Effect of Coapplied Glyphosate, Pyrithiobac, Pendimethalin, orS-Metolachlor on Cotton Injury, Growth, and Yield

Field studies were conducted in Louisiana and Mississippi in 2009 and 2010 to evaluate coapplications of glyphosate, pyrithiobac, and residual herbicides on growth and yield of glyphosate-resistant cotton. Treatments were a factorial arrangement of glyphosate (0 and 860 g ae ha−1), pyrithiobac (0 and 470 g ai ha−1), and two residual herbicides (pendimethalin [1,120 g ai ha−1],S-metolachlor [1,070 g ai ha−1], and no residual herbicide). Cotton injury was greatest 3 d after treatment (DAT) and decreased at each evaluation interval until 28 DAT when pyrithiobac was coapplied with glyphosate. Cotton injury ranged from 4 to 17% through 14 DAT when pyrithiobac was applied alone (no residual herbicide) or with pendimethalin, but injury decreased to ≤ 3% after 14 DAT. Cotton injury 3 to 21 DAT following pyrithiobac plusS-metolachlor ranged from 4 to 31%, butS-metolachlor alone injured cotton 1 to 7%. When pyrithiobac was included, cotton injury followingS-metolachlor was 3 to 15% greater than that following pendimethalin from 3 to 14 DAT. Pendimethalin did not reduce plant height at 21 or 42 DAT compared with treatments receiving no residual herbicide, butS-metolachlor reduced plant heights 5 and 4% at 21 and 42 DAT, respectively. Although cotton injury was severe in some cases and persisted until 21 DAT, the injury did not cause reductions in yield. This indicates the early-season cotton injury was transient, and cotton was able to recover from the injury with no observed differences in yield.