Simulating evolution of glyphosate resistance in Lolium rigidum II: past, present and future glyphosate use in Australian cropping P NEVE*, A J DIGGLE  , F P SMITH à & S B POWLES* *Western Australian Herbicide Resistance Initiative, School of Plant Biology, University of Western Australia, Crawley, WA, Australia,  Western Australian Department of Agriculture, South Perth, WA, Australia, and àCSIRO Sustainable Ecosystems, Wembley, WA, Australia Received 20 November 2002 Revised version accepted 23 July 2003 Summary Glyphosate is a key component of weed control strategies in Australia and worldwide. Despite wide- spread and frequent use, evolved resistance to gly- phosate is rare. A herbicide resistance model, parameterized for Lolium rigidum has been used to perform a number of simulations to compare predicted rates of evolution of glyphosate resistance under past, present and projected future use strategies. In a 30-year wheat, lupin, wheat, oilseed rape crop rotation with minimum tillage (100% shallow depth soil disturbance at sowing) and annual use of glyphosate pre-sowing, L. rigidum control was sustainable with no predicted glyphosate resistance. When the crop establishment system was changed to annual no-tillage (15% soil disturbance at sowing), glyphosate resistance was predicted in 90% of populations, with resistance becoming apparent after between 10 and 18 years when sowing was delayed. Resistance was predicted in 20% of populations after 25–30 years with early sowing. Risks of glyphosate resistance could be reduced by rotating between no-tillage and minimum- tillage establishment systems, or by rotating between glyphosate and paraquat for pre-sowing weed control. The double knockdown strategy (sequential full rate applications of glyphosate and paraquat) reduced risks of glyphosate and paraquat resistance to <2%. Introduction of glyphosate-resistant oilseed rape sig- nificantly increased predicted risks of glyphosate resistance in no-tillage systems even when the double knockdown was practised. These increased risks could be offset by high crop sowing rates and weed seed collection at harvest. When no selective herbicides were available in wheat crops, the introduction of glypho- sate-resistant oilseed rape necessitated a return to a minimum-tillage crop establishment system. Keywords: Lolium rigidum, herbicide, paraquat, model, management, tillage, glyphosate-resistant crops. Introduction Glyphosate, a broad spectrum, non-selective herbicide, is the world’s most important and widely used herbicide (Baylis, 2000; Woodburn, 2000). Until recently, its lack of selectivity meant that it could not be used for weed control within crops. However, this is no longer the case as genetic transformation has enabled genes conferring glyphosate resistance to be introduced into a number of crop species (Padgette et al., 1996; Wilcut et al., 1996). In the past few years transgenic glyphosate-resistant Glycine max L. (soyabean), Brassica napus L. (oilseed rape) and Zea mays L. (maize) have been rapidly and widely adopted in North and South America. The predominant use of glyphosate in current Aus- tralian grain production systems is for broad spectrum, non-selective weed control prior to crop sowing. Since the 1970s in Australia, there has been substantial adoption of reduced tillage crop establishment systems and most farmers practice direct drilling in which pre- sowing weed control is achieved with herbicides and soil disturbance occurs at crop sowing only (Pratley & Rowell, 1987). The degree of soil disturbance at sowing varies from 100% in minimum-tillage systems which use Correspondence: P Neve, Western Australian Herbicide Resistance Initiative, School of Plant Biology, University of Western Australia, Crawley, WA 6009, Australia. Tel: (+61) 8 9380 7872; Fax: (+61) 8 9380 7834; E-mail: pneve@agric.uwa.edu.au Ó European Weed Research Society Weed Research 2003 43, 418–427