Abstract: Field studies were conducted to determine if varying carrier volume proportionally with herbicide dosage, thus maintaining constant herbicide concentration in the carrier, would change the response of sweet corn to glyphosate and of cotton to 2,4-D when compared with using a constant carrier volume where herbicide concentration would vary and be more dilute. For all the parameters measured, more sweet corn injury occurred if the concentration of glyphosate was constant in all volumes of spray. The glyphosate no-effect level for sweet corn was determined to be 0.046 kg/ha when using the variable carrier volume but was over four times greater (0.185 kg/ha) when applied at the constant carrier volume of 281 L/ha. Cotton response to 2,4-D was similar, with the constant herbicide concentration in the carrier at the lower volumes causing greater injury. The response of seed cotton yield was not different when comparing constant to variable carrier volume. The highly sensitive growth stage of cotton at the time of application (bud formation before blooming) may explain this result. These studies demonstrate the need to use carrier volumes that are proportional to the herbicide dosage, thus maintaining constant herbicide concentration in the carrier, when conducting simulated herbicide drift research. Failure to do so could underestimate the potential for injury.

Nomenclature: Glyphosate; 2,4-D; cotton, Gossypium hirsutum L. ‘Acala 1517-95’; sweet corn, Zea mays L. ‘Sweetie 82’.

Additional index words: Crop injury, nontarget crops, off-site movement.

Abbreviations: DAT, days after treatment; OM, organic matter; NMSU-PSRC, New Mexico State University Plant Sciences Research Center.