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Issue No. 15, Article 2/July 6, 2007

Identification of an Illinois Waterhemp Population Not Controlled with Glyphosate

We've all heard (repeatedly) that change is inevitable. Some of us who are engaged in public service professions are further advised that (a) diversity is 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 within most weed species. While the changes that result from intense selection are sometimes difficult to manage, they often lead us (in the long term) to improved understanding of weeds and how to better manage them.

Evolved resistance to herbicides is a phenomenon that highlights the genetic diversity of plant 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.

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 cliché reminds us, change is inevitable.

Weed scientists at the University of Illinois recently have worked with an Illinois waterhemp population that has demonstrated greatly reduced sensitivity to glyphosate. It is altogether likely that we will label this population "glyphosate-resistant" pending completion of additional research. Greenhouse experiments have revealed that many plants survived treatment with 3 lb per acre of glyphosate, equivalent to about 85 fluid ounces of a 4.5 lb per gallon glyphosate formulation. Subsequent field research has confirmed greenhouse results, indicating many (but not all) waterhemp plants survived treatment with glyphosate at rates exceeding the maximum labeled in-crop application rate. Indeed, a few plants survived treatment with 12 lb ae glyphosate. Additionally, our field research has suggested this population is also 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.

As indicated in a previous issue of this Bulletin, (Issue 12, "Postemergence Herbicide Applications in Soybean: Pull the Trigger or Wait for Rain?"), we have been notified of several other instances from across Illinois in which waterhemp was not controlled by an initial application of glyphosate. It is altogether possible that other Illinois waterhemp populations are demonstrating a response to glyphosate similar to the population we have been working with during the past several months. If these populations are to be retreated, the following suggestions and observations are offered for your consideration.

(1) There are only four herbicide active ingredients that control waterhemp postemergence in soybean: glyphosate, acifluorfen, lactofen, and fomesafen. Our previous research has indicated the vast majority of Illinois waterhemp populations demonstrate resistance to ALS-inhibiting herbicides, thus diminishing the effectiveness of these products. We are unaware of any new herbicide active ingredients for postemergence use in soybean that are being developed by the herbicide manufacturing industry.

(2) If glyphosate will be part of the respray, you may want to increase the application rate to the maximum allowed for a single in-crop application. Why? If the waterhemp survived the initial glyphosate application for reasons other than resistance to glyphosate, a higher application rate will be needed to control the (now) larger plants.

(3) What about tank-mixing other products with glyphosate? The choices are limited, and the results are very difficult to predict. In our field research this season, we did include tank mixes of glyphosate with acifluorfen, lactofen, and fomesafen and were able to increase waterhemp control over glyphosate alone. However, these treatments were applied to waterhemp plants between 1 and 6 inches tall, and we anticipate reduced control when plants exceed 6 inches in height. Also, we know some Illinois waterhemp populations are resistant to PPO-inhibiting herbicides. Tank-mixing a low rate of 2,4-DB with glyphosate has generally proven ineffective against large waterhemp.

Unfortunately, there is no easy or simple remedy to this problem. We have a great deal of work to do before we better understand why this waterhemp population is not controlled with glyphosate. We will keep you updated as we learn more.--Aaron Hager

Aaron Hager

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