Effect of row spacing on weed management in glufosinate-resistant cotton

Transgenic, herbicide-resistant cultivars and equipment to spindle-pick 38-cm rows has renewed interest in narrow-row cotton production. Field experiments were conducted at four locations in North Carolina during 2004 and 2005 to evaluate weed management systems in glufosinate-resistant cotton planted in 38- and 97-cm rows. Weeds included broadleaf signalgrass, goosegrass, fall panicum, large crabgrass, Palmer amaranth, smooth pigweed, pitted morningglory, and tall morningglory. Greater than 90% control of annual grasses and Amaranthus spp. in 2004 and Ipomoea spp. in both years was obtained in narrow-row cotton with glufosinate applied early POST (EPOST) and mid-POST (MPOST) to two- and six-leaf cotton, respectively. With good early-season control by glufosinate and rapid canopy closure, there was little benefit from pendimethalin, fluometuron, or pyrithiobac applied PRE, S-metolachlor or pyrithiobac mixed with glufosinate applied MPOST, or trifloxysulfuron applied late POST (LPOST) to 11-leaf cotton in 2004. In 2005, with larger weeds at initial application, glufosinate EPOST and MPOST did not adequately control annual grasses and Amaranthus spp. Pendimethalin PRE increased control to greater than 90% and increased yields 59 to 75%. Pendimethalin PRE followed by S-metolachlor or pyrithiobac mixed with glufosinate at MPOST was no more effective than pendimethalin alone. Without PRE herbicides, trifloxysulfuron applied LPOST increased Amaranthus but not annual grass control. Cotton row spacing had no effect on cotton yield and little effect on weed control.