No. 4 Article 6/April 17, 2009

Residuals and Resistance: On the Minds of Weed Management Practitioners

Extension weed scientists across much of the United States have expended considerable time and effort exhorting the benefits and advantages of integrating multiple weed management tactics, including the use of soil-residual herbicides, into glyphosate-resistant corn and soybean production systems. Two advantages of using soil-residual herbicides are reducing early-season weed interference that can lead to loss of crop yield potential and reducing the intensity of selection for herbicide-resistant weed populations. Other benefits, including enhanced control of volunteer glyphosate-resistant corn in soybean and improved efficacy against weed species less sensitive to glyphosate, can be realized by integrating soil-residual herbicides and/or tank-mix partners in glyphosate-resistant cropping systems.

Recent evidence suggests that practitioners are in fact moving toward more integrated weed management in glyphosate-resistant soybean. During the 2009 University of Illinois Corn & Soybean Classics, attendees (about 1,150 across six meetings) were asked questions on a variety of agronomic topics. Two weed science-related questions were posed, with data gathered using the Turning Point anonymous response system. The first question was "If you grow glyphosate-resistant soybean, which of the following scenarios best describes the number of different herbicides you use to control weeds?" Four choices for an answer were offered:

a) Glyphosate only
b) A tank-mix partner with glyphosate
c) A soil residual followed by glyphosate
d) A soil residual followed by glyphosate and a tank-mix partner

Across all locations, 717 responses were collected. Beforehand, we (admittedly) supposed "glyphosate only" would be the predominant answer, but we were (pleasantly) surprised when it received only 28% of responses (199 out of 717). While we have no comparable "historical" data against which to compare, national-level surveys conducted after the first year that glyphosate-resistant soybean was commercially available indicated 8 out of 10 farmers did not use soil-residual herbicides in their weed management programs in glyphosate-resistant soybean.

The issue of glyphosate resistance in weeds also appears to be on the minds of weed management practitioners. The second question we asked at the Classics was "Do you believe glyphosate-resistant weeds will change the way you manage weeds in glyphosate-resistant cropping systems within the next 5 years?" Three choices for an answer were offered:

a) Yes
b) No
c) Don't know

Across all locations, 877 responses were collected. An overwhelming 91% of respondents said they believe glyphosate-resistant weeds will change the way they manage weeds in glyphosate-resistant cropping systems within the next 5 years.

We hope these data are indicative that the fallacy of glyphosate's alone being able to resolve all weed problems in corn and soybean is becoming increasingly obvious. The dynamic and adaptable nature of weeds has (again) demonstrated how difficult it can be to adequately manage weeds long-term with a singular approach. Weed scientists at the University of Illinois are collaborating with colleagues in the USDA/ARS Invasive Weed Management Unit and at Western Illinois University and Southern Illinois University to find solutions to the new challenges presented by these adaptable pests.--Aaron Hager

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