Efficacy of Halauxifen-Methyl for Management of Glyphosate-Resistant Weeds and Soybean Crop Safety

Halauxifen-methyl is a new synthetic auxin herbicide for broadleaf weed control prior to planting of corn, cotton, and soybean. Experiments were conducted to evaluate the efficacy of herbicide programs containing halauxifen-methyl for control of glyphosate-resistant (GR) horseweed and other problematic weed species in comparison to existing herbicides programs utilized in no-till GR soybean systems. Halauxifen-methyl-, dicamba-, and saflufenacil-based herbicide programs controlled GR horseweed, while glufosinate and 2,4-D herbicide programs resulted in lower GR horseweed control. The addition of ALS and PPO inhibitor herbicides with soil residual activity increased GR horseweed control of herbicide programs with halauxifen-methyl and glufosinate. Halauxifen-methyl applied alone or in tank-mixtures controlled both GR horseweed and common ragweed. However, the addition of either 2,4-D or dicamba was necessary to increase the weed control spectrum of halauxifen-methyl in preplant burndown applications. Synthetic auxin herbicides such as 2,4-D and dicamba have preplant interval restrictions for soybean. Research was conducted to evaluate the potential of halauxifen-methyl to cause soybean phytotoxicity when applied at five different preplant intervals. Soybean phytotoxicity from halauxifen-methyl applied at any preplant interval did not occur in 2015. Soybean phytotoxicity ranging from 1 to 15% was observed in 2016 at 14 days after planting for applications made at 14, 7, and 0 days before planting. However, soybean plants quickly recovered and phytotoxicity was negligible at 21 days after planting. Research was conducted to evaluate the efficacy of halauxifen-methyl and dicamba alone and in tank-mixtures with 2,4-D, glyphosate, and chlorimuron + tribenuron for control of fall-emerged GR horseweed. Fall applications of halauxifen-methyl, 2,4-D, and dicamba control fall-emerged horseweed and reduce GR horseweed height in the spring. However, control of winter annual weeds can increase spring horseweed emergence. Halauxifen-methyl controls GR horseweed and common ragweed in spring preplant burndown applications and fall-emerged horseweed in fall burndown applications; however, tank-mixtures are required to increase the weed control spectrum of the herbicide. Halauxifen-methyl can cause minor phytotoxicity to soybean unifoliates, although it did not impact soybean stands or grain yield. The potential use of halauxifen-methyl for weed management in preplant burndown applications will depend on market price, adoption of dicamba- and 2,4-D-tolerant crops, and the spectrum of weeds present at burndown