Effect of Shading, Cultivar, and Application Timing on Cotton Tolerance to Glufosinate

The increasing presence of glyphosate-resistant (GR) weeds in the Midsouth, and inconsistent crop injury and moisture dependence of residual herbicides has created a need for effective post-emergence options. Cotton cultivars with tolerance to glufosinate have been widely adopted by growers throughout the Midsouth because glufosinate provides an effective option for controlling GR weeds like Palmer amaranth (Amaranthus palmeri (S.) Wats.]. The objective of this study was to determine if differences exist in tolerance of PhytoGen® and Liberty Link® cultivars to glufosinate applied at different growth stages in the presence and absence of low-light conditions. At two weeks after cotton emergence (WAE), tolerance to glufosinate differed by cultivar, although some injury was observed on Liberty Link cotton. Injury was often greatest when applied at the one-leaf stage to PhytoGen® cultivars, but by four to five weeks after treatment, all cultivars showed similar potential to recover. In general, cotton plants that were shaded three days prior to applying glufosinate were injured to a greater extent than non-shaded plants. Similarly, seed cotton yields were reduced in shaded plots by 72 and 76 g m-1 of row in 2012 and 2013, respectively. This research indicates that there is greater risk for early-season injury from glufosinate if applied to young cotton experiencing prolonged cloudy conditions prior to application; albeit, this injury does not translate into seed cotton yield loss for the three cultivars evaluated, compared to an untreated control. Hence, it is recommended that growers make timely applications of glufosinate to optimize weed control, even when conditions have been less than ideal for cotton growth prior to application.

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