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Issue No. 3, Article 3/April 11, 2008

Managing the Consequences of Long-Term Weed Control

Are the outcomes of weed control practices always identical to the outcomes of weed management programs? The following article was written for the proceedings of the 2008 University of Illinois Crop Protection Technology Conference. It provides many more philosophical considerations than empirical data, but we hope it might invoke some dialogue and discussion. You might note some themes and passages similar to those in "Glyphosate-Resistant Waterhemp in Illinois: Recommendations for Management" (issue 1 of the Bulletin, March 21, 2008).

Plant species considered to be weeds have caused myriad maladies to befall human society since the beginning of recorded time, including hunger caused by losses in crop productivity and yield, dramatic reductions in the aesthetic value of countless landscapes, significant and sometimes permanent loss of ecological diversity, physical ailments of humans and livestock alike, and untold expenditures of financial resources aimed toward their control. Yet despite the best of human efforts to keep these undesirable plants in check, weeds continue to plague multiple aspects of daily life.

Those who are considered weed management practitioners of agronomic cropping systems know all too well how difficult it can be to remove weeds from the cropping landscape. History is replete with examples of how weeds have evolved to evade many of the tools designed for their management. Papers in previous proceedings of the Crop Protection Technology Conference have illustrated some of these adaptations, including changes in the emergence characteristics of giant ragweed (once considered an early-emerging species) in response to long-term crop production practices, increased occurrence of weed species not previously well characterized (such as hophornbeam copperleaf), and selection of herbicide-resistant biotypes (such as waterhemp with resistance to three herbicide modes of action).

Some of these adaptations could perhaps be described as analogous to corn yield potential: many plant genes and environmental factors contribute to the observed response. Other adaptations in weeds are the result of the intense human selection for traits that ensure survival of the species in the "artificial" environment of agronomic cropping systems. For example, waterhemp was once very sensitive to many ALS-inhibiting herbicides but now demonstrates high-level resistance via an altered herbicide target site selected by repeated use of these herbicides. Some weeds have even adapted to the age-old practice of hand weeding; intense weeding of flooded rice fields has selected for a barnyardgrass biotype that closely mimics rice in appearance and thus escapes being hand-eliminated from the crop.

These examples illustrate how weed species have adapted to changes in production practices. In some cases weeds adapt in response to a single selection factor. Other times the adaptation results from multiple changes in production practices. Whether single or multiple factors are involved, it is important to remember that weeds will continue to adapt and challenge us.

The introduction and commercialization of glyphosate-resistant soybean varieties and corn hybrids has in many ways dramatically altered the weed management practices of farmers across much of the Midwest. Estimates place the adoption of herbicide-resistant soybean varieties and corn hybrids (principally glyphosate-resistant) at approximately 90% and 37%, respectively, of U.S. soybean and corn acreage, according to the USDA National Agricultural Statistics Service report of June 2006. Glyphosate-resistant crops offer many advantages to farmers, but as the cited examples illustrate, overreliance on a single management option can lead to new weed management challenges. Until 2007, glyphosate-resistant weeds were the least represented herbicide-resistant biotypes among all major herbicide mode-of-action families. In 2007, however, the frequency of glyphosate-resistant weed biotypes surpassed that of dinitroaniline-resistant weed biotypes.

A "philosophical" consideration sometimes discussed among academic weed scientists is the difference, real or perceived, between what is accomplished through weed management compared with weed control. The weed science community has no "Webster's of Weed Science Terminology" from which to seek definitive answers, so debate often becomes spirited, and conjectures among pontificators abound. The great "author" Merriam Webster offers several possible ways to define control and manage. Careful scrutiny and (biased) selection of possible definitions provide the following:

  • control: to reduce the incidence or severity of, especially to innocuous levels
  • manage: to handle or direct with a degree of skill

The weed spectrum in many Illinois soybean fields is such that a single control strategy (e.g., a single postemergence herbicide application) may not always provide consistent control. Over the past decade, many practitioners have become very proficient at controlling weeds but perhaps less proficient at managing them. Potentially serious repercussions are poised to plague Illinois soybean farmers in 2008 due to the widespread adoption of weed control in lieu of weed management. A specific consequence of widespread weed control is the selection of Illinois waterhemp biotypes resistant to glyphosate. A pertinent question to consider is this: How will Illinois farmers manage a waterhemp population that may no longer be susceptible to glyphosate or diphenylether herbicides, the only postemergence soybean herbicide options that control waterhemp?--Aaron Hager

Aaron Hager

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