Physiological response of soybean (Glycine max) and two weed species to thifensulfuron and bentazon combinations

Greenhouse and laboratory studies were conducted to determine the effects of bentazon on the crop safety, efficacy, foliar absorption, and translocation of thifensulfuron when it is applied to soybean, velvetleaf, and common lambsquarters. A metabolism study was conducted on soybean. Thifensulfuron applied at 1.1 g ai ha−-1with 28% urea ammonium nitrate (2.5% v/v) and BAS-0904805 (Dash) adjuvant (0.63% v/v) reduced the growth of velvetleaf and common lambsquarters by an average of 91 and 84%, respectively. The addition of 420 g ai bentazon ha−-1had no effect on thifensulfuron efficacy in the weed species. Soybean dry weights were decreased by 58% when thifensulfuron was applied at 2.2 g ha−-1but were decreased by only 36% when bentazon, at 420 g ha−-1, was added. In the absorption study, the addition of bentazon reduced foliar absorption of14C-thifensulfuron into velvetleaf and common lambsquarters 8 and 24 h after treatment (HAT), but absorption into soybean was not affected. Bentazon reduced the translocation of14C from thifensulfuron out of the treated leaves of velvetleaf and common lambsquarters by at least 16 and 11%, respectively, beyond 24 HAT. Soybean translocated 18, 29, and 26% of absorbed14C out of the treated leaflets 24, 72, and 168 HAT, respectively. These translocation values were reduced to 7, 12, and 11%, respectively, when bentazon was tank-mixed with thifensulfuron. Soybean metabolized 35% of recovered14C-thifensulfuron by 24 HAT. Addition of bentazon did not change this level of metabolism. These studies suggest that the physiological basis for the decrease of soybean injury from thifensulfuron, when it is tank-mixed with bentazon, is decreased thifensulfuron translocation.