Multiple resistant biotypes of Palmer amaranth occur throughout the United States and are becoming more common in the upper Midwest.

Herbicide resistance falls into six herbicide groups for this aggressive weed species (Shyam et al. 2021). The best course of action is to assume that Palmer amaranth is resistant to at least one class of herbicide and diversify your weed management strategies accordingly.     

If you have concerns regarding herbicide resistance in Palmer amaranth, please contact the University of Minnesota Extension Crop Production Specialist in your region or discuss with your crop consultant. You can also contact the Minnesota Department of Agriculture Noxious Weed Program. More information is available on the University of Minnesota Extension Herbicide Resistance Management Web Site.

Testing Laboratories

Two labs offer testing to determine if Palmer amaranth plants are glyphosate or PPO resistant: University of Illinois Plant Clinic and National Agricultural Genotyping Center. Contact either lab to learn more about the services they provide and how to properly collect and ship samples. A grow out would be required to test for all the different mechanisms of resistance.

Herbicide resistant weeds increase the cost of production and require a strategy for management.

Herbicide resistant weeds compete with crops and increase the cost of production because they require new strategies for management. Producers with Palmer amaranth infestations must diversify their weed management strategies and take an aggressive approach to reducing, and if possible, eliminating infestations over time.

  • More weeds per acre increases competition with crops for resources and significantly reduces yields – some estimates as high as 70 – 90%.
  • Selecting specific genetics and chemistry to control infestations may increase the cost of planting seed and herbicides for control.
  • Labor intensive activities need to be implemented including season-wide field scouting, hand pulling, hoeing, cleaning equipment before entering new fields.
  • New technologies like drones, spectral imaging, and artificial intelligence are being explored in a variety of applications, but will add cost to modern production practices.