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Herbicide Resistant Weeds

A couple of weeks ago, the UMN Institute of Ag Professional held a 2-day introductory crops workshop covering a wide variety of topics from diseases in corn and soybeans to entomology. The session that I found most interesting and perplexing was the session on herbicide resistant weeds from Jeff Gunsolus, Extension Agronomist & Weed Science with the Department of Agronomy and Plant Genetics.

Herbicide resistance in weeds is not a new problem. Records date back to 1968 that indicate weed resistance to herbicides in the triazine family. The magnitude of herbicide resistance has grown to a point where now there 11 main weeds that are resistant to several different herbicide site of action (SOA). SOA refers to the biochemical site within the plant which the herbicide directly interacts. These main weeds including common waterhemp, palmer amaranth, horseweed, giant ragweed, common ragweed, common lambsquarter, kochia, italian ryegrass, barnyard grass, johnsongrass, and giant foxtail. 

How does weed resistance to herbicides occur? Low levels of weeds that are resistant to herbicides are in the natural populations of the weeds. When a field is sprayed to remove the weeds, those resistant weeds remain, grow and produce seed. If the same herbicide continues to be used, the resistant weeds will reproduce resulting in a higher percentage of the weed populations being resistant.

Being proactive is the best way to avoid and properly manage herbicide resistant weed problems. By taking the necessary steps of expanding our knowledge of weed identification and SOA, we can be better prepared and even prevent serious issues with herbicide resistant weeds in the future. Below are a summary of several strategies to consider for avoiding and properly managing herbicide resistant weeds. A more complete article about herbicide resistant weeds can be found at
• Use herbicides only when necessary at appropriate levels included on the label
• Rotate herbicide with different SOA
• Use herbicide mixtures with different SOA
• Practice crop rotation
• Scout and control weed escapes and practice good sanitation of equipment to prevent the spread of resistant weeds

Taking action to avoid or properly manage herbicide resistant weeds is essential to prevent more serious resistant issues in the future. Please visit to utilize the SOA of herbicides look up feature and to learn more about herbicide resistant weeds. For more information on herbicide resistant weeds, contact the Steele County Extension office at 507-444-7685.


Karen Anderson
Extension Educator, Ag Production Systems
(507) 444-7685
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