Response of Difficult-to-Control Palmer Amaranth Accessions to Ten Herbicide Groups.

Palmer amaranth [Amaranthus palmeri (S.) Wats.] is one of the most troublesome soybean weeds [Glycine max (L). Merr.]. Chemical control options are limited with the evolution of resistance to 9 herbicide sites of action (SOAs) in Palmer amaranth. In 2021, a greenhouse experiment was conducted at the University of Arkansas System Division of Agriculture's Milo J. Shult Agricultural Research and Extension Center in Fayetteville, Ark., to evaluate the response of Arkansas Palmer amaranth populations to commonly used row crop herbicides with differing SOAs. Two experimental runs were conducted as a completely randomized design with 3 spatial replications. Three difficult-to-control accessions (A2019, A2020, and B2020) along with 1 standard susceptible accession (SS2001) were treated with 10 different herbicide SOAs, including pendimethalin and S-metolachlor as a preemergence application and imazethapyr, 2,4-D, dicamba, atrazine, diuron, glyphosate, glufosinate, fomesafen, paraquat, mesotrione, and tembotrione as a postemergence application. All difficult-to-control accessions were observed to have at least 20 percentage points less mortality than a susceptible standard to 5 herbicide SOAs. Mortality of A2019 accession was at least 20 percentage points less than the susceptible standard to herbicides from 9 differing SOAs. Additionally, A2020 and B2020 showed reduced sensitivity to 5 herbicide SOAs. Furthermore, atrazine and paraquat provided >86% mortality and are still viable options for controlling challenging Palmer amaranth populations. [ABSTRACT FROM AUTHOR]