During the last decade, opinions about the potential for weeds to evolve resistance to glyphosate have changed dramatically. The prevailing industry perspective in the early 1990’s was that due to the complexity of the glyphosate mechanism of action and other purported reasons, weeds would never evolve resistance to glyphosate.

Many university weed scientists, including the Iowa State University Extension Weed Management Specialists, disagreed with that position and have attempted to inform Iowa growers objectively about the risks of herbicide resistant weeds in general, and glyphosate resistant weeds, specifically. Glyphosate resistant horseweed (marestail) is widely distributed across the eastern United States. Recent press announcements have heralded the discovery of glyphosate resistance in common ragweed, Palmer amaranth, and common waterhemp. Recently, the agchem industry has done an excellent job of recognizing the risks of glyphosate resistant weeds and has initiated stewardship programs to preserve the utility of glyphosate. However, there does not appear to be a great following by growers to proactively adopt tactics to delay the evolution of glyphosate resistant weed populations. So why are growers not feverishly acting to thwart the evolution of glyphosate resistant weeds?

Historically, weeds have evolved resistance to a number of herbicide families. Triazine resistance occurred globally despite predictions of this impending problem. Similarly, ALS herbicide adoption by growers in the 1980’s caused the evolution of ALS resistant weeds in the 1990’s. In each case, another “silver bullet” to resolve the newest chink in the weed management armor was anticipated by growers . The current issue with evolved resistance to glyphosate will not likely be resolved by the same solution of past problems. A recent survey conducted by Dr. Bill Johnson, Purdue University illustrates the problem nicely. Growers expect a new “silver bullet”. The article describing the survey results can be found at and in it, a number of disturbing grower practices are reported. I suspect that Iowa growers have similar attitudes and thus will encounter similar problems with regard to the evolution of glyphosate resistance.

Prepared by Micheal D. K. Owen, extension weed management specialist, Department of Agronomy, Iowa State University

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