Weed control and heliothine management in transgenic cotton (Gossypium hirsutum) cultivars and the physiological response of selected weed species to glyphosate

Field studies were conducted from 2000 through 2003 to evaluate weed control and heliothine management in transgenic cotton. All herbicide systems controlled prickly sida, barnyardgrass, and large crabgrass. Palmer amaranth was controlled better with glyphosate systems than with bromoxynil or conventional herbicide systems. Heliothine infestations were low and infestations did not reduce yields. Yield and economic gross margins of Bt/glyphosate-resistant (BR) cotton cultivars, although non-significant, were the highest over the 3-yr period.;Greenhouse studies were conducted to evaluate the effect of Palmer amaranth seedbank density on the efficacy of pendimethalin plus fluometuron applied preemergence (PRE). Increased Palmer amaranth seedbank density decreased the efficacy of pendimethalin plus fluometuron. Palmer amaranth survivors were increased 3- to 7-fold when seedbank density was doubled. A field study was also conducted to evaluate cotton seeding rate, glyphosate rate, and glyphosate application timing on fruiting pattern and yield of glyphosate-resistant cotton. Cotton seeding rate did not consistently affect the number of vegetative nodes to the first fruiting branch. Increased seeding rate generally decreased the number of vegetative nodes and bolls. Glyphosate applied to cotton with more than four leaves reduced boll retention on the first five fruiting nodes and increased boll retention on higher fruiting nodes.;Roundup Ready FlexRTM cotton will be available commercially. Greenhouse studies were conducted to evaluate the efficacy of glyphosate and insecticide tank mixtures on cotton bollworm (Helicoverpa zea) and selected weed species. H. zea survival and weed control was affected by glyphosate and insecticide combinations. Adding carbaryl to glyphosate decreased control of most weed species.;Shikimate accumulation patterns in selected weed species in response to glyphosate treatment were also evaluated. Plants treated with glyphosate increased in shikimate concentration over time. Shikimate concentrations at 9 DAT followed trends similar to translocation percentages of 14C-glyphosate absorbed 3 DAT. Shikimate concentrations were usually in the order of pitted morningglory < hemp sesbania < barnyardgrass 9 DAT.