In traditional simulated herbicide drift research, dose response is evaluated using a constant carrier volume. The influence of carrier volume was evaluated in field experiments with drift rates representing 12.5 and 6.3% of the use rates of 1,120 g ai/ha glyphosate (140 and 70 g/ha, respectively) and 420 g ai/ha glufosinate (53 and 26 g/ha, respectively). Corn and soybean were exposed to herbicide rates applied in constant carrier volume of 234 L/ha and in proportional carrier volumes of 30 L/ha for the 12.5% rate and 15 L/ha for the 6.3% rate. Averaged across herbicides, corn height reduction 14 d after treatment (DAT) was greater for the 12.5% rate when applied in proportional 30 L/ha carrier volume (45%) compared with constant 234 L/ha carrier volume (28%). The 6.3% rate reduced corn height 38% when applied in proportional 15 L/ha carrier volume but not when applied in 234 L/ha carrier volume. When carrier volume was changed from constant to proportional, corn injury 14 DAT increased from 33 to 51% for the 12.5% rate and 18 to 38% for the 6.3% rate. Compared with constant spray volume, corn yield reduction was 1.5 times greater for the 12.5% rate but 4 times greater for the 6.3% rate when spray volume was varied proportionally to the herbicide rates. Differential response due to carrier volume was not observed when herbicides were applied to soybean. Soybean was injured more by glyphosate than by glufosinate, but recovery was rapid and yield was not negatively affected. Results suggest that drift research using constant spray volume may underestimate the yield reduction expected for sensitive crops exposed to glyphosate or glufosinate.

Nomenclature: Glufosinate; glyphosate; corn, Zea mays L. ‘Dekalb 687’; soybean, Glycine max (L.) Merr. ‘DPL 3588’.

Additional index words: Crop injury, herbicide drift, off-target movement.

Abbreviations: DAT, days after treatment.