HERBICIDE RESISTANCE IN THE CANADIAN PRAIRIE PROVINCES - 5 YEARS AFTER THE FACT

Herbicide resistance was first recognized as a problem on the Canadian Prairies in 1988 when trifluralin-resistant green foxtail (Setaria vi ridis) was reported in Manitoba, and chlorsulfuron-resistant chickweed (Stellaria media) and kochia (Kochia scoparia) in Alberta and Saskatc hewan, respectively. Since then, the number of resistant weeds has inc reased to include wild oats (Avena fatua) resistant to triallate and t o aryloxyphenoxypropionate and cyclohexanedione (group 1) herbicides, green foxtail to group 1 herbicides, Russian thistle (Salsola pestifer ) and wild mustard (Sinapis arvensis) to sulfonylurea and imidazolinon e (group 2) herbicides, and wild mustard to growth regulator (group 4) herbicides. The levels and patterns of cross-resistance to chemicals in groups 1 and 2 vary widely among different populations, with resist ance factors [resistant to susceptible (R:S) ratios] derived from dose response curves typically ranging from < 2 to > 150. Group 1 resistan ce in green foxtail and group 2 resistance in chickweed and kochia pop ulations are due to reduced sensitivities of the target enzymes, acety l coenzyme-A carboxylase (ACCase) and acetolactate synthase (ALS), res pectively. The mechanisms of resistance in the other species including wild oats resistant to ACCase inhibitors (group 1) and to triallate/d ifenzoquat (group 8) are unclear. At present, the only instance of mul tiple resistance in western Canada is green foxtail resistant to chemi cals in both groups 1 and 3 (ACCase inhibitors and dinitroanilines). F uture concerns focus mainly on the increasing seriousness of group 1 a nd 8 resistance across the Prairies, and on the possibility of selecti ng for multiple resistance in weeds such as green foxtail for which th ere are few remaining effective control options.