New Resistance Nightmares

Glyphosate-Resistant Weeds in the U.S. (Progressive Farmer image by Dr. Kevin Bradley, University of Missouri)

Few issues have rocked crop production quite like weeds that resist glyphosate.

The first case of glyphosate resistance hearkens back to 1998 in a population of rigid ryegrass in California. Horseweed (marestail) soon bolted out the gate and, as maps from University of Missouri weed scientist Kevin Bradley show, glyphosate resistance had spread to 17 different weed species in 39 states by 2019.

The latest nightmare is many weeds now possess the ability to withstand herbicides from multiple chemical families. In Illinois, some populations of common waterhemp have exhibited resistance to herbicides from seven different site-of-action groups.

University of Illinois weed scientist Aaron Hager says most farmers equate resistance with foliar-applied herbicides. However, populations of waterhemp recently defeated certain soil-applied Group 15 products.

New research shows weeds are also genetically altering the way they resist herbicides by employing both target-site and metabolic tactics. Fighting back will require farmers to use multiple approaches beyond rotating herbicides. The key will be keeping weeds from ever setting seed, Hager says.

Seize control of herbicide resistance and find a site-of-action herbicide chart at

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