Interactions between glyphosate and imazethapyr on four annual weeds

Greenhouse and field experiments were conducted to evaluate the efficacy of various combinations of imazethapyr (0, 23, 46, and 70 g ai/ha) with glyphosate (0, 210, 420, 630, and 840 g ae/ha) on Setaria faberi, Amaranthus rudis, Abutilon theophrasti, and Ipomoea hederacea control. Additivity was the most frequently observed interaction and no synergistic interaction occurred throughout this study. The combination of imazethapyr with glyphosate at 210 g/ha caused an antagonistic response on Setaria faberi. Glyphosate at 420 g/ha with or without imazethapyr provided at least 95% control of Setaria faberi. The interaction between glyphosate and imazethapyr was additive on Amaranthus rudis control. Eight of the twenty-one herbicide combinations were antagonistic on Abutilon theophrasti control. Antagonistic interactions occurred when 46 or 70 g/ha of imazethapyr was added to 420 or 630 g/ha of glyphosate; while no antagonistic interactions were noted when glyphosate rate was 840 g/ha. The interactions on Ipomoea hederacea control were additive when the glyphosate rate was at least 420 g/ha. Glyphosate at 210 g/ha plus imazethapyr at 46 or 70 g/ha caused antagonistic interactions on Ipomoea hederacea control. Weed control tended to be more variable when the glyphosate rate was 210 g/ha and the imazethapyr rate was 46 or 70 g/ha. In general, the addition of imazethapyr to low rates of glyphosate improved control of Amaranthus rudis and Ipomoea hederacea and did not improve control of Setaria faberi and Abutilon theophrasti.