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.