Managing resistance in Palmer amaranth

Pigweed species are spreading 

Pigweeds, like Palmer amaranth, have been growing in western Nebraska for many years and most growers are familiar with redroot pigweed. In recent years, redroot pigweed has not been as prevalent as in the past due to an increase in common lambsquarters, which emerges earlier in the spring and has more tolerance to glyphosate.

Palmer amaranth is native to the lower Rio Grande Valley of Texas where cotton is an important crop. It was commonly found in the southern United States, and in recent years had expanded its range into Kansas.

Temperature is the leading ecological factor determining which pigweed species will dominate. Palmer amaranth responds negatively to the low temperatures typical in the northern United States. Its ideal temperature range is between 85° F and 95° F and plant growth declines dramatically between 50°F and 60°F. Growth rate, biomass, and seed production of Palmer amaranth are greater than redroot pigweed at temperatures between 65° F to 95° F.

Palmer amaranth seed is moved via farm equipment, especially combines, or as a contaminant in crop seed or livestock feed. An interesting case in point occurred in 2011 when cottonseed cake used as a protein source for livestock was shipped north. Cotton fields infested with Palmer amaranth were harvested and as seed was removed from cotton fibers, Palmer amaranth seed was mixed in the seed cake. Cattle ate the protein and deposited the Palmer amaranth seed in an environment favorable for expansion.

Palmer amaranth has expanded its range into western and northern Nebraska for several reasons:

• Higher temperatures,
• Reduced use of herbicides at planting,
• Its ability to develop resistance to herbicides, and
• A decrease in preplant tillage has provided an environment favorable to the weed.

Palmer amaranth growing on a farm may already be resistant to some herbicide families. Triazine-resistant Palmer amaranth has already been identified in Nebraska and UNL researchers are testing for additional resistance development. As is always recommended, avoid relying on a single herbicide mode of action for weed control.

• Use production practices that do not spread the weed.
• Rotate herbicide modes of action to reduce the potential for resistance development.

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