Survey of Protoporphyrinogen Oxidase and Glutamine Synthetase Resistance In Alabama Amaranthus Populations

Abstract

Amaranthus species, particularly A. palmeri and A. tuberculatus, are among the most common and troublesome weeds nationally. In Alabama, statewide screenings have not been conducted since 2016, but producers have still reported reduced herbicide efficacy. This research aimed to conduct a statewide screening of Amaranthus spp. and document new resistance evolution in glufosinate and saflufenacil. Collected populations displayed reduced germination vigor in the greenhouse, and a germination study was needed to produce viable seedlings for screenings. The most effective treatment for germination was a 96-hour wet cold stratification, either with a seed soak or without. Using the optimized germination method a statewide screening identified reduced control to glyphosate, saflufenacil, and glufosinate. Saflufenacil dose responses confirmand resistance in three Amaranthus populations: one A. tuberculatus population carrying the ΔGly-210 PPX2L mutation and two A. palmeri populations lacking known target-site mutations, suggesting non-target-site resistance. Glufosinate resistance was also detected in one A. tuberculatus population; likely resistance is caused by non-target site mechanisms. Field studies in 2024 and 2025 in a producer field in Moulton, Alabama, and the Tennessee Valley Research and Extension Center evaluated cereal rye and oats in combination with at-plant herbicide programs. In 2024, rye and oats reduced Amaranthus density by up to 93% compared to fallow, coinciding with higher biomass production. However, severe winter conditions in 2025 reduced biomass and limited weed suppression. Together, these studies provide updated weed management strategies for Alabama producers regarding herbicide resistance in Amaranthus species and the confirmation of new cases of herbicide resistance.