Comparison of Glyphosate Products in Glyphosate-Resistant Cotton (Gossypium hirsutum) and Corn (Zea mays)1

Wide-spread planting of glyphosate-resistant (GR) crops in the United States and glyphosate patent expiration has led to a proliferation of glyphosate products. Growers have questioned their advisors on efficacy and crop tolerance of the many products available. Field experiments were conducted to evaluate eight glyphosate products in 2002 and ten in 2003, representing isopropylamine, diammonium, and potassium salts, applied POST and postemergence-directed (PDIR) at 630 and 1,680 g ae/ha for GR corn and GR cotton tolerance and weed control. There were no differences among products for control of six annual grass and 10 annual broadleaf weed species. No injury to corn from any glyphosate product at 630 or 1,680 g/ha or to cotton from 630 g/ha was noted at any of seven locations. ClearOut 41 Plus™, an isopropylamine salt of glyphosate, and Roundup WEATHERMAX™, a potassium salt of glyphosate, applied POST at 1,680 g/ha injured cotton 27 to 30% and 10 to 17%, respectively, at 3 of 7 locations. No cotton injury was noted with Glyfos®, Glyfos® X-TRA, Glyphomax™, Gly Star™ Original, Roundup Original™, Roundup UltraMAX®, Touchdown®, or Touchdown Total™. No differences were noted among glyphosate products or between rates for corn or cotton yield or cotton fiber properties.Nomenclature: Glyphosate; corn, Zea mays L. ‘DK687’, ‘DKC 69-71’; cotton, Gossypium hirsutum L. ‘DP 458 B/RR’, ‘ST 4892BR’.Additional index words: ClearOut 41 Plus™, crop tolerance, diammonium salt of glyphosate, Glyfos®, Glyfos® X-TRA, Glyphomax™, Gly Star™ Original, isopropylamine salt of glyphosate, potassium salt of glyphosate, Roundup Original™, Roundup UltraMAX®, Roundup WEATHERMAX™, Touchdown®, Touchdown Total™, weed control.Abbreviations: EPSPS, 5-enolpyruvylshikimate-3-phosphate synthase; GR, glyphosate-resistant; PDIR, postemergence-directed; PEP, phosphoenolpyruvate; WAP, wk after postemergence application; WAP1, wk after first postemergence application; WAP2, wk after second postemergence application; WAPD, wk after postemergence-directed application.