Field studies were conducted in 2001 in Lewiston, NC, and in 2002 at Clayton and Lewiston, NC, to investigate the response of nontransgenic cotton to simulated glyphosate drift in a weed-free environment. Nontransgenic cotton variety ‘Fibermax 989’ was planted in a conventional seedbed at all locations. Glyphosate treatments were applied early postemergence (EPOST) at the four-leaf growth stage of cotton at 0, 8.7, 17.5, 35, 70, 140, 280, 560, and 1,120 g ai/ha and represent 0, 0.78, 1.55, 3.13, 6.25, 12.5, 25, 50, and 100% of the commercial use rate, respectively. Rates as low as 140 g/ha caused lint yield reductions depending on year and location. When averaged over all locations, lint yield reductions of 4, 49, 72, and 87% compared with nontreated cotton were observed with glyphosate rates of 140, 280, 560, and 1,120 g/ha, respectively. Visual injury and shikimic acid accumulation were evident at glyphosate rates greater or equal to 70 g/ha. Collectively, visual injury and shikimic acid accumulation at 7 d after EPOST treatment might be used as a diagnostic indicator to predict potential yield reductions from simulated glyphosate drift.

Four field studies were conducted at the Peanut Belt Research Station near Lewiston Woodville, NC, in 2000, 2001, and 2002 to evaluate crop tolerance, weed control, grain yield, and net returns in glyphosate-resistant corn with various herbicide systems. Preemergence (PRE) treatment options included no herbicide, atrazine at 1.12 kg ai/ha, or atrazine plus metolachlor at 1.68 kg ai/ha. Postemergence (POST) treatment options included glyphosate at 1.12 kg ai/ha as either the isopropylamine salt or the diammonium salt, either alone or in mixtures with mesotrione at 105 g ai/ha plus crop oil concentrate at 1% (v/v) or halosulfuron at 53 g ai/ha plus 0.25% (v/v) nonionic surfactant. All response variables were independent of glyphosate formulation. Addition of metolachlor to atrazine PRE improved large crabgrass and goosegrass control but did not always improve Texas panicum control. POST control of these annual grasses was similar with glyphosate alone or in mixture with halosulfuron or mesotrione. Glyphosate POST controlled common lambsquarters and common ragweed 89 and 93%, respectively. Glyphosate plus halosulfuron POST provided more effective yellow nutsedge control than glyphosate POST. Atrazine PRE or atrazine plus metolachlor PRE followed by any glyphosate POST treatment controlled Ipomoea spp. at least 93%. Glyphosate plus mesotrione in total POST systems always provided greater control of Ipomoea spp. than glyphosate alone. The highest yielding treatments always included glyphosate POST, either with or without a PRE herbicide treatment. Similarly, systems that included any glyphosate POST treatment had the highest net returns.