No. 1 Article 5/March 21, 2008

Glyphosate-Resistant Waterhemp in Illinois: Recommendations for Management

Illinois farmers have another adversary to consider this year in their annual battle against weeds. Glyphosate-resistant waterhemp is poised to bring new challenges (i.e., problems) to agronomic cropping systems in Illinois, and it might be especially troubling for soybean producers. Weed scientists at the University of Illinois have worked together to develop specific recommendations to help farmers better manage this problem in the 2008 soybean crop. These recommendations were developed using data on waterhemp biology, ecology, and control generated by scientists over the past 15 years. We first presented the recommendations in the 2008 University of Illinois Corn & Soybean Classics; the following information is modified from the resulting proceedings.

Over the past decade, many practitioners have become very proficient at controlling waterhemp but perhaps less proficient at managing it. 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.

Glyphosate-resistant waterhemp seedlings in an Illinois soybean field, 2007.

A pertinent question to consider is this: How will Illinois soybean farmers manage a waterhemp population no longer susceptible to glyphosate or diphenylether herbicides, the only postemergence soybean herbicide options for waterhemp control?

Why should we pose such a question when we're only on the cusp of the new growing season? University of Illinois weed scientists have conducted field, greenhouse, and laboratory research with an Illinois waterhemp population that is resistant to glyphosate. It is altogether likely that other populations of glyphosate-resistant waterhemp exist across the state.

If the herbicide-resistance profile of a particular waterhemp population is known, appropriate changes in herbicide selection and utilization (particularly postemergence soybean herbicides) can be made well before the beginning of the growing season. However, apart from the general assumption that most Illinois waterhemp populations are resistant to ALS-inhibiting herbicides, the vast majority of populations remain uncharacterized with respect to their susceptibility to the limited number of postemergence soybean herbicides that control waterhemp. We speculate that glyphosate-resistant and PPO-resistant waterhemp biotypes might be encountered across large geographical areas of central and south central Illinois during the 2008 growing season. Additionally, it is altogether possible that waterhemp biotypes resistant to both glyphosate and PPO inhibitors soon will be discovered. These biotypes represent a worst-case scenario, in that there are no postemergence herbicide options for their control in soybean.

The University of Illinois offers the following recommendations for managing glyphosate-resistant waterhemp in the 2008 soybean crop. The recommendations assume glyphosate-resistant soybean are planted. The considerations and justifications employed to develop the recommendations are based on current and previous research on waterhemp biology and management and are discussed after each recommendation. The recommendations are specific to herbicides, but weed management practitioners should strongly consider the benefits of practices that increase the competitive ability of the soybean crop (such as practices that hasten crop establishment and canopy development).

Recommendation 1. Apply a full rate (according to label guidelines for soil type and organic matter content) of a soil-residual herbicide no sooner than 7 days before planting or no later than 3 days after planting.



Recommendation 2. The initial postemergence application of glyphosate (alone at 0.75 to 1.0 pound acid equivalent per acre) must be made when waterhemp is 3 to 5 inches tall.


Recommendation 3. Fields must be scouted 7 days after the initial glyphosate application to determine treatment effectiveness.


Recommendation 4. If waterhemp control is inadequate and retreatment is necessary, consider applying a PPO-inhibiting herbicide (lactofen, fomesafen, or acifluorfen) at a full labeled rate (with recommended additives) as soon as possible.


Recommendation 5: Rescout the treated field within 10 to 14 days to determine effectiveness of the PPO-inhibiting herbicide treatment. If scouting reveals that plants treated with a second herbicide application might survive, implement whatever tactics are available or feasible to rogue these surviving plants from the field before they reach a reproductive growth stage.


These recommendations illustrate the need for an integrated approach to waterhemp management. Integrated weed management introduces multiple tactics to control weeds and to slow the rate at which weeds can adapt to a single control tactic. Introducing an integrated approach to waterhemp management into glyphosate-resistant cropping systems may well stave off some potential new challenges, enhancing the long-term effectiveness of this valuable weed control technology.--Aaron Hager


Hager, Aaron G., Wax, Loyd M., Stoller, Edward W., and Bollero, Germán A. 2002a. Common waterhemp (Amaranthus rudis) interference in soybean.Weed Science 50(5):607-610.

Hager, Aaron G., Wax, Loyd M., Bollero, Germán A., and Simmons, F. William. 2002b. Common waterhemp (Amaranthus rudis Sauer) management with soil-applied herbicides in soybean (Glycine max [L.] Merr.). Crop Protection 21(4):277-283.

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