Tolerance of Cotton to the Herbicide Glyphosate1

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Abstract
An increasing group of weed pests in U.S. cotton (Gossypium hirsutum L.) are the perennial grasses which can be controlled with glyphosate [N‐phosphonomethyl) glycine], but cotton is also sensitive to this herbicide. Field studies were initiated to determine if certain cotton genotypes could be selected with sufficient tolerance to glyphosate to allow its use in a cotton‐weed control program. In 1974,405 genotypes were evaluated for tolerance to glyphosate at rates of 0.84, 1.68, and 3.36 kg/ha applied as an over‐the‐top treatment in the early bloom stage. In 1975, 24 genotypes selected from 1974 studies were screened at rates of 0.56 and 0.84 kg/ha of glyphosate ap lied over‐the‐top of cotton plants. Six genotypes selected from previous studies were evaluated with glyphosate applied over‐the‐top or directed under cotton plants 50 to 62 cm in height in the very early square stage. Glyphosate toxicity varied with cotton genotypes and increased with increasing rates. In 1976 and 1977, all genotypes that received an over‐the‐top treatment of glyphosate yielded significantly less seed cotton than the untreated checks. Glyphosate applied as a directed post‐emergence treatment did not cause significant injury symptoms in treated cotton, however, none of the treated genotypes produced seed cotton yields equal to their untreated controls. Cotton injury was only 10% or less when the herbicide was directed to the base of the plants as compared with 65 to 80%, injury when the herbicide was applied over‐the‐top. Two genotypes (DES 04‐11 and DES 04‐606) yielded over 100% of their untreated controls when one or two separate applications of 0.56 kg/ha of glyphosate was directed to the base of the cotton plants in 1977. The increase in yield of these two genotypes was not significant, but indicated a level of glyphosate tolerance which could be used in the development of commercial cultivars possessing enough tolerance to glyphosate to allow its use for perennial weed control in cotton.