and identified areas of concentration upon which future researchers may focus. Introduction of novel herbicides and continued analysis of their activity in rice allows for development of alternate methods of sustainable weed control to contend with the rise of herbicide resistance amid the common weeds of California rice agriculture.Rice is the major calorie source for a large proportion of the world’s population and is one of the most commonly grown agricultural commodities in the world . California is the second largest rice-growing state in the USA, with approximately 200,000 ha of rice, most of which is concentrated in the Sacramento Valley. The majority of rice in California is produced using a continuously flooded, i.e., water-seeded system, where rice is pre-germinated and aerially seeded into fields with a 10-to 15 cm existing flood . The flooded conditions in which California rice is grown favor flood-adapted, competitive grass weeds such as watergrass species Beauv. spp.and bearded sprangletop [Leptochloa fusca Kunth ssp. fascicularis N. Snow] . The continuously flooded system also promotes sedges such as rice field bulrush [Schoenoplectus mucronatus Palla] and small flower umbrellas edge as well as aquatic broadleaf weeds such as ducksalad [Heteranthera limosa Willd.] and redstems . Weeds are the greatest biological constraint to rice yields, and farmer inputs towards weed management are expected to increase as herbicide resistance spreads worldwide . The potential yield lost to weed infestation is species dependent, and the practice of continuous rice monoculture in California has resulted in an abundance of highly competitive weeds that negatively impact rice yields . In California rice fields, weedy grasses are the largest predictors of overall yield loss . Late watergrass [Echinochloa phyllopogon . Koss] competition has caused rice yield losses as high as 59% . Studies in Arkansas have shown rice yield losses to be 79% from competition with barnyardgrass [Echinochloa crus-galli Beauv.] and 36% from bearded sprangletop competition . In the Midsouth region, yield losses due to ducksalad infestations can reach 30% . Most California rice herbicides are limited in the spectrum of weeds controlled and the length of residual activity, cannabis curing requiring herbicide treatment plans to consist of multiple herbicides to enact weed control over a range of weeds . Effective weed control in the state relies on combinations of herbicides to enact a complete spectrum of weed control Continuous use of herbicides with the same mode of action aids in the development of herbicide resistance in rice fields .

However, due to high costs of development and registration, few additional herbicides are currently available for California rice growers, particularly herbicides that target grass weeds . As of today, there are 13 registered active ingredients for water-seeded rice in California that belong to 9 modes of action . The rises in herbicide resistance have made weed management more difficult and more costly to California rice growers . Herbicide resistance has also been a major biological issue, with confirmed resistance from various populations of watergrass species and bearded sprangletop . California arrowhead and small flower umbrellas edge were the first confirmed cases of herbicide resistance in rice to bensulfuron-methyl, an ALS-inhibitor, in 1993 . Eight other rice weed species have since been identified with resistance to commonly used herbicides, some with resistance to more than one mode of action . A direct result of herbicide resistance development to more than one mode of action is the necessity of using combinations of different modes of action to combat weeds in rice systems. Pyraclonil is a broad-spectrum herbicide with protoporphyrinogen oxidase inhibitor mode of action that is new to California. Carfentrazone, which is a currently registered protox-inhibitor, is a viable herbicide for California water-seeded rice but lacks activity on grass weeds . Pyraclonil is presently in use in Japan and has shown efficacy against sulfonylurea-resistant broadleaf biotypes of Lindernia procumbens Borbas, grasses, and sedges . Currently, there is no record of protox inhibitor resistance in California rice weeds. Protox inhibition takes place inside the chloroplasts of plant cells. As the last enzyme in the common tetrapyrrole biosynthesis pathway prior to heme and chlorophyll synthesis, protoporphyrinogen IX oxidase catalyzes the oxidation of protoporphyrinogen IX to protoporphyrin IX . Pyraclonil inhibits the conversion of protogen to proto by blocking protox activity. When protox is inhibited, excess protogen accumulates in the chloroplast until protogen leaks to cytoplasm . In cytoplasm, leaked protogen is oxidized into proto and is unable to reenter the chloroplast . When proto is exposed to light and molecular oxygen in the cytoplasm, it produces toxic oxygen species, which are responsible for lipid peroxidation and membrane disruption, resulting in overall plant death . A formulation of pyraclonil has been developed by Nichino America Inc. as a preemergent granular form that is suitable for aerial application in California water-seeded rice agroecosystems.Therefore, the objectives of this research were to determine the grass, sedge, and broadleaf control of pyraclonil alone and in partnership with other commonly used herbicides in water-seeded rice systems and determine the rice response to the granular formulation of pyraclonil.Field experiments were conducted during the 2019 and 2021 growing seasons at the Rice Experiment Station in Biggs, CA, USA . Soils at the study site are characterized as Esquon-Neerdobe silty clay with a pH of 5.1, and 2.8% organic matter. The study site weed seedbank has been previously described in Brim-DeForest et al. and contains watergrass species, bearded sprangletop, rice field bulrush, small flower umbrellas edge, ducksalad, and redstem.Seeds of medium-grain rice cultivar ‘M-206’ were soaked in water for 24 hours for pregermination and then drained and aerially seeded at a rate of 168 kg ha-1 into a 10 cm flooded field. Seeding dates were June 13, 2019, and June 1, 2021. Plots were 3 m by 6 m and surrounded by small levees to prevent herbicide cross contamination to other plots . Pyraclonil was applied as a granular formulation of 1.89% pyraclonil at a rate of 0.3 kg ai ha -1 at day of seeding . Pyraclonil was also applied in combination with propanil, clomazone, benzobicyclon plus halosulfuron, thiobencarb, bispyribac-sodium, penoxsulam, and florpyrauxifen-benzyl .Treatment applications were timed on rice emergence or development stages according to manufacturer labels. Granular herbicides were evenly broadcast by hand. Foliar applied herbicides were applied with a CO2-pressurized boom sprayer with a 2 m boom equipped with six 8003XR flat-fan nozzles calibrated to deliver 187 L ha-1 at 180 kPa. For the combination treatments including propanil, the spray mixture included 2.5% v/v crop oil concentrate . For the combination treatment including bispyribac-sodium, the spray mixture included a multifunction adjuvant of 0.37 ml ha-1 . Several of the contact herbicide treatments required the 10 cm permanent flood to be lowered in order to reveal the weeds. For the treatments containing propanil, bispyribac-sodium, and florpyrauxifen-benzyl, the plots were drained to reveal 70% of the weeds prior to that herbicide application and were reflooded to 10 cm 48 hours after application, according to the manufacturer labels.Visual ratings measuring weed control were conducted for watergrass species, bearded sprangletop, rice field bulrush, small flower umbrellas edge, ducksalad, and redstem at 14 and 42 DAT . Ratings consisted of a 0 to 100 scale, where 0 = no weed control, and 100 = no weeds present, or full control. Visual crop phytotoxicity ratings were conducted at 14 and 42 DAT on a 0 to 100 scale, where 0 = no injury and 100 = plant death, as compared to the non-treated control plots. Phytotoxicity ratings consisted of stunting and chlorosis ratings. Rice grain was harvested from each plot with a small-plot combine with a swath width of 2.3 m . Rice grain yield for both years was adjusted to 14% moisture.

Currently, six resistance loci have been mapped in tomato . The Ty-1 and Ty-3 genes are allelic and were introgressed from S. chilense . The Ty-2 gene originated from S. habrochaites and encodes a nucleotide-binding site-leucine-rich repeat containing protein . The Ty-4 gene is located on the long arm on chromosome 3, and was introgressed from S. chilense . The recessive Ty-5 gene encodes the messenger RNA surveillance factor Pelota, and is believed to have been introgressed from S. peruvianum . Finally, the Ty-6 gene is located on the long arm of chromosome 10, and was introgressed from S. chilense . The Ty-1 is a partially dominant resistant gene that was introgressed from the S. chilense line LA1969 . The Ty-1 locus is composed of three allelic variants, and Ty–1-like alleles are present in both resistant and susceptible tomato lines . Expression analyses of these alleles revealed that the Ty–1 gene expression was significantly higher compared to that of the ty- 1 allele, and that these differences correlate with the resistant phenotype . The Ty-1 gene encodes an RNA-dependent RNA polymerase of the γ-class, but with an atypical DFDGD amino acid motif in the catalytic domain located at the Cterminus . The γ-RdRp is found in plants and fungi, but no clear function has been established for these proteins . Investigation of the Ty-1 function has shown that the accumulation of TYLCV in inoculated tissues and the long-distance movement of the virus were impaired in Ty-1 lines,suggesting a role in suppression of cell-to-cell movement . However, vertical farm equipment it was later demonstrated that TYLCV-specific small interfering RNA levels were higher in resistant Ty-1 plants compared with those in susceptible ty-1 plants .

Furthermore, the presence of these siRNA was correlated with cytosine methylation of the viral genome . This suggested that the Ty-1 gene is involved in transcriptional gene silencing . So far, little is known about the functional properties of the Ty-1 protein, and the nature of the signal transduction pathway involved in the defense response. Here, we generated cDNA libraries from samples of inoculated stems and newly emerged leaves of the near isogenic lines LA3473-R and LA3474-S, respectively. We used an RNASeq approach to identify differentially expressed genes during the resistant and susceptible responses to TYLCV infection. Finally, we cloned and sequenced the Ty-1 gene and determine some properties of the Ty-1 protein to provided further insight into how some of the newly identified DEGs participate in the TYLCV-mediated defense response. These results will be discussed in terms of the mechanism of resistance and associated signal transduction pathways.NILs of tomato with the Ty-1 gene or ty-1 gene were used for these experiments . Seeds of these NILs were obtained from the Tomato Genetics Resource Center, UC Davis. Tomato plants of both lines were grown in a greenhouse, allowed to be self-pollinated and fruits were harvested. Seeds were extracted and treated with 2.7% sodium hypochlorite for 30 min and rinsed with MilliQ water. Treated seeds were planted into sunshine mix 1 potting mix in a controlled environment chamber .To confirm the resistant and susceptible phenotypes of plants produced from increased seeds, NILs LA3473-R and LA3474-S, respectively, seedlings at the three- to five-leaf-stage were agroinoculated with cell suspensions of a strain of Agrobacterium tumefaciens containing a binary plasmid with the multimeric infectious clone of TYLCV from California by needle puncture inoculation of the stem just beneath the shoot apex .

The positive control in these experiments were seedlings of the susceptible tomato plants cv. Glamour agroinoculated with TYLCV, whereas the negative control was seedlings agroinoculated with the empty vector . Inoculated plants were maintained in a controlled environment chamber, and symptom development was assessed visually and recorded at 7, 14 and 21 d post infection based on the severity scores described in Lapidot et al. . Absolute viral DNA accumulation was quantified at 7, 14 and 21 dpi by quantitative polymerase chain reaction tests according to the protocol described by Mason et al. . Total genomic DNA was extracted from newly emerged leaves according to the method of Dellaporta et al. . A virus-specific primer pair for qPCR detection was designed to direct the amplification of an ~150 base pair fragment from the capsid protein gene of TYLCV . The specificity of this primer pair was predicted based on BLAST search , and confirmed experimentally by conventional PCR with DNA extracts of tomato plants agroinoculated with TYLCV and non-inoculated plants. The PCR-amplified TYLCV fragment was cloned into pCR-Blunt II-TOPO to generate a standard for qPCR assays. Recombinant plasmids containing the cloned PCR-amplified TYLCV fragment were quantified with a NanoDrop1000 spectrophotometer , and plasmid copy number was adjusted to 107 copies/µl with the Avogadro’s constant . Standard curves for qPCR were prepared with tenfold serial dilutions ranging from 101 to 106 copies of plasmid DNA. These standard curves were used to estimate the viral Cn for each sample. The qPCR was conducted on a QuantStudio™ 6 Flex Real-Time PCR System with 100 ng of total genomic DNA in a 20-μl reaction mix with the SsoFast EvaGreen Supermix kit . To detect the presence of the Ty-1 or ty-1 genes in the NILs LA3473-R and LA3474-S, respectively, restriction fragment length polymorphism analyses of the amplified fragments with a marker to Ty-1/ty-1 alleles were performed.

Here, leaf samples were collected from non-inoculated leaves of LA3473-R and LA3474-S plants and total genomic DNA was extracted as previously described. PCR tests were performed with the C2_At5g61510- F/C2_At5g61510-R primer pair , which direct the amplification of an ~1.0 kb fragment. PCR-amplified fragments were purified with the QIAquick gel extraction kit and digested with the five-base-cutting enzyme HinfI and the reaction analyzed by agarose gel electrophoresis.For the RNA-sequencing experiments, a cell suspension of an A. tumefaciens strain containing the multimeric infectious clone of TYLCV-[US:CA:06] or the empty vector was used for agroinoculation of seedlings of the NILs LA3473-R and LA3474-S as previously described. A total of three plants per treatment were inoculated per experiment, and the experiment was repeated three times. Tissue samples of the resistant and susceptible lines were individually collected and pooled together in each treatment from inoculated stems and, from other inoculated plants, from newly emerged leaves for a total of eight-time points . Samples were frozen immediately in liquid nitrogen. High-throughput RNA-seq library preparation was performed according to the method described in Kumar et al. . Briefly, double-stranded complementary DNA was prepared with random hexamer priming, and the resulting cDNA was fragmented, end-repaired and A-tailed. DNA barcodes for multiplexing Illumina DNA-Seq libraries were added to the cDNA fragments during adapter ligation, and the adapter-ligated cDNA libraries were enriched with 13 cycles of PCR amplification followed by size selection of ~200-500 bp fragments. A total of 96 cDNA libraries we generated, which included the eight-time points each for the resistant and susceptible lines and for the three independent experiments. Finally, barcoded libraries for each independent experiment were pooled together and sequenced on two lanes on Illumina HiSeq 2000 platform at the UC Davis Genome Center.Raw sequence data were filtered/trimmed for low-quality reads and technical sequences with Trimmomatic . Trimmed RNA-Seq reads were aligned to the high quality, non-redundant database generated from the tomato genome by the International Tomato Annotation Group with Hisat2 . The latest ITAG database was obtained from Sol Genomics Network . Feature Count was used to count the number of reads mapped to each gene . DEGs were identified with Cuffdiff, a method that estimates the relative transcript abundance . Gene expression levels were normalized with fragments per kilobase of exon per million mapped reads values, and the false discovery rate was used to determine the differentially expressed p-value threshold. In addition, an independent DEGs analysis was performed with DEseq and EdgeR methods , commercial indoor growing systems and data were visualized with Galaxy . In the present study, genes were considered to be differentially expressed only when their absolute value of log2 fold change was >1.5 and p-value was <0.05. To identify the potential function of DEGs involved in the Ty-1 resistance response to TYLCV infection, the functional classes of DEGs were identified with gene ontology enrichment analyses with PANTHER within the Gene Ontology project .

In order to investigate the transcriptional changes in tomato stems and leaves during TYLCV infection, we performed RNA-Seq experiments with inoculated stems and systemically infected leaves in the NILs LA3473-R and LA3474-S. Overall, ~1140 million read pairs were obtained for 96 libraries, with an average of ~12 million read pairs per library . The reads from these libraries were trimmed and then aligned to the tomato genome in the ITAG database. Time course comparisons of DEGs in the NILs revealed that the largest transcriptional changes in LA3473-R occurred at early stages of infection in inoculated stem tissues. A total of 797 and 660 genes were differentially expressed at 12 and 24 hpi, respectively, and dropped to 42 DEGs at 48 hpi . In the systemically infected leaves, there were substantially fewer DEGs, with 0, 44 and 10 genes differentially expresses at 7, 10 and 14 dpi, respectively . Most of the transcriptional changes observed in the LA3474-S plants occurred from 12 to 48 hpi in inoculated stem tissues and at 7 dpi systemically infected leaves. A total of 264 , 540 and 355 genes were differentially expressed at 12, 24 and 48 hpi, respectively . In systemic infected leaves, 356 genes were differentially expressed at 7 dpi, whereas 99 and 2 genes were differentially expressed at 10 and 14 dpi, respectively . Interestingly, a similar number of DEGs were induced in both resistant and susceptible responses, with 1553 and 1616 DEG in the resistant and susceptible lines, respectively. However, the percentage of upregulated genes in the susceptible LA3474-S line was higher, whereas more genes were downregulated in the resistant LA3473-R line . In order to gain insight into the mechanism or signal transduction pathway involved in resistance to TYLCV infection, we analyzed the transcriptome of resistant vs susceptible tomato plants at the eight time points during the TYLCV infection response following agroinoculation. DEGs in the resistant vs susceptible response to TYLCV infection were identified with three different statistical algorithms, and the analyses revealed 10751 genes differentially expressed by Cuffdiff, 5051 by Deseq, and 1462 by EdgeR, respectively . Moreover, a total of 679 genes and 58 lncRNAs were identified as differentially expressed in LA3473-R during TYLCV infection by all three methods . A large number of genes were differentially expressed at 24 hpi , but the largest transcriptional changes occurred at 7 dpi in systemically infected leaves, a time when TYLCV is accumulating in emerging leaves . In contrast, a total of 50 and 17 genes were differentially expressed at 10 and 14 dpi, respectively . To further study the TYLCV-mediated defense and susceptible responses in tomato during early and late infections, DEGs in protein families involved in stress responses in plants were selected for further analyses . Of the DEG identified by all three methods, 12 genes were associated with protein families involved in tolerance to abiotic stress response and plant-pathogen interactions, such as WRKY transcription factors, nucleotide binding site-leucine rich repeats proteins, receptor-like protein kinases , leucine-rich repeat receptor kinases and chloroplast proteins. In comparisons between resistant and susceptible responses to TYLCV infection, the mannose-1- phosphate guanyltransferase was upregulated 3.1-fold at 48 hpi, the heavy metal transport/detoxification protein upregulated 5.4-fold at 10 dpi and the WRKY transcription 46 upregulated 5.8-fold at 14 dpi . Comparisons between susceptible and the control revealed that Ycf68 chromosomal protein was upregulated 4.1-fold at 14 dpi; whereas the WRKY transcription factors 45 , 46 and 55 , two NBS-LRR genes , two LRR-RKs genes and one RLK gene were downregulated 1.6 to 6.7-foldat 14 dpi . Additionally, two lncRNAs were downregulated 2.5 to 4.0- folds at 14 dpi in the susceptible response to TYLCV infection . For a better understanding of the transcriptional responses to TYLCV infection in LA3473- R plants and to reveal putative functions of DEGs, GO enrichment analyses were performed with PANTHER and the tomato reference genome sequence. In these analyses, >80% of the DEGs, respectively, were annotated. Within the biological process class, the majority of DEGs belong to the category of cellular processes, metabolic processes and biological regulation .

Barb goatgrass has a higher rating due to its more limited distribution and, therefore, greater opportunity for containment than medusahead. Both of these species commonly occur with other exotic annual grasses . As annual grasses, barb goatgrass and medusahead produce seed at the end of the growing season for subsequent germination in the fall. Inhibiting this seed production is crucial to suppressing or controlling both weeds. Unfortunately, barb goatgrass and medusahead are prolific seed producers, making management to reduce their abundance possible but eradication difficult and unlikely. Barb goatgrass compounds this problem because in addition to producing seed for the following season, it also produces a dormant set of seed that will not germinate until the second growing season after seed production, making 2 years of control necessary to reduce infestations. For these reasons, grazing has not yet proven to be a fully successful strategy for depleting stands of barb goatgrass. Mowing prior to seed maturity may be a better mechanism to control barb goatgrass. Fortunately, medusahead populations can be drastically reduced with a single event that suppresses seed production, lending it to both grazing and mowing treatment methods. Both of these methods require appropriate timing so that the mechanism matches the biological susceptibility of the plant for successful management. The timing of growth events, including major biological changes in plant growth and development through the season, is referred to as phenology. As applied to managing invasive annual grasses, the important events to consider include germination, vertical grow rack vegetative growth phases, and a series of reproductive growth phases that end in the production of viable seed.

As barb goatgrass and medusahead develop and change visually, other changes are less apparent, though important to consider, related to the nutritional quality for livestock grazing, ability of the individual plant to recover from defoliation , and the ability of the seed to continue to develop and later germinate after it is detached from the plant. While these changes are roughly predictable, variation from year to year, across regions, and even within pastures occurs due tovariation in weather, climate, landscape, presence of grazing, soil differences, and genetics. This means that predicting the timing for control can be somewhat imprecise, making the use of phenology observations imperative to optimize control treatments. University of California research conducted over multiple years describes how observations of plant growth stage can help to optimally time grazing and mowing treatments.The stages in table 1 provide a framework to describe how both of these species progress and change in their physical characteristics over the course of the growing season. The 12 stages are broken into vegetative stages V1 to V3, reproductive stages R4 to R9, a mature stage M10, a summer dry stage D11, and dead residual in the subsequent growing season at stage L12. Defining these stages helps to optimize the timing of grazing and mowing control treatments by defining the windows of susceptibility.During the first two vegetative stages the plants are very small and inconspicuous. They are often unnoticed when viewed on a landscape level and provide very little forage for grazing. When plants are grazed during these stages they will readily recover with new flowering stems; thus, little to no control is achieved. In order to impact plants heavily enough to prevent further reproductive stages from progressing, targeted grazing of the infested areas should begin during the late vegetative stage 3 or boot stage . Plants are affected by grazing from stage V3 until the reproductive stage 4 , when awns fully emerge, and neither grass species is palatable to grazing animals thereafter .

Crude protein of the vegetative grasses drops significantly from approximately 10 to 11 percent at the V3 stage to 7 to 8 percent at the R4 stage in the R8 stage , the seed is able to continue to develop and become viable if spikes detach from the plant. This can be visually approximated as the point that plant leaves and stem are turning brown, but the seeds and seed head are still green. The plant is fully mature once shades of red, brown, and green are apparent on the seed head . The window for treatment is much longer for mowing than grazing since palatability is not an issue.To better understand the timing of phenology of these two grasses in California, UC researchers measured the proportion of barb goatgrass and medusahead at various times through the growing season. Included were 18 locations in 11 counties from Shasta to Monterey at elevations from 80 to 990 feet above sea level during the growing season from 2006 to 2010. Whereas medusahead was present at all sites, barb goatgrass was present in samples taken from Tehama County to Yolo County. The limited range of sample locations for barb goatgrass reflected the more restricted range of this species compared with medusahead .Medusahead was sampled over a larger geographic range than barb goatgrass, and we therefore expected and observed more variability in the timing of phenology of medusahead relative to barb goatgrass. Medusahead became susceptible to grazing treatments from early or mid-April to early or mid-May, depending on the site and year. The transition from grazing susceptibility to only mowing susceptibility reliably occurred in early May and was usually complete in mid-May. Medusahead tends to enter the stage of maturity that is too late for effective mowing treatment between the last week of May and the first week of June. Some cooler locations in the Central Coastal valleys can be about a week later. In addition, there seem to be some locations with warmer winter temperatures and less moisture overall where this transition to maturity occurs much earlier, such as early May. Select sites in Glenn, San Joaquin, and Shasta Counties were consistently 2 to 4 weeks earlier than other sites. This may be due to a number of factors, including locally warmer spring temperatures. Barb goatgrass became susceptible to grazing treatments from mid-March to mid-April; however, grazing is not recommended at this time due to the existence of a dormant seed bank and less complete control compared with mowing, making this approach challenging. In 2007, barb goatgrass began developing notably later; 2007 was the only year when barb goatgrass collections made in late March and early April were not yet susceptible to grazing. During other years, the first collections made in early April were already susceptible to livestock grazing. The transition from susceptibility of grazing to only mowing reliably occurred in early May and was complete by mid-May. Generally, at all sites, barb goatgrass entered the stage of maturity that is too late for effective mowing treatment around June 1, with a variance of about 5 days. One year seemed to have more variability, with several sites maturing earlier and others sites later than other years. At the pasture scale, some individuals and patches of medusahead will mature more quickly than others due to variation in soils, slope, and aspect. In more-uniform pastures, most individual and patches of plants may be well synchronized, while in morevariable pastures there may be a greater range of stages at any given time in the spring. This can lead to patchy areas, with some grasses that are too early for treatment success and others that are too late for treatment success. Variable effects within pastures are amplified when they are not grazed. In these cases, managers will need to keep in mind that grazing should be considered a long-term approach that will not be fully successful every year .While the start of susceptibility varies substantially, commercial vertical hydroponic systems ranging from late March to early May, the duration of susceptibility for both barb goatgrass and medusahead tends to vary across its range and from year to year, but in somewhat predictable patterns.

The period of susceptibility of medusahead to targeted grazing is 2 to 3 weeks, while susceptibility to mowing of both species is about 5 weeks. The length of these periods varies by year, location, precipitation, and soils . Cooler spring seasons tend to lengthen the period of susceptibility relative to drier, warmer sites and years. Livestock stocked at moderate or light stocking rates will avoid medusahead and goatgrass particularly as plants transition fromV3 to R4. To overcome this aversion, heavy stocking rates that far exceed rates considered normal for annual rangelands are required to encourage enough consumption to impact seed production. For grazing to be successful, plants must be impacted enough to prevent the onset of seed set. This is possible because these two species tend to mature so late in the season that soil moisture is not present in adequate amounts for plant recovery , although this is not the case every year. The rate is determined by the forage biomass present, but it often exceeds 1 to 2 animal units per acre during the critical V3 and early R4 stages. This high rate is necessary because of the short window and amount of forage that needs to be removed. The objective is to consume forage biomass to, or even slightly below, 500 pounds per acre . This makes treatment site-specific within most ranches because of the difficulty of fencing, watering, and providing enough cattle to impact an area, as well as the increased management time. The short period of susceptibility for successfully implementing a grazing treatment can be overcome with mowing treatments. When implemented correctly, mowing is more likely to be a successful treatment than grazing because the timing will almost, but not always, prevent subsequent seed production. However, mowing faces other challenges, including the inability to cover steep terrain, rocks that damage the mower, and fire potential, which are very common scenarios on California rangelands. It is important to ensure that these treatments are applied as effectively as possible to get the most benefit out of them. By understanding the phenology of barb goatgrass and medusahead, these treatments can be better planned so that the timing and intensity of treatment corresponds to when they will most effectively reduce the abundance of these noxious annual grasses in the future.Grazing and mowing are two of many successfully tested weed control methods that can be used in combination with other methods for medusahead and barb goatgrass control. Using multiple methods is highly suggested for barb goatgrass areas. Ultimately, treatment decisions are site and management specific. Specifics on the effectiveness and implementation of the various other treatment options, such as burning and herbicide application, are available from sources such as Davy et al. 2008, DiTomaso and Kyser 2013, and Kyser et al. 2014. In addition, articles by Aigner and Woerly and James et al. have also assessed the effectiveness of different treatment tools on barb goatgrass and medusahead, respectively.Crops that grow vigorously can often outcompete weeds. Weeds grow best where competition is sparse; for instance, between rows or in gaps in a crop stand. Crops that are well adapted to their planted areas are often better competitors since they will tend to occupy a site rapidly. If you increase the density of the crop by decreasing the in-row spacing or by reducing the space between rows you will improve the crop’s competitiveness. A close-planted crop will close the canopy more rapidly, reducing the weeds’ ability to compete. Some crops compete effectively with weeds if given an early competitive advantage, while others never establish a competitive canopy. The use of transplants give the crop an advantage over the weeds because transplants enter the field larger and more developed that the weeds. With help from subsequent cultivation or hand weeding operations, a transplanted crop can develop a full canopy and crowd out weeds.Practices that reduce the production of weed seed also reduce weed pressure and can help keep weeding costs down over time. In an ideal situation, no weed would be allowed to go to seed. Any that do go to seed can aggravate weed problems for many years to come. As an example, common purslane seed has been shown to remain viable for over 20 years in the soil, and black mustard seed survives for over 40 years. The longevity of weed seed, together with the large numbers of seed produced by individual plants , can lead to the long-term build-up of enormous seed banks in the soil. If you make it a policy to remove weeds prior to seed production, you can reduce weed pressure in subsequent seasons.