Soybean plant responses to glyphosate and manganese in glyphosate-resistant production systems

Glyphosate-resistant (GR) soybean is widely accepted in the United States. Recent research in Indiana and elsewhere has observed that post-emergence foliar applications of glyphosate may cause or exacerbate manganese (Mn) deficiency and then limit yield of GR soybeans on low Mn availability soils. The objectives of this study were to: (1) determine responses of GR soybeans to glyphosate application when supplemental Mn is and is not applied, (2) determine the effectiveness of starter-banded and foliar Mn in GR soybean production systems with various frequencies of glyphosate application, and (3) better understand the spatial variation in inherent soil properties that affect soybean micronutrient uptake and soybean response to supplemental Mn fertilizer. A field study involving wide-row (76-cm) soybean production was conducted at three Indiana locations ranging in mean soil-test Mn from 2.8 to 13.7 mg/kg in 2007 and 2008. Main-plot treatments were four levels of glyphosate application frequency (zero, pre-emergence only, pre-emergence plus single post-emergence, and pre-emergence plus double postemergence). Sub-plot treatments were three levels of banded Mn (0, 2.8 and 5.6 kg/ha). A foliar Mn application (0.56 kg/ha) was also evaluated following only the pre-emergence plus single post-emergence glyphosate application. Full weed control was achieved on all plots by pre-emergence residual herbicide application and hand weeding so that possible glyphosate and Mn treatment effects on soybean would not be confounded by differential weed competition. Leaf Mn concentrations, measured after sampling the top fully expanded trifoliate leaves three or four times during the growing season, changed with time but rarely in response to treatments. Glyphosate applications never reduced leaf and seed Mn concentrations relative to the control plots without glyphosate. Banded Mn fertilizer did not improve leaf Mn concentrations; however, seed Mn concentrations increased in response to banded Mn at all three locations in 2007. Foliar Mn application increased leaf Mn concentrations substantially, although this beneficial effect was short lived. Other leaf and seed nutrient (zinc, iron, nitrogen, phosphorus, potassium) concentrations rarely responded significantly to either glyphosate or Mn treatments. Soil spatial variability substantial impacted soybean response in our approximately 2 ha experiments despite five replications. Leaf Mn concentrations was often highly correlated with individual plot soil pH (negatively) and Mehlich-3 extractable soil Mn (positively). Overall, the application of glyphosate to soybean without weed competition did not reduce yield; but neither banded nor foliar Mn supplementation increased soybean yield