CORN Newsletter: 2005-01, January 11, 2005

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C.O.R.N Newsletter 2005-01
January 11, 2005 - January 25, 2005

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Preserving the Value of Glyphosate
by Mark Loux


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The following article was co-authored by the following, all of whom conduct extension education in weed management in the Midwest: Bob Hartzler and Mike Owen, Iowa State University; Chris Boerboom, University of Wisconsin; Kevin Bradley, University of Missouri; Jeff Gunsolus, University of Minnesota; Bill Johnson, Purdue University; Jim Kells and Christy Sprague, Michigan State University; Mark Loux and Jeff Stachler, Ohio State University; Aaron Hager and Dawn Nordby, University of Illinois; Bryan Young, Southern Illinois University

It is well known that glyphosate-resistant horseweed (also known as marestail) populations have been selected in Roundup Ready soybean and cotton cropping systems. Resistance was first reported in Delaware in 2000, a mere 5 years after the introduction of Roundup Ready soybean. Since that initial report, glyphosate-resistant horseweed is now reported in 12 states and is estimated to affect 1.5 million acres in Tennessee alone.

A person could ask if this is any indication of what might lie ahead. On one hand, it has been proposed in a popular advertisement that glyphosate-resistant weeds are unlikely to occur if glyphosate is frequently used, as long as glyphosate is applied at full rates. The recommendations in this advertisement, in part, are based on several long-term university studies of Roundup Ready cropping systems. We feel a person should ask whether or not these studies can prove that resistance will or won’t happen. It is our belief that these studies are not large enough to test if resistance will develop. For example, the University of Wisconsin has a 7-year Roundup Ready cropping system trial. This trial has horseweed in the no-till plots. Despite burndown and in-crop treatments with glyphosate, glyphosate-resistant horseweed has not developed in this study. Since this trial did not find glyphosate-resistant horseweed, does this mean that glyphosate-resistant horseweed cannot develop? Does it mean that the resistant horseweed in Ohio or Tennessee or Delaware is not truly resistant? Obviously not. Small scale trials cannot prove that some event will not occur when a larger scale is considered. In total, these Roundup Ready cropping system trials may only be testing continuous glyphosate use on perhaps 50 acres, which is an extremely small area compared to the total acres relying on glyphosate as a primary management tool. The true trial to determine if a rare event like glyphosate-resistant weeds will develop is actually being tested on the tens of millions of acres of Roundup Ready corn, soybean, and cotton that are sprayed each year.

Many weed scientists across the Midwest have warned of the potential for additional glyphosate-resistant weeds if a “high selection pressure” is maintained. In this case, “high selection pressure” refers to the repeated use of glyphosate without interruption by herbicides with other modes of action or other weed management practices. This potential was confirmed at the recent North Central Weed Science Society Meeting where glyphosate-resistant common ragweed weed was reported. This is the first report of glyphosate-resistance in common ragweed. It was identified in a Missouri no-till soybean field that has been in continuous soybean production (with some double crop wheat) for many years and in Roundup Ready soybean since 1996. This field had a high selection pressure for glyphosate resistance with one or more glyphosate applications per year. This is the second example of a glyphosate-resistant weed developing in a Roundup Ready cropping system. We do not know which weed species will be the next to develop glyphosate resistance or when it will occur, but high selection pressure will likely result in additional cases of resistance.

Midwest weed scientists appreciate the value that glyphosate and Roundup Ready crops offer to growers. We hope that growers and crop advisors will evaluate how they use glyphosate and the Roundup Ready technologies to gain the value of these technologies without increasing the risk of resistance. Ideally, we recommend:

1) tank mixing glyphosate with another mode of action like 2,4-D in burndown treatments when glyphosate will be applied in the subsequent crop,
2) alternating glyphosate use with other herbicide modes of action between years, and
3) incorporating appropriate integrated weed management practices such as cultivation.

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C.O.R.N. is a summary of crop observations, related information, and appropriate recommendations for Ohio Crop Producers and Industry. C.O.R.N. is produced by the Ohio State University Extension Agronomy Team, State Specialists at The Ohio State University and Ohio Agricultural Research and Development Center. C.O.R.N. Questions are directed to State Specialists, Extension Associates, and Agents associated with Ohio State University Extension and the Ohio Agricultural Research and Development Center at The Ohio State University.

Information presented above and where trade names are used, they are supplied with the understanding that no discrimination is intended and no endorsement by Ohio State University Extension is implied. Although every attempt is made to produce information that is complete, timely, and accurate, the pesticide user bears responsibility of consulting the pesticide label and adhering to those directions.

All educational programs conducted by Ohio State University Extension are available to clientele on a nondiscriminatory basis without regard to race, color, creed, religion, sexual orientation, national origin, gender, age, disability or Vietnam-era veteran status.

Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Keith L. Smith, Director, Ohio State University Extension.

TDD # 1 (800) 589-8292 (Ohio only) or (614) 292-1868


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