Palmer amaranth, or Palmer pigweed, is a dioecious broadleaf annual weed species that has created significant problems for growers in the Mid-South over the past 10 years due to its resistance to glyphosate and its prolific seed production. A single female plant can produce one million seeds, depending on growing condition and competition.
It continues to cause significant problems in the Mid-South and has rapidly spread throughout the Midwest. As of 2015, glyphosate-resistant Palmer amaranth has been confirmed in Georgia, North Carolina, Arkansas, Tennessee, South Carolina, New Mexico, Mississippi, Alabama, Missouri, Louisiana, Illinois, Ohio, Kentucky, Michigan, Virginia, Kansas, Texas, Arizona, Delaware, Indiana, Pennsylvania, Florida, Maryland, New Jersey, and Wisconsin.
Palmer amaranth is highly competitive in row crops because of its wide germination window and aggressive growth under favorable weather conditions. In addition to being an obligate outcrossing species, Palmer amaranth has been shown to rapidly evolve resistance to numerous herbicide modes of action (ALS-inhibitors, EPSP synthase inhibitors, Photosystem II-inhibitors, HPPD-inhibitors and Microtubule-inhibitors). When left unmanaged, Palmer amaranth can cause significant yield losses.
In Nebraska, HPPD-inhibitor resistance has already been confirmed in Palmer amaranth. This problem was associated primarily with seed corn production in south-central Nebraska. It is possible that this has become wider spread since the initial report. Applicators and growers should use these products judiciously if they suspect resistance.
In greenhouse studies conducted last winter, we were able to screen 82 Palmer amaranth populations from southwest Nebraska to evaluate glyphosate resistance. Dose response analysis was performed to estimate the I90 (effective dose required to cause 90% injury to the population) values for each Palmer amaranth population.
The populations, when screened in the greenhouse, showed low levels of resistance in many of the populations that were arbitrarily collected, and a few populations showed high levels of resistance (Figure 1). While the level of resistance may not directly translate to the field, the trends in the greenhouse provide a clear message that glyphosate alone is neither a long-term solution for management of this species nor a recommended practice.
Continuing to rely on glyphosate alone for weed management will speed up the evolution of glyphosate-resistant weeds and diminish the effectiveness of glyphosate-based crops and weed control programs. Control of glyphosate-resistant Palmer amaranth will require an integrated approach, including:
• Practices of zero-tolerance, or mitigating seed banks from being replenished at the end of the year;
• Use of soil residual herbicides with different modes-of-action followed by labeled post-emergence herbicides other than glyphosate as needed throughout the growing season.
• Growers also should consider crop rotation when possible to help diversify the weed management tools being used to combat Palmer amaranth.