Management of Palmer Amaranth (Amaranthus palmeri) in Second-Generation Glyphosate-Resistant Cotton

Field experiments were conducted in 2005 and 2006 to evaluate glyphosate timings and use of residual herbicides to control Palmer amaranth ( Amaranthus palmeri S. Wats.) in second-generation glyphosate-resistant cotton ( Gossypium hirsutum L.).  Glyphosate treatments based on timing were used , in addition to preplant incorporated and postemergence-topical residual herbicides , to determine effective Palmer amaranth management systems.  In 2005, complete control (100%) of Palmer amaranth was achieved across all glyphosate postemergence-topical systems.  Less than complete control (94 to 98%) was observed with postemergence-directed glyphosate-resistant systems either with or without trifluralin preplant incorporated compared to second-generation glyphosate-resistant postemergence-topical systems.  In 2006, similar Palmer amaranth control (95 to 99%) was observed across all second-generation glyphosate-resistant systems following trifluralin preplant incorporated and no benefit was observed from pyrithiobac or S -metolachlor tank-mixed with glyphosate.  When trifluralin was not used, pyrithiobac or S -metolachlor tank-mixed with glyphosate early postemergence-topical followed by glyphosate improved control compared to glyphosate alone.  Delaying early-season glyphosate applications did not reduce cotton lint yield; however, when no glyphosate was used cotton yields were reduced.