Field studies were conducted at three locations in North Carolina to evaluate clomazone preemergence (PRE) in glyphosate-tolerant cotton. Cotton was injured by fluometuron PRE at one location. Clomazone PRE controlled common ragweed, goosegrass, large crabgrass, and prickly sida better than trifluralin preplant incorporated (PPI). Trifluralin controlled Palmer amaranth better than clomazone. The addition of a late post-directed (LAYBY) treatment of cyanazine plus MSMA improved the control of goosegrass. Glyphosate at 0.8 kg ai/ha, used postemergence as needed, controlled tall morningglory and entireleaf morningglory at least 84%. Common ragweed was controlled with all herbicides, except trifluralin PPI. Prickly sida was controlled 94% or greater in glyphosate-containing systems. Cotton yields and net returns were similar for all glyphosate systems, regardless of soil-applied herbicides and LAYBY treatment options.

Studies were conducted at Clayton, Lewiston-Woodville, and Rocky Mount, NC, to evaluate weed and cotton response to herbicide systems in glyphosate-resistant cotton in 1995 and 1997. Herbicide systems evaluated included various combinations of soil-applied (trifluralin and fluometuron) and postemergence (POST) (glyphosate or pyrithiobac) herbicides with or without late postemergence-directed (LAYBY) treatments of cyanazine plus MSMA. Glyphosate-resistant cotton injury was less than 5% with all herbicide treatments. Glyphosate POST systems were as efficacious in weed control as other herbicide systems. Depending on location, glyphosate and pyrithiobac POST systems usually required cyanazine plus MSMA LAYBY for season-long control of common lambsquarters, goosegrass, large crabgrass, pitted morningglory, prickly sida, and Texas panicum. Glyphosate POST applied as needed provided weed control equivalent to soil-applied plus POST herbicides, although lint yield was slightly reduced depending on location. Herbicide systems that included soil-applied herbicides required one to two treatments of glyphosate POST and post-directed for season-long weed control and high cotton lint yields, whereas the same herbicide systems without soil-applied herbicides required two to three glyphosate treatments. In all herbicide systems, a residual soil-applied or LAYBY herbicide treatment increased yield compared with glyphosate POST only systems. Location influenced weed control and cotton yield. Generally, as herbicide inputs increased, yield increased.