A field experiment was conducted during three cropping seasons to compare weed control and cotton yield provided by conventional (CV), glufosinate-resistant (LL), and glyphosate-resistant (RR) weed management systems under standard (102 cm) and narrow (38 cm) row spacing grown in conventional and conservation tillage systems. The conventional tillage and/or CV cotton received a PRE application of pendimethalin. The CV, LL, and RR cotton varieties received two POST applications of pyrithiobac, glufosinate, and glyphosate, respectively, at two- and four-leaf cotton growth stages. A final (LAYBY) application of trifloxysulfuron was applied to 38-cm row cotton while a LAYBY POST-directed spray of prometryn plus MSMA was used in 102-cm row cotton. The LL and RR weed management systems controlled at least 97% of large crabgrass, Palmer amaranth, sicklepod, and smallflower morningglory, while the CV system controlled 89, 73, and 87 to 98% of large crabgrass, smallflower morningglory, and Palmer amaranth, respectively. Sicklepod control increased from 85% in 102-cm rows to 95% in 38-cm rows in the CV herbicide system. Yellow nutsedge and pitted morningglory control exceeded 98% and was not affected by tillage, row spacing, or weed management system. Cotton yield was not affected by row spacing any year, by tillage in 2005, or by weed management system in 2004 and 2005. In 2006, yield in the RR weed management system was 27 and 24% higher than LL and CV weed management systems, respectively. In 2004, yield of conventional tillage cotton was 18% higher than conservation tillage cotton, but in 2006 the yield in conservation tillage was 12% higher than conventional tillage.

Nomenclature: Glufosinate; glyphosate; monosodium methanearsonate (MSMA); pendimethalin; prometryn; pyrithiobac; trifloxysulfuron; large crabgrass, Digitaria sanguinalis (L.) Scop. DIGSA; Palmer amaranth, Amaranthus palmeri S. Wats. AMAPA; pitted morningglory, Ipomoea lacunosa L. IPOLA; sicklepod, Senna obtusifolia (L.) H.S. Irwin & Barneby SENOB; smallflower morningglory, Jacquemontia tamnifolia (L.) Griseb. IAQTA; yellow nutsedge, Cyperus esculentus L. CYPES; cotton, Gossypium hirsutum L.