Evaluation of thiencarbazone‐methyl for use in acetolactate synthase‐resistant soybean

To combat herbicide‐resistant weeds, new options are needed to effectively rotate herbicide modes of action and slow the development of additional herbicide resistance. Thiencarbazone‐methyl (TCM), an acetolactate synthase (ALS)‐inhibiting herbicide, is currently being evaluated for preemergence (PRE) and postemergence (POST) activity on many troublesome summer annual and perennial weeds. Multiple experiments were conducted to assess the utility of this product in soybean (Glycine max) production systems. Field experiments were conducted across Arkansas in 2016 and 2017 to determine the tolerance of ALS‐resistant soybean varieties to PRE and POST applications of TCM at either 0.03 or 0.06 lb acre–¹. For all locations, no significant interactions or main effects were observed for any parameters evaluated. Overall, soybean injury remained consistently low, with <3% injury at 21 days after treatment (DAT) on all ALS‐resistant varieties. However, the ALS‐susceptible variety showed high levels of injury for both PRE (75%) and POST (98%) applications at 21 DAT. Two additional experiments were conducted in 2016, 2017, and 2018 to evaluate the efficacy of PRE and POST applications of TCM in ALS/glufosinate‐resistant soybean. For PRE applications, residual activity of TCM, imazethapyr, chlorimuron, nicosulfuron, halosulfuron, and flumetsulam at labeled rates for soybean or corn were evaluated. TCM at 0.06 lb acre–¹ was effective in controlling ivyleaf morningglory (91 %) and broadleaf signalgrass (97%) at 21 DAT, but was not effective in controlling ALS‐resistant Palmer amaranth (58%). In the POST experiment, TCM at 0.03 or 0.06 lb acre–¹ was compared to imazethapyr, chlorimuron, nicosulfuron, and halosulfuron with and without glufosinate (0.5 lb acre–¹). Alone, TCM produced higher control of ivyleaf morningglory (99 to 100%) and broadleaf signalgrass (94 to 99%) at 21 DAT compared to all other ALS‐inhibiting herbicides. However, TCM alone was not effective in controlling ALS‐resistant Palmer amaranth (55 to 61%) at 21 DAT, but when combined with glufosinate control increased to 95 to 99%. The results from this research suggest that TCM could have utility in soybean weed control systems due to the high level of resistance exhibited by ALS‐resistant soybean varieties and the efficacy of TCM on common soybean weed species at both the PRE and POST application timings.