THE OCCURRENCE OF HERBICIDE-RESISTANT WEEDS WORLDWIDE

The 1995/6 International Survey of Herbicide-Resistant Weeds recorded 183 herbicide-resistant weed biotypes (124 different species) in 42 co untries. The increase in the number of new herbicide-resistant weeds h as remained relatively constant since 1978, at an average of nine new cases per year worldwide. Whilst 61 weed species have evolved resistan ce to triazine herbicides, this figure now only accounts for one-third of all documented herbicide-resistant biotypes. Triazine-resistant we eds have been controlled successfully in many countries by the use of alternative herbicides. Due to the economic importance of ALS and ACCa se inhibitor herbicides worldwide, and the ease with which weeds have evolved resistance to them, it is likely that ALS and ACCase inhibitor -resistant weeds will present farmers with greater problems in the nex t five years than triazine-resistant weeds have caused in the past 25 years. Thirty-three weed species have evolved resistance to ALS-inhibi tor herbicides in 11 countries. ALS-inhibitor-resistant weeds are most problematic in cereal, corn/soybean and rice production. Thirteen wee d species have evolved resistance to ACCase inhibitors, also in 11 cou ntries. ACCase inhibitor resistance in Lolium and Avena spp. threatens cereal production in Australia, Canada, Chile, France, South Africa, Spain, the United Kingdom and the USA. Fourteen weed species have evol ved resistance to urea herbicides. Isoproturon-resistant Phalaris mino r infesting wheat fields in North West India and chlorotoluron-resista nt Alopecurus myosuroides in Europe are of significant economic import ance. Although 27 weed species have evolved resistance to bipyridilium herbicides, and 14 weed species have evolved resistance to synthetic auxins, the area infested and the availability of alternative herbicid es have kept their impact minimal. The lack of alternative herbicides to control weeds with multiple herbicide resistance, such as Lolium ri gidum and Alopecurus myosuroides, makes these the most challenging res istance problems. The recent discovery of glyphosate-resistant Lolium rigidum in Australia is a timely reminder that sound herbicide-resista nt management strategies will remain important after the widespread ad option of glyphosate-resistant crops.