Greenhouse and laboratory experiments were conducted to determine if the synergistic interaction between imazethapyr and thifensulfuron in sulfonylurea-tolerant (STS) soybean involved enhanced absorption and translocation or reduced metabolism of one or both herbicides. Thifensulfuron at 4.4 g ha−1 and imazethapyr at 70 g ha−1 caused 0 and 28% injury to STS soybean 7 DAT, respectively, while the combination of both herbicides caused 50% injury 7 DAT. Imazethapyr had no effect on absorption of 14C-thifensulfuron into the first trifoliolate. Imazethapyr did not affect absorption, translocation, or metabolism of 14C-thifensulfuron. Metabolism of 14C-thifensulfuron was rapid, with less than 10% remaining after 24 h, and was not affected by imazethapyr. Likewise, thifensulfuron did not affect the absorption, translocation, or metabolism of 14C-imazethapyr. Therefore, the synergism between thifensulfuron and imazethapyr does not involve changes in the absorption, translocation, or metabolism of either herbicide.

Greenhouse and laboratory experiments were conducted to determine if the synergistic injury to sulfonylurea-tolerant soybean (STS soybean) caused by applications of thifensulfuron plus imazethapyr involves an interaction between the herbicides at the ALS enzyme. In vitro ALS from STS soybean (Asgrow 3200) was 10 times more tolerant to thifensulfuron than ALS from non-STS soybean (Williams 82). In vivo ALS was 30% of maximum activity 6 HAT in STS soybean treated with 4.4 g ha−1 thifensulfuron, but 0% of maximum in similarly treated non-STS soybean. Increasing thifensulfuron from 4.4 to 141 g ha−1 did not increase in vivo ALS inhibition in STS soybean, indicating a highly resistant ALS isozyme. Both cultivars responded similarly to imazethapyr in terms of visual injury, in vitro ALS inhibition, and in vivo ALS inhibition. Soybean injury and in vivo ALS inhibition increased with imazethapyr rate in both cultivars. The relationship between imazethapyr inhibition of ALS in vivo and soybean injury was curvilinear. In vitro ALS assays revealed no synergistic interaction between thifensulfuron and imazethapyr at the enzyme level. In vivo ALS activity was reduced to 7 and 10% of controls 24 and 48 HAT by the tank mixture compared to 56 and 84% for thifensulfuron alone and 20 and 28% for imazethapyr alone. These data show that the synergistic increase in soybean injury caused by tank mixing thifensulfuron with imazethapyr results from the cumulative inhibition of ALS, not a synergistic inhibition of the enzyme.

Sulfonylurea-tolerant soybean (STS soybean) tolerance to imazethapyr (70 g/ha) plus thifensulfuron (4.4 g/ha) tank mixture was evaluated in 1993 and 1994 at two locations in Illinois. Thifensulfuron injury was reduced from 27% in non-STS soybean to 1% in STS soybean 14 days after treatment (DAT). STS and non-STS soybean did not differ in response to imazethapyr alone with < 6% injury 14 DAT. Injury from the tank mix was reduced from 40% in non-STS soybean to 24% in STS soybean 14 DAT. However, a synergistic interaction between the herbicides exists in STS soybean. The yield of injured soybean was not reduced in either STS or non-STS soybean. Injury to STS soybean from the tank mix was decreased by reducing the rate of imazethapyr from 70 g/ha to 52.5 or 35 g/ha. Reducing the thifensulfuron rate mixed with imazethapyr at 70 g/ha did not decrease injury in STS soybean. Control of smooth pigweed, common lambsquarters, and common cocklebur was not affected by reducing the rate of either herbicide in the tank mixture.