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Glyphosate Resistance Management - Is it Worth the Effort?  
Bob Hartzler

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December 16, 2005 - In recent weeks several states have reported glyphosate resistance in Palmer amaranth (Amaranthus palmeri) or common waterhemp (Amaranthus tuberculatus). Of particular concern is that glyphosate has significant advantages over other available control practices for these weeds, thus their spread could have serious repercussions on cropping systems highly reliant on Roundup Ready crops and glyphosate. The continued reports of glyphosate resistance in new weed species might lead some to conclude that resistance is inevitable, and thus there is little value to incorporating practices intended to slow the evolution of resistance.

A recent paper (Weesink et al. 2005) out of the weed science group in Western Australia discusses the economics of utilizing weed management strategies for the purpose of delaying the onset of glyphosate resistance. Although the management strategy discussed in the paper is not appropriate for the cropping systems utilized in the Midwest, I believe the authors provide some valuable insights regarding glyphosate resistance management. In a summary of resistance strategies for most selective herbicides, they stated the two alternatives are either 'just say no' or 'accept the inevitable'. These tactics are based on the assumption that abstinence is the only effective way to prevent resistance. They cite previous economic analyses which concluded there was no economic advantage to taking preemptive action to delay the onset of herbicide resistance. However, they concluded that for glyphosate a third option, 'short term pain for long-term gain', could be the optimal approach for managing weeds.

So why is glyphosate resistance different? Why not simply 'use it till we lose it'? According to the Australian researchers, the difference is that the frequency of glyphosate resistance genes is much lower than that of resistance genes for other herbicides. Due to the low frequency of glyphosate resistance, it may be possible to significantly prolong the time required for the resistance gene to increase to problematic levels (control failures). While we don't have a resistance avoidance tactic as effective as the double knockdown (1) described in the paper, I believe the relative rarity of glyphosate resistance in weed populations and the availability of effective herbicide alternatives to glyphosate make resistance management a worthwhile endeavor in our corn-soybean production system.

There are numerous approaches to managing glyphosate resistance, therefore creating a continuum of risk levels. The 'just say no' approach would be one extreme, where a farmer decides simply not to use glyphosate. The other end of the spectrum, 'accept the inevitable', would be continuous planting of Roundup Ready crops with sole reliance on glyphosate to control weeds. The two primary resistance management strategies applicable to most Midwest farms are: 1) incorporate other herbicides into the system, and 2) rotate between Roundup Ready and non-Roundup Ready crops. Avoiding continuous planting of Roundup Ready crops is the simplest approach to managing resistance since it completely eliminates selection pressure from glyphosate in the 'off' years. The benefit of using alternative herbicides during years Roundup Ready crops are planted is more difficult to measure since their impact on glyphosate selection pressure varies with the alternative herbicides' effectiveness on the weeds present in the field (related article). Farmers should evaluate where their weed management system falls on this risk continuum, and hopefully adopt systems that place them somewhere in the middle of the two extremes in order to preserve the integrity of glyphosate.


(1) Knockdown is 'Australian' for a burndown herbicide treatment. Double knockdown is the application of a second knockdown treatment one to two weeks after the first to control any weeds that survived the first application. The first knockdown herbicide would be glyphosate, the second treatment would be with paraquat.

Weersink, A., R.S. Llewellyn and D.J. Pannell. 2005. Economics of preemptive management to avoid weed resistance to glyphosate in Australia. Crop Protect. 25:659-665.

Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University

For more information contact:
ISU Extension Agronomy
2104 Agronomy Hall
Ames, Iowa 50011-1010
Voice: (515) 294-1923
Fax: (515) 294-9985
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Common chemical and trade names are used in this publication. The use of trade names is for clarity by the reader. Inclusion of a trade name does not imply endorsement of that particular brand of herbicide and exclusion does not imply nonapproval.