Responses of selected weeds and glyphosate-resistant cotton and soybean to two glyphosate salts

Studies were conducted in 2000 and 2001 to investigate responses of glyphosate- resistant cotton, glyphosate-resistant soybean, and selected weed species to postemergence applications of isopropylamine (Ipa) and diammonium (Dia) salts of glyphosate at selected rates ranging from 0.42 to 3.36 kg ae/ha. No differences were detected between either glyphosate salts or application timings for cotton injury, cotton lint yield, micronaire, fiber length, fiber strength, or fiber uniformity. In a weed-free soybean study, no differences in soybean injury occurred between early-postemergence treatments of the two glyphosate salts. Injury from late-postemer- gence treatments did not exceed 12% with glyphosate-Ipa or 9% with glyphosate-Dia at 3.36 kg/ ha. Soybean treated with glyphosate-Ipa yielded 3,050 kg/ha, whereas soybean treated with gly- phosate-Dia yielded 2,880 kg/ha, when averaged over glyphosate rate and application timing. In a soybean study that included weed control as a variable, weed control at 14 d after treatment (DAT), and soybean yield was independent of glyphosate salt. Control of common ragweed, ivyleaf morningglory, pitted morningglory, and large crabgrass at 28 DAT was similar at 0.84 kg/ha of either glyphosate salt.