National Summit on Strategies to Manage Herbicide-Resistant Weeds: Proceedings of a Workshop (2012)

Herbicide Resistance in Weeds: What is the Nature of the Problem?

Micheal D. K. Owen
Iowa State University

Weeds represent the most important pest complex threat to global food security; they cause more loss of productivity and economic cost to humanity when compared to all other pest complexes combined. Furthermore, more agricultural acres are treated with herbicides to address the production issues caused by weeds than the acreage treated with all other pesticide classes combined. However, weeds have not been seen as a serious problem, given the general success of using herbicides to manage widespread weed problems. When genetically engineered herbicide resistance was introduced into many major crops, the concerns for weeds further declined, as weed control was perceived as simple and convenient. The historic perspective would suggest that weed control should not be considered as either simple or convenient given the ability of weeds to evolve resistance to all herbicides, including those for which genetically engineered traits were developed in crops. Currently there are 388 herbicide-resistant weed biotypes represented by 208 weed species.¹ These evolved resistances include resistance to all of the commercially available herbicide mechanisms of action. One or more of these resistances may be represented in a given species. Over the last decade, resistance in (currently) 23 weed species has evolved to glyphosate, and the production of corn, cotton, and soybean is threatened by the increasing numbers of glyphosate-resistant weeds in an increasing number of crop fields. It is important to recognize that the ever-increasingly important problem with herbicide-resistant weeds is not an herbicide problem. Furthermore, evolution of herbicide resistance in weeds is not a genetically engineered crop problem. The evolution of herbicide resistance in weeds represents a behavioral problem with the management and application of herbicides.

The changes in weed populations brought forward by the use of herbicides represent the selection of the fittest as described by Charles Darwin more than 100 years ago. The evolution of herbicide-resistant weed biotypes, however, illustrates Darwinian

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¹Numbers current as of May 2012.

evolution in fast forward. Given the predominance of using herbicides to control weeds, to the exclusion of other more diverse tactics and strategies, the production of crops has become simplified and places an incredible selection pressure on the weed populations that exist within the crop production systems. Essentially, agriculture is very quickly selecting for the pests that are best adapted to the tactics and strategies used to control them.

Specific weeds with evolved resistances to many herbicide mechanisms of action are becoming increasingly important. While there are several extremely important weed species that have evolved resistance to herbicides, common waterhemp (Amaranthus tuberculatus syn. rudis) is of particular note, given its widespread presence in the Midwest United States. Common waterhemp exhibits a number of characteristics that make it particularly “weedy”; these characteristics include but are not limited to a dioecious reproductive habit (obligate outcrossing), high seed productivity, opportunistic germination, and the demonstrable ability to evolve resistance to herbicides. Common waterhemp populations have evolved resistance to triazine herbicides, the ALS-inhibitor herbicides, the protoporphyrinogen oxidase (PPO)-inhibitor herbicides, the hydroxyphenylpyruvate dioxygenase (HPPD)-inhibitor herbicides, the growth-regulator herbicides, and glyphosate. There are specific populations of common waterhemp that have evolved multiple resistances to as many as five different herbicide mechanisms of action. Given that, to date, most of the evolved herbicide resistances are dominant traits; these evolved traits will spread quickly. The speed is enhanced by a failure of those involved in agriculture to adopt alternative strategies for the management of weeds.

In the opinion of the author, it is clear that, thus far, agriculture is not accepting the importance of herbicide resistance nor willing to react to the need for changes in weed-management tactics and strategies. There continue to be disconnects between the long-term perspectives of herbicide-resistant weeds and the short-term concerns about profitability for agricultural producers as well as within agricultural chemical companies. There are also important questions about whether herbicide resistance can and should be regulated. However, there should be no question about whether herbicide resistance should be managed. Evolved resistance to herbicides in important weed species will continue to be an increasing problem that changes at an increasing rate unless stewardship is implemented in all crop-production systems immediately.

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