Glufosinate Rate and Timing for Control of Glyphosate-Resistant Rhizomatous Johnsongrass (Sorghum halepense) in Glufosinate-Resistant Cotton (Gossypium hirsutum)

Johnsongrass was documented as resistant to glyphosate in 2010. Consequentially, strategies to mitigate and/or manage glyphosate-resistant (GR) johnsongrass in Louisiana cotton are needed. Field experiments were conducted in Louisiana from 2011 through 2013 to evaluate the number and timing of glufosinate applications for control of GR johnsongrass in glufosinate-resistant(GLR) cotton. Treatments included two or three glufosinate applications during the season with the initial application applied 2 or 4 wk after planting, and sequential applications applied 2 or 3 wk after the initial application. In treatments with two applications, 880 g ha-1 of glufosinate followed by (fb) 590 g ha-1 of glufosinate were applied. Glufosinate at 590 g ha-1 was sequentially applied for treatments with three applications. Following two applications, delaying the sequential application from 2 to 3 wk after the initial application reduced control from 95 to 85% 14 d after final glufosinate treatment (DAT), but no differences in control with sequential timings was observed following three applications. Similar results were observed 21 DAT, except with three applications separated by 3 wk, which controlled johnsongrass 97% compared to 86% when sequential applications were spaced 2 wk apart. In addition, three applications spaced 3 wk apart reduced johnsongrass heights to 22% of the nontreated. Cotton yield was increased following two applications that were initiated 4 wk after application (WAP) (1000 kg ha-1) compared to 2 WAP (620 kg ha-1). Data indicate three glufosinate applications separated by 3 wk will provide season-long GR johnsongrass control and maximize cotton yield in GLR cotton.