Field studies were conducted in five states at six locations from 2002 through 2003 to evaluate weed control and cotton response to early POST (EPOST), POST/POST-directed spray (PDS), and late POST-directed (LAYBY) systems using glyphosate-trimethylsulfonium salt (TM), s-metolachlor, trifloxysulfuron, prometryn, and MSMA. Early POST applications were made from mid May through mid June; POST/PDS applications were made from early June through mid July; and LAYBY applications were made from early July through mid August. Early season cotton injury and discoloration was minimal (< 1%) with all treatments; mid- and late-season injury was minimal (< 2%) except for trifloxysulfuron POST (11 and 9%, respectively). Annual grasses evaluated included barnyardgrass, broadleaf signalgrass, goosegrass, and large crabgrass. Broadleaf weeds evaluated included entireleaf morningglory, pitted morningglory, sicklepod, and smooth pigweed. For the EPOST, POST/PDS, and LAYBY applications, weeds were at cotyledon to 10 leaf, 1 to 25 leaf, and 2 to 25 leaf stage, respectively. Annual broadleaf and grass control was increased with the addition of s-metolachlor to glyphosate-TM EPOST systems (85 to 98% control) compared with glyphosate-TM EPOST alone (65 to 91% control), except for sicklepod control where equivalent control was observed. Annual grass control was greater with glyphosate-TM plus trifloxysulfuron PDS than with trifloxysulfuron POST or PDS, or trifloxysulfuron plus MSMA PDS (90 to 94% vs. 75 to 83% control). With few exceptions, broadleaf weed control was equivalent for trifloxysulfuron applied POST alone or PDS alone or in combination with glyphosate-TM PDS or MSMA PDS herbicide treatments (81 to 99% control). The addition of a LAYBY herbicide treatment increased broadleaf weed control by 11 to 36 percentage points compared with systems without a LAYBY. Cotton lint yield increased 420 kg/ha with the addition of s-metolachlor to glyphosate-TM EPOST treatments compared with systems without s-metolachlor EPOST. Cotton lint yield was increased 330 to 910 kg/ha with the addition of a POST herbicide treatment compared with systems without a POST/PDS treatment. The addition of a LAYBY herbicide treatment increased cotton lint yield by 440 kg/ha compared with systems without a LAYBY.
Nomenclature: Glyphosate-TM, MSMA, prometryn, s-metolachlor, trifloxysulfuron, barnyardgrass, Echinochloa crus-galli (L.) Beauv. ECHCG, broadleaf signalgrass, Brachiaria platyphylla (Griseb.) Nash. BRAPP, entireleaf morningglory, Ipomoea hederacea var. integriuscula Gray. IPOHG, goosegrass, Eleusine indica (L.) Gaertn. ELEIN, arge crabgrass, Digitaria sanguinalis (L.) Scop. DIGSA, pitted morningglory, Ipomoea lacunosa L. IPOLA, sicklepod, Cassia obtusifolia L. CASOB, smooth pigweed, Amaranthus hybridus L. AMACH, cotton, Gossypium hirsutum L. ‘DP 458 RR/BG’, ‘DP 555 RR/BG’, ‘FM 989 RR/BG’, ‘PM 2344 RR/BG’, ‘ST 4793 RR’