The introduction of glyphosate tolerant soybean varieties in 1996 resulted in lower input costs and simplicity of weed control for growers. Nearly all soybean acres in production today have glyphosate and/or other stacked trait tolerances. Since 2017 for example, Alabama soybean producers have seen a steady increase in the number of acres dedicated to dicamba technology, and future projections are still increasing. It took only a decade after this initial introduction before glyphosate resistance was detected in Palmer amaranth. Palmer amaranth currently has confirmed resistance to eight herbicide sites of action. Glufosinate, dicamba, and PPO-inhibitor herbicides are just a few of the latest herbicides to which pigweed species have confirmed resistance. Additionally, the pigweed species are strong competitors and prolific seed producers. A single plant can produce up to 500,000 seed in a single season, making it easy for resistance to spread. Collaborative efforts have been underway in southeastern states to determine the landscape distribution of resistant Amaranth species as well as the specific herbicide modes of action to which these species are resistant. No similar effort has been made in Alabama since 2015 for the benefit of soybean growers and long-term herbicide stewardship.
At least 40 statewide populations of pigweed species (Palmer, smooth, waterhemp, etc.) will be identified and geo-tagged in production fields throughout the 2023 growing season. Upon weed maturity in fall (October-December), female amaranth seedheads will be collected across the main crop producing regions of Alabama. Seedheads will be dried, threshed, and mature seed separated from the chaff before being inventoried for greenhouse evaluation.
Multiple foundation herbicides used in soybean production will be applied at standard rates both preemergence and postemergence to seed and seedlings. Qualitative and quantitative evaluations will be recorded up to 3 weeks after treatment to determine the level (if any) of herbicide resistance. Results will be analyzed and immediately used to inform statewide weed management recommendations through Extension efforts for the 2024 cropping season.

1. Collect at least 40 accessions of pigweed species across Alabama that indicate some instance of herbicide tolerance. 2. Develop methods of seed germination to improve the resistance screening process. 3. Conduct resistance screening to commonly-used soybean herbicides in greenhouse trials

Cases of suspected herbicide resistance will ultimately lead to modified weed management recommendations to be delivered through Extension programming to soybean growers across the state. Results from the greenhouse screening and genetic analysis will be presented at the 2025 WSSA and SWSS conferences. An Extension publication titled "Identification Guide of Mature Amaranthus Species" is under review and can be used to enhance field identification and enhance management programs

Updated June 13, 2024:

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Based on suspected herbicide resistance cases, modifications will be made to currently-approved weed management recommendations in order to improve their inputs and return on investment for increased yields. By providing a guide for identification of Amaranth species, growers and consultants alike can be more efficient in their scouting techniques for troublesome weed populations.