POSTEMERGENCE ACTIVITY OF SULFENTRAZONE - EFFECTS OF SURFACTANTS AND LEAF SURFACES

Sulfentrazone was foliar applied at 34 and 56 g ai ha(-1) alone or in combination with surfactants to soybean cultivars Hutcheson and Centen nial and to sicklepod, coffee senna, smallflower morningglory, velvetl eaf, and yellow nutsedge. The most sensitive weeds, including coffee s enna, smallflower morningglory, and velvetleaf, were severely injured by the lowest rate when sulfentrazone was applied with surfactants. Su lfentrazone provided the highest control of yellow nutsedge with X-77, Soybeans were not severely injured by sulfentrazone applied alone, bu t 55% foliar injury occurred when the herbicide was applied with X-77. However, the seedlings were not killed, Sicklepod was the most tolera nt of the weeds tested, In the absence of surfactants, the order of ra diolabeled sulfentrazone absorption by the foliage was Centennial (5.8 %) = Hutcheson (8.5%) = coffee senna (10.4%) < yellow nutsedge (17.0%) < velvetleaf (22.3%) = smallflower morningglory (24%), Sicklepod leav es did not retain droplets containing sulfentrazone when no surfactant was used. Species with the highest foliar absorption also showed the greatest phytotoxic response to the herbicide. Addition of surfactants to the spray mixture enhanced the foliar absorption and overall phyto toxicity of sulfentrazone in the weeds. An inverse relationship was de tected between the foliar absorption of sulfentrazone without surfacta nts and the amount of cuticular wax present on the leaves. No such cor relation was observed when surfactants were used. Thus, surfactants ov ercame the barrier to absorption imposed by the cuticular wax and, und er these conditions, selectivity apparently became dependent upon spec ies-specific cellular tolerance to sulfentrazone.