Comparison of Weed Control Technologies for Protoporphyrinogen Oxidase‐resistant Palmer Amaranth

Confirmation of protoporphyrinogen oxidase (PPO)-resistant Palmer amaranth (Amaranthus palmeri S.Watson) in 2015 limited weed control options for soybean [Glycine max (L.) Merr.] producers. With already prevalent 5-enolpyruvyl-shikimate-3-phosphate synthase-, acetolactate synthase-, beta-tubulin-, and very-long-chain fatty acid-resistant Palmer amaranth, there are few remaining effective pre-emergence and postemergence herbicides labeled in soybean. The recent commercialization of soybean varieties resistant to dicamba and 2,4-D give producers effective options to control multiresistant Palmer amaranth. To evaluate new and existing technologies and postemergence herbicide options, two experiments were conducted in 2017. These experiments were conducted in Marion and Crawfordsville, AR, two locations with confirmed PPO-, 5-enolpyruvyl-shikimate-3-phosphate synthase-, acetolactate synthase-, betatubulin-, and very-long-chain fatty acid-resistant Palmer amaranth. Each technology was tested over three pre-emergence programs with distinct levels of control (74, 88, and 95%). At 28 d after pre-emergence, the first postemergence applications were made when Palmer amaranth was approximately 4 in in height. Fourteen days later, visible weed control ratings showed that glufosinate-, 2,4-D-, and dicamba-resistant programs resulted in at least 95% control of Palmer amaranth across all pre-emergence treatments, better than the 83% control provided by the glyphosate-resistant soybean program. Visible control ratings taken 14 d after the second postemergence treatment showed separation between the glufosinate- (97%), 2,4-D- (97%), and the dicamba-resistant (96%) programs from the glyphosate-resistant program (59%). The results indicate that effective preemergence programs followed by postemergence programs containing glufosinate, 2,4-D, or dicamba will provide extended control of PPO-resistant Palmer amaranth but this may not be feasible in a glyphosate-resistant soybean system.