Palmer Amaranth’s Expanding Resistance Threat Demands a Broader Weed Management Approach
A recent article featured on GROW (Getting Rid of Weeds) highlights a troubling trend for growers and crop advisors across the country: Palmer amaranth continues to spread into new regions while evolving resistance to key herbicide tools. As one of the most aggressive and economically damaging weeds in U.S. agriculture, Palmer amaranth is presenting new challenges for farmers from the Pacific Northwest to the Northeast.
Once primarily confined to the Southwest, Palmer amaranth has steadily expanded across much of the United States through contaminated seed, equipment movement, livestock feed, birdseed, and other pathways. The weed’s biology makes it particularly difficult to manage. It can grow up to three inches per day, produce as many as 500,000 seeds per female plant, and reduce crop yields by up to 95% when left uncontrolled. Compounding the problem, Palmer amaranth populations have developed resistance to multiple herbicide modes of action, making traditional management programs less effective.
GROW highlights emerging concerns in the Pacific Northwest, where researchers are documenting widespread glyphosate-resistant Palmer amaranth populations. According to University of Idaho weed scientist Dr. Albert Adjesiwor, more than 70% of Idaho’s documented Palmer amaranth populations have shown resistance to glyphosate. This poses a significant challenge for growers relying on glyphosate-based systems in crops such as corn, sugarbeets, and alfalfa.
Fortunately, growers in the region are responding proactively. Many are incorporating residual herbicides, diversifying crop rotations, planting cover crops, using weed electrocution technologies, and even employing hand-weeding crews to prevent seed production and further spread. Adjesiwor notes that lessons learned from Southern states that have battled Palmer amaranth for years are helping Western farmers avoid repeating the same mistakes.
Meanwhile, researchers in the Northeast are confronting a different resistance challenge. Recent studies identified Palmer amaranth populations in New York with high levels of resistance to atrazine. Scientists determined that these populations possess non-target-site resistance mechanisms, sometimes referred to as metabolic resistance. This form of resistance can be particularly concerning because it allows weeds to detoxify herbicides and may reduce the effectiveness of multiple herbicide classes, limiting the benefits of simple herbicide rotation strategies.
Dr. Thierry Besançon of Rutgers University emphasizes that herbicides alone will not provide a long-term solution. Instead, growers should integrate multiple tactics, including residual herbicides, cover crops, crop rotation, sanitation practices, and early detection efforts. Cleaning equipment before moving between fields is especially important, as even a single contaminated combine or implement can introduce Palmer amaranth into previously clean areas.
For ag retailers, the article reinforces the growing value of integrated weed management programs. As herbicide resistance becomes increasingly complex, retailers have an opportunity to help growers build diversified strategies that combine chemistry with cultural, mechanical, and preventative practices. Early scouting, accurate weed identification, residual herbicide planning, cover crop recommendations, and stewardship guidance will all play increasing roles in successful Palmer amaranth management.
Learn More at GROW
Herbicide-resistant Palmer amaranth is no longer a regional problem. To learn more about the latest resistance developments and integrated management recommendations from researchers across the country, read the full article on GROW (Getting Rid of Weeds).
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