Herbicide Program Alternatives for Glyphosate-Resistant Palmer Amaranth (Amaranthus palmeri) Control in Cotton

Over the past several years, glyphosate-resistant Palmer amaranth (Amaranthus palmeri) has emerged has one of the most severe threats to reduced tillage cotton production in the southeast.  Field studies were conducted in 2008 to evaluate different combinations preemergence and postemergence herbicides for glyphosate-resistant Palmer amaranth control in South Carolina.  Preemergence treatments included pendimethalin at 1.0 lb ai/A, alachlor at 1.0 lb ai/A, fluometuron at 1.0 lb ai/A, and fomesafen at 0.375 lb ai/A.  Postemergence treatments included glyphosate at 0.75 lb ae/A, s-metolachlor at 1.3 lb ai/A, and pyrithiobac at 0.095 lb ai/A.   Preemergence treatments were applied shortly after planting.  Postemergence treatments were applied at the 1 to 2 and 4 to 5 leaf stage.  An untreated check was included for comparison.  The study was arranged as a randomized complete block design with 4 replications.  Individual plot sizes were 12.7 by 40 ft.    Since rainfall is sporadic during cotton planting in South Carolina, overhead irrigation was provided in this study.  In all treatments receiving a preemergence treatment (alachlor, pendimethalin, fomesafen, and fluometuron), Palmer amaranth was controlled greater than 95%.  In addition, s-metolachlor applied before Palmer amaranth emergence increased the length of residual control of alachlor, pendimethalin, fomesafen, and fluometuron (100% control).  Pyrithiobac provided excellent control of Palmer amaranth in this study (>95% control).  This indicated that the populations were not ALS-resistant.  Overall, cotton yield was not impacted by any of the herbicide treatments even though some injury was noted on the s-metolachlor and fomesafen treatment early in the season (10% or less injury).  Where irrigation or rainfall is timely, preemergence programs will continue to play an important role in managing glyphosate-resistant Palmer amaranth.  If preemergence herbicides fail to control glyphosate-resistant Palmer amaranth (i.e., not activated by irrigation or rainfall), then pyrithiobac is the only topical treatment currently available to control escapes.  Currently, populations of ALS-resistant Palmer amaranth are increasing in South Carolina.  This leaves fomesafen and flumioxazin (same mode of action) as the only control options in glyphosate-tolerant cotton (both depend on water for activation).