Weed Control and Cotton Response to Combinations of Glyphosate and Trifloxysulfuron1

Greenhouse and field studies were conducted to evaluate potential interactions between glyphosate and trifloxysulfuron on barnyardgrass, browntop millet, hemp sesbania, seedling john­ songrass, pitted morningglory, prickly sida, sicklepod, and velvetleaf control as well as cotton injury and yield. In the greenhouse, glyphosate at 840 g ae/ha controlled all weed species 62 to 99%, which was better than trifloxysulfuron at 2.5 or 5 g ai/ha. Control of four-leaf pitted morningglory and hemp sesbania was 80 to 88% when glyphosate and trifloxysulfuron were mixed compared with 62 to 66% control with glyphosate alone. Mixing trifloxysulfuron with glyphosate did not affect control of other species compared with glyphosate alone. In the field, glyphosate controlled barnyardgrass, prickly sida, sicklepod, seedling johnsongrass, and velvetleaf 68 to 100%. Trifloxysulfuron controlled hemp sesbania, seedling johnsongrass, and sicklepod 65 to 88%. All other species were controlled 36 to 72% with glyphosate and 10 to 60% with trifloxysulfuron. Combinations of glyphosate (840 g/ha) and trifloxysulfuron (5 g/ha) were applied postemergence over-the-top and postemergence­ directed to three-, six-, and nine-leaf glyphosate-resistant cotton in the field. Cotton injury at 2 wk after treatment (WAT) was less than 13% for all herbicide treatments and less than 5% by 3 WAT. Herbicides did not affect the percent of open bolls or nodes per plant. Seed cotton yield ranged from 1,430 to 1,660 kg/ha, and only the sequential over-the-top applications of trifloxysulfuron reduced cotton yield compared with the weed-free, nontreated cotton. Nomenclature: Glyphosate; trifloxysulfuron; barnyardgrass, Echinochloa crus-galli (L.) Beauv. #3 ECHCG; browntop millet, Brachiaria ramosa (L.) Stapf # PANRA; entireleaf morningglory, Ipo­ moea hederacea var. integriuscula Gray # IPOHG; hemp sesbania, Sesbania exaltata (Raf.) Rydb. ex A. W. Hill # SEBEX; johnsongrass, Sorghum halepense L. Pers. # SORHA; pitted morningglory, Ipomoea lacunosa L. # IPOLA; prickly sida, Sida spinosa L. # SIDSP; sicklepod, Senna obtusifolia (L.) Irwin & Barnaby # CASOB; velvetleaf, Abutilon theophrasti Medik. # ABUTH; cotton, Gos­ sypium hirsutum L. Additional index words: CGA-362622, crop injury, glyphosate-resistant cotton, herbicide interac­ tions, pesticide interactions, tank mixtures. Abbreviations: ALS, acetolactate synthase; EPOST, early postemergence; fb, followed by; GRC, glyphosate-resistant cotton; LPOST, late postemergence; MPOST, midpostemergence; PD, postemer­ gence-directed; POST, postemergence; POT, postemergence over-the-top; WAT, weeks after treat­ ment. INTRODUCTION and sedge weeds in glyphosate-resistant crops such as corn (Zea mays L.), cotton, and soybean [Glycine max Glyphosate provides broad-spectrum postemergence (L.) Merr.] (Askew and Wilcut 1999; Ateh and Harvey (POST) control of annual and perennial broadleaf, grass, 1999; Culpepper et al. 2000; Faircloth et al. 2001; Gon­ zini et al. 1999; Johnson et al. 2000; Reddy and Whiting 1 Received for publication December 8, 2003, and in revised form August 2000; Tharp and Kells 1999; Wilcut and Askew 1999; 25, 2004. 2 First and third authors: Research Biologist and Plant Physiologist, USDAYoung et al. 2001). Glyphosate-resistant cotton (GRC) ARS, Southern Weed Science Research Unit, 141 Experiment Station Road, was commercialized in 1997, and the area planted to P.O. Box 350, Stoneville, MS 38776; Second author: Research Weed Biolo­ gist, USDA-ARS, National Soil Dynamics Laboratory, 411 Donahue Drive, GRC in the United States has increased from 323,750 ha Auburn, AL 36832. Corresponding author’s E-mail: [email protected]. in 1997 (Heering et al. 1998) to 4,470,000 ha in 2001 3 Letters following this symbol are a WSSA-approved computer code from (NCC 2002). Glyphosate is applied postemergence overComposite List of Weeds, Revised 1989. Available only on computer disk from WSSA, 810 East 10th Street, Lawrence, KS 66044-8897. the-top (POT) of GRC up to the four-leaf growth stage