Management of Glyphosate-Resistant Palmer amaranth in Corn and Soybean in New York
Palmer amaranth (a native to the southwestern United States) is a summer annual broadleaf weed that has recently invaded into the Northeastern region, including New York. According to recent Weed Science Society of America (WSSA) surveys, Palmer amaranth was ranked 1st among the 10 most troublesome weeds in grass & broadleaf crops, pasture, turf, fruits and vegetable crops in the United States. The evolution and widespread occurrence of herbicide resistance among Palmer amaranth populations makes it challenging for effective management.
Identifying characteristics
Palmer amaranth is often confused with other pigweed species. The most prominent characteristics to identify Palmer amaranth seedlings include hairless leaves with petiole (stalk that connects the leaf to the stem) longer than the leaf blade. In addition, the seedhead of female Palmer amaranth is spiny, prickly that can grow up to 2 to 3 feet tall. For more details on Palmer amaranth identification, read Cornell University palmer amaranth identification fact sheet.
Images above: Palmer amaranth seeds (left) and seedling (right).
Images above: Fully grown female plant in soybean (left) and infestation in alfalfa/corn field border in New York (right).
Palmer amaranth biology
Palmer amaranth is a dioecious weed species (separate male and female plants) with high outcrossing and hybridization potential. Palmer amaranth emerges best from the topsoil surface (0.5-inch depth) and exhibits an extended emergence period. Seedlings can grow 1 to 2 inches per day if the weather conditions are optimum (high temperature and sufficient soil moisture). Depending upon weather conditions and crop competition, matured Palmer amaranth plants can grow 1.5-6.5 ft tall with a single female plant capable of producing >0.5 million seeds. Most of the seeds produced are dormant and persist in the soil seedbank for 3 to 5 years. Palmer amaranth competition can reduce grain yields by 91% in corn, 79% in soybeans, up to 40% in grain sorghum, and 77% in dry beans.
Herbicide-resistant Palmer amaranth in the United States and New York
Palmer amaranth populations resistant to up to 10 different herbicide site of action (SOA), including inhibitors of acetolactate synthase (ALS, Group 2), microtubule assembly (Group 3), synthetic auxins (Group 4), photosystem II (Groups 5 and 6), EPSPS (Group 9), glutamine synthetase (Group 10), protoporphyrinogen oxidase (PPO, Group 14), very long-chain fatty acid (VLCFA, Group 15), 4-hydroxyphenylpyruvate dioxygenase (HPPD, Group 27) have been reported in the United States. Glyphosate-resistant Palmer amaranth populations (42 to 67-fold resistant) have recently been identified from soybean fields in Genesee, Orange, and Stueben Counties of New York1. The amplification of the target gene (25 to 135 more copies of the ESPSP gene) has been found to confer glyphosate resistance in Palmer amaranth populations1. During 2024 growing season, several additional Palmer amaranth populations were observed in corn, soybean, pumpkins, and alfalfa fields in Ontario and Yates Counties. In addition to glyphosate, resistance to ALS, PS II, and HPPD inhibitors have been suspected in these populations.
Management options
Sanitation and cleaning. Follow proper sanitation and cleaning of farm equipment after use in fields with Palmer amaranth infestation to prevent its further spread to new fields or areas.
Cultural practices. Rotation of annual field crops with competitive perennials, cover crops, and use of inversion tillage can help to reduce Palmer amaranth populations significantly. The use of best agronomic practices, including proper selection of competitive crop variety/hybrid, optimum seeding rates, row spacing, planting times, and proper fertility and irrigation management can increase corn and soybean competitiveness with Palmer amaranth.
Mechanical practices. Two pre-sowing blind cultivations in combination with use of tine weed or rotary hoes can help reduce Palmer amaranth populations. Manual collection and destruction of Palmer amaranth seedheads can reduce subsequent Palmer amaranth infestations.
Chemical options.
Soybean: Preemergence herbicides which are premix of 2 to 3 active ingredients such as chlorimuron, S-metolachlor, acetochlor, metribuzin, flumioxazin and postemergence herbicides such as glufosinate (Liberty link soybean), 2,4-D (E3 soybean), lactofen, and fomesafen can effectively control glyphosate-resistant Palmer amaranth.
Corn: Preemergence herbicides which are premix of 2 to 3 active ingredients such as atrazine, S-metolachlor, acetochlor, mesotrione, and postemergence herbicides such as glufosinate (Liberty Link corn), 2,4-D (Enlist corn), dicamba, tembotrione, tolpyralate, etc. can provide effective control of glyphosate-resistant Palmer amaranth.
Soil residual herbicides (acetochlor, S-metolachlor) applied postemergence in corn and soybean can provide extended overlapping residual activity for season-long control of Palmer amaranth
Citations:
1Butler-Jones AL, Maloney EC, McClements M, et al. (2024) Confirmation of glyphosate-resistant Palmer amaranth (Amaranthus palmeri) populations in New York and responses to alternative chemistries. Weed Science 72(5):508-516
More information:
- Visit the weed management sections for corn and soybeans in the 2025 Cornell Guide for Integrated Field Crop Management
- For more information, please contact: Vipan Kumar-vk364 [at] cornell.edu (vk364[at]cornell[dot]edu)