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Widespread occurrence of multiple herbicide resistance in Western Australian annual ryegrass (Lolium rigidum) populations

Mechelle J. Owen A C , Michael J. Walsh A , Rick S. Llewellyn A B and Stephen B. Powles A
+ Author Affiliations
- Author Affiliations

A Western Australian Herbicide Resistance Initiative, School of Plant Biology, University of Western Australia, 35 Stirling Hwy, Crawley, WA 6009, Australia.

B Current address: CSIRO Sustainable Ecosystems, Urrbrae, Post: PMB 2 Glen Osmond, SA 5064, Australia.

C Corresponding author. Email:

Australian Journal of Agricultural Research 58(7) 711-718
Submitted: 25 August 2006  Accepted: 28 March 2007   Published: 26 July 2007


In 2003, a random survey was conducted across the Western Australian wheatbelt to establish the frequency and distribution of herbicide resistance in ryegrass populations infesting crop fields. Five hundred cropping fields were visited at crop maturity, and ryegrass seed was collected in 452 of these fields. Subsequently, each crop field population was screened with herbicides of various modes of action that are commonly used for ryegrass control in Australian cropping systems. Most of these ryegrass populations were found to be resistant to the ACCase-inhibitor herbicide diclofop-methyl (68%) and the ALS-inhibitor herbicide sulfometuron (88%). A comparison of resistance levels in the same agronomic zones surveyed 5 years earlier determined that there had been an increase of 20 percentage points in the frequency of resistance over this 5-year period. This survey also determined that the majority (64%) of populations were found to be multiple resistant to both diclofop-methyl and sulfometuron. The distribution patterns of the collected populations indicated that there were higher frequencies of resistant and developing resistance populations occurring in the intensively cropped regions of the wheatbelt, which had greater herbicide selection pressure. Of concern is that 24% and 8% of populations were found to be developing resistance to trifluralin and clethodim, respectively. Currently these herbicides are heavily relied upon for control of ACCase and ALS herbicide resistant ryegrass. Nearly all populations remain susceptible to glyphosate. Ryegrass across the WA wheatbelt now exhibits multiple resistance across many but not all herbicides, posing severe management and sustainability challenges.

Additional keywords: resistance survey, resistance evolution.


This project was funded by the Grains Research and Development Corporation. We thank the members of the WAHRI research team (staff and students) who assisted in this field survey, Bolette Palle for invaluable technical assistance, and Frank D’Emden for assistance with data analysis.


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