To be effective, postemergence herbicides must be absorbed and translocated to sites of action in proper form and quantity. Any factor that interferes in this process may account for differential sensitivity. Adjuvant effects on foliar absorption of BAY MKH 6561 by jointed goatgrass and downy brome were evaluated under growth chamber conditions. Absorption of BAY MKH 6561 by jointed goatgrass and downy brome without adjuvants was 41 and 30% of applied, respectively, 48 h after treatment (HAT). Herbicide absorption with methylated seed oil (MSO) was significantly higher than with nonionic surfactant (NIS) 24 and 48 HAT. The addition of urea ammonium nitrate (UAN) to MSO and NIS significantly increased absorption over MSO and NIS alone 24 HAT, but absorption was similar to that obtained with MSO 48 HAT. Averaged across adjuvant combinations, jointed goatgrass and downy brome absorbed 90 and 89% of applied BAY MKH 6561, respectively, 48 HAT. BAY MKH 6561 translocation and metabolism in jointed goatgrass, downy brome, and winter wheat were also evaluated. More 14C-BAY MKH 6561 translocated to shoot and root tissue in downy brome than in jointed goatgrass and winter wheat. Root exudation accounted for 26% of root-translocated BAY MKH 6561 in jointed goatgrass, 31% in downy brome, and 43% in winter wheat. Winter wheat, jointed goatgrass, and downy brome metabolized 82, 65, and 50% of absorbed 14C-BAY MKH 6561 12 HAT, respectively, and 97% metabolism occurred in all species 48 HAT. Exponential decay equations predicted a 7-h BAY MKH 6561 half-life in winter wheat, 10-h half-life in jointed goatgrass, and 13-h half-life in downy brome. Jointed goatgrass absorbed amounts of 14C-BAY MKH 6561 that were similar to those absorbed by downy brome, but jointed goatgrass was intermediate in translocation and metabolism compared to winter wheat and downy brome. Therefore, differential translocation and metabolism may explain differential field susceptibility observed between winter wheat, jointed goatgrass, and downy brome.
Nomenclature: Jointed goatgrass, Aegilops cylindrica Host., AEGCY; downy brome, Bromus tectorum L., BROTE; wheat, Triticum aestivum L. ‘TAM 107’; BAY MKH 6561, methyl 2-[[[(4,5-dihydro-4-methyl-5-oxo-3-propoxy-1H-1,2,4-triazole-1-yl)carbonyl]amino]sulfonyl]benzoate, sodium salt.