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Roundup resistant rigid ryegrass
by Bob Hartzler

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October 22 -  Rigid ryegrass (Lolium rigidum) is a major weed of southern Australia.   The weed is unique in its propensity for selection of resistant biotypes.  It has resistance to most of the herbicide families used in Australia.  Biotypes with multiple resistance, therefore making all herbicides cleared for use in wheat ineffective, have forced Australian wheat farmers to change production practices due to the lack of effective herbicides.  In 1997 it was first reported that a population of rigid ryegrass with resistance to Roundup was identified.  An article in the Sept.-Oct. 1998 issue of Weed Science provided more details regarding Roundup resistance in rigid ryegrass (Powles, S. B., D. F. Lorraine-Colwill, J. J. Dellow, and C. Preston.   1998.  Evolved resistance to glyphosate in rigid ryegrass.  Weed Sci.   46:604-607).  This article will summarize some of the information in this article.

The resistant biotype was obtained from an orchard near Orange, New South Wales, Australia.  The site had intensive selection pressure, with two or three applications per year of Roundup (2 to 4 qt/A) for 15 years.  The Roundup was used to control weeds within rows of trees.  Seeds were collected from suspected resistant plants in December 1995 for use in the experiments.  Four experiments were conducted, three using the isopropylamine salt of glyphosate (Roundup) and own with the trimethylsulfonium salt (Touchdown).  Similar results were found with both formulations.  On all occasions the resistant population was found to be between 7 and 11-fold resistant to Roundup compared to the susceptible population.

Response of a susceptible and resistant rigid ryegrass biotype to Roundup.  Plants were grown in pots and  sprayed at the two to three leaf stage.  Measurements were taken 21 days after treatment.

Roundup Rate Percent survival Dry Wt (mg)/Plant
(Pints/Acre) Susceptible Resistant Susceptible Resistant
0 100 100 190 180
0.6 88 100 106 -
1.3 20 100 30 -
2.6 5 88 20 110
5.3 0 85 0 65
10.6 0 48 0 58
21.1 0 18 0 20

Adapted from Powles, S. B., D. F. Lorraine-Colwill, J. J. Dellow, and C. Preston.  1998.  Evolved resistance to glyphosate in rigid ryegrass.  Weed Sci.  46:604-607.

At this time the mechanism of resistance that allows rigid ryegrass to survive Roundup has not been reported.  This species has numerous mechanisms that provide resistance to other classes of herbicides, including several altered sites of action (ALS, ACC-ase) and enhanced metabolism (ALS, ACC-ase, dinitroanilines, photosystem 2, amides).  In addition, a second case or Roundup resistant ryegrass has been reported in Australia, this time in an agronomic setting with less intensive selection pressure than occurred with the biotype reported here.

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.