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Agronomy Day 2007

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Glyphosate-Resistant Weeds in Illinois: How to Improve Your Chances

tour c
Aaron Hager Aaron Hager
Extension Weed Science
Department of Crop Sciences
Dawn Nordby Dawn Nordby
Extension Weed Science
Department of Crop Sciences

We’ve all been told that change in inevitable. Some of us who are engaged in public service professions are further advised that: a) diversity is always good, and b) we should embrace change as an avenue toward new understanding. While these clichés are favorites of some administrators and daytime talk show hosts, they also have (some) relevancy to weed management in agronomic crops. For instance if we rely heavily on a limited number of tools to control weeds in row crops, the weeds ultimately will adapt and change in some manner to better enhance the probability for long-term survival of the species. These adaptations and changes come about because of the genetic diversity inherent with most weed species. While the changes that result from intense selection are sometimes difficult to manage, they often lead us to improved understanding of weeds and how to better manage them.

fig 1
Many waterhemp survived exposure to 3 lb glyphosate.

Weed resistance to herbicides is a phenomena that highlights the genetic diversity of plants species we consider weeds. Repeated selection, brought about by relying on a limited number of herbicide sites-of-action, causes a “shift” in the population of a particular species by effectively eliminating all susceptible members and leaving only those that can survive in the presence of the herbicide. Herbicide-resistant weeds have plagued Illinois farmers and weed control practitioners for decades, and there does not appear to be an end to the problem anywhere on the horizon.

fig 2
Waterhemp recovery 14 days after treatment with 3 lb glyphosate.

Across the United States, glyphosate-resistant weeds have increased both in acreage infested and number of weed species with resistant populations. Two species recently labeled with the moniker glyphosate-resistant are waterhemp and giant ragweed, initially identified in Missouri and Ohio, respectively. While Illinois farmers have dealt with glyphosate-resistant populations of horseweed (a.k.a. marestail) for several years, these other glyphosate-resistant summer annual weeds have been confined outside the state’s borders. However, as the well-worn clichés reminds us, change is inevitable.

fig 3
This waterhemp plant was photographed 7 days after treatment with 3 lb glyphosate.

Weed scientists at the University of Illinois recently have worked with an Illinois waterhemp population that has demonstrated greatly reduced sensitivity to glyphosate. Greenhouse experiments have revealed that many plants survived exposure to 3 lb per acre of glyphosate, equivalent to approximately 86 fluid ounces of a 4.5 lb per gallon glyphosate formulation. Subsequent field research generally has confirmed greenhouse results in that many (but not all) waterhemp plants survived exposure to glyphosate, some plants surviving up to 6 lb ae glyphosate. Additionally, field research has suggested this population also is resistant to ALS-inhibiting herbicides; results have been less clear with respect to the population’s response to foliar-applied triazine and PPO-inhibiting herbicides.

Utilizing herbicides with other sites of action can reduce the selection intensity for glyphosate-resistant waterhemp. Our field research has demonstrated excellent control of this particular waterhemp population with various soil-residual herbicides, including sulfentrazone and flumioxazin.