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.