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Roundup resistant weeds: Can it happen?
by Mike Owen

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February 10, 1997 - In Volume 8 of Resistant Pest Management, the biannual newsletter of the Pesticide Research Center at Michigan State University, there were two articles discussing the development and existence of weed populations that are resistant to glyphosate herbicide. Given the interest that Iowa growers have expressed in the use of Roundup Ready soybeans, and the general feeling that there is a low risk of weeds developing resistance to glyphosate, these articles are extremely interesting.

The first article is "Fewer Constraints than Proclaimed to the Evolution of Glyphosate-Resistant Weeds and was written by Dr. Jonathan Gressel, Professor of Plant Genetics at the Weizmann Institute of Science in Rehovot, Israel. Dr. Gressel points out that there are numerous examples of weed biotypes that demonstrate variable response to low rates of glyphosate. Further, he lists the known mechanisms that could be important in plants and lead eventually to glyphosate resistance. Dr. Gressel suggests that a major change in any one mechanism might confer resistance to plants treated with field rates of glyphosate. While glyphosate does not demonstrate residual characteristics typically found in herbicides which have influenced the development of resistant weed populations, the consistent and repeated use of glyphosate would "simulate a biological persistence akin to other herbicides that selected for resistance". Finally, Dr. Gressel cautions that there are in reality, very few constraints to weeds becoming resistant to glyphosate. He strongly suggests the need for resistant weed management strategies if glyphosate is to be available for general use in the future.

The second article is by Dr. Brian Sindel from the University of New England in Australia and is titled "Glyphosate Resistance Discovered in Annual Ryegrass". Dr. Sindel indicates that herbicide resistant weeds are an increasing problem for Australian agriculture and Australia has more resistant weeds than any other country in the world. Reports that a farmer in northern Victoria found an annual ryegrass that was not controlled by glyphosate. The area had been treated with glyphosate ten times in 15 years. The resistance has been confirmed by the Charles Sturt University at Wagga Wagga. Dr. Sindel makes the point that while this first report of glyphosate resistance is not a disaster, but rather a warning that unless proper management is included with the use of glyphosate, the development of glyphosate resistant weeds may increase.

Prepared by Mike Owen, 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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