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Fluid Journal : Fluid Journal 2005-2007
FALL 2007 Fluid Journal 21 inhibits vital physiological functions to cause death of susceptible plants and microorganisms. Many commercial crop plants have been genetically modified to partially compensate for the physiological disruption caused by glyphosate so that they tolerate the chemical. Although glyphosate resistance in genetically modified corn and soybeans reduces the efficiency of manganese (Mn) uptake and physiological efficiency 10 to 50 percent, depending on the genetic nutrient uptake efficiency of the particular transformed variety or hybrid, other physiological effects of glyphosate (reduced uptake and translocation of Fe, K, and Mn, physiological immobilization of Mn, reduced root nodulation and N-fixation, drought stress, early maturity, and disease increase) may go largely unnoticed in commercial practice without a near-isogenic comparison available. Glyphosate tolerant (Roundup Ready®) crops required the application of almost 50 percent more Mn to meet their physiological sufficiency than conventional soybean varieties. Glyphosate in root exudates of weeds and GM tolerant plants changes the rhizosphere biology to reduce the availability of Fe, Mn, and other essential mineral nutrients for crop uptake. Glyphosate-induced Mn deficiency can compromise plant resistance mechanisms mediated through the shikimate pathway so that diseases such as take-all, Fusarium head scab and root rot, Corynesporium root rot, and numerous other diseases increase after glyphosate is applied for weed control. Our research the past 15 years has been directed at developing strategies to ameliorate the deleterious effects of glyphosate Figure 1. Effect of glyphosate* on nutrient uptake and translocation by "non-target" plants, Eker, et al. 2006. (* 2.5% of recommended herbicidal rate of glyphosate.) on nutrient efficiency and disease susceptibility that will optimize its benefit as a herbicide. Strategies Strategies to ameliorate Mn immobilization by glyphosate include: • Mn enrichment (fertilization and biological enhancement) • Detoxification of glyphosate in root exudates or soil residual. • Modification of cultural practices to enhance Mn availability. Our initial hope was to use glyphosate as a carrier for several micronutrients that are deficient in various Indiana soils and thereby improve the economic viability of micronutrient fertilization by eliminating the need for a separate trip across the field. It became apparent early on that tank mixes of glyphosate and various mineral elements were incompatible and reduced the efficacy of the glyphosate as a herbicide (Table 1) while providing little available nutrient advantage or plant response. Ammonium sulfate ("water softener") and different glyphosate formulations can reduce the antagonism and immobilization of Mn to improve weed control. Normal uptake and translocation of foliar- applied Mn was not observed until at least eight days after glyphosate was applied. The greatest soybean yield response on high organic soils was with both Mn and Cu applied 8 to 12 days after the glyphosate. The highest yields for corn were obtained by foliar-applying Zn 15 days after glyphosate was applied in northwestern Indiana, while tank mixes of various Zn products reduced yields as much as 35 bu/A under heavy weed pressure because of reduced herbicidal efficacy of the glyphosate-Zn complex that formed (Table 2). Zinc significantly increased Roundup Ready corn yield only when applied 15 days after the glyphosate in an irrigated field. Bio-enrichment. We have been unable to obtain consistent responses to preplant or at-plant Fe Mn Zn Fe Mn Zn Root uptake Translocation to shoot 100 80 60 40 20 0 % Control +Glyphosate
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