Cotton Response and Palmer Amaranth Control with Mixtures of Glufosinate and Residual Herbicides

Recommendations to control glyphosate-resistant Palmer amaranth (Amaranthus palmeri S. Wats.) in cotton (Gossypium hirsutum L.) typically include glufosinate applied postemergence (POST) and residual herbicides applied both preemergence and POST. Residual herbicide options for POST application are limited primarily to pyrithiobac and the chloroacetamides herbicides acetochlor and S-metolachlor. Labeling for pyrithiobac warns of injury when mixed with metolachlor. No published information is available on crop or weed response to mixtures of glufosinate plus acetochlor, with or without pyrithiobac. Tolerance of WideStrike® cotton and Palmer amaranth control with glufosinate applied alone to 1- to 2-leaf cotton, glufosinate mixed with pyrithiobac or micro-encapsulated acetochlor or S-metolachlor, and three-way combinations of glufosinate plus acetochlor or S-metolachlor plus pyrithiobac were evaluated in field experiments. These treatments were followed by a second application of glufosinate and diuron plus MSMA directed at layby. Prior to the second application, glufosinate early POST alone controlled Palmer amaranth 77%. Pyrithiobac mixed with glufosinate increased control 10 to 11%, whereas acetochlor and S-metolachlor increased control 12 to 14%. Control was similar with glufosinate plus acetochlor with or without pyrithiobac, whereas combinations of glufosinate plus S-metolachlor plus pyrithiobac were 4 to 5% more effective than glufosinate plus S-metolachlor. Pyrithiobac increased cotton necrosis 3 to 4% and reduced growth 5% 7 d after application compared to glufosinate alone. Acetochlor and S-metolachlor increased necrosis 14 to 18% and reduced growth 7 to 10%. Necrosis was similar with glufosinate plus acetochlor with or without pyrithiobac. Pyrithiobac added to glufosinate plus S-metolachlor increased necrosis 3 to 4%. Injury was transient, and no differences in lint yield were noted among herbicide treatments.