Experiments were conducted in the greenhouse to determine the effects of adjuvant and herbicide concentrations on imazamox, imazethapyr, nicosulfuron, and ICIA 0604 phytotoxicity, independent of spray retention. Equal amounts of herbicide were applied to oat in a single 0.5-μl drop or four 0.5-μl drops. Changes in adjuvant concentration influenced herbicide phytotoxicity more with application in one concentrated drop rather than in four dilute drops. Overall, herbicide phytotoxicity was greater when low adjuvant concentrations were applied in four dilute drops compared with a single concentrated drop. But when the same total amount of adjuvant was applied in one or four drops, herbicide phytotoxicity in a single drop was generally equal to or greater than in four drops. These results suggest that high herbicide phytotoxicity with high herbicide and adjuvant concentrations in low spray volumes in the field was primarily because of increased herbicide absorption rather than spray retention.

Nomenclature: ICIA 0604 (proposed common name, tralkoxydim), 2-[1-(ethoxyimino)propyl]-3-hydroxy-5-(2,4,6-trimethylphenyl)cyclohex-2-enone; imazamox; imazethapyr; nicosulfuron; oat, Avena sativa L.

Additional index words: Absorption, spray coverage, spray volume.

Abbreviations: MVO, methylated vegetable oil; NIS, nonionic surfactant.