Three Avena fatua (wild oat) populations resistant to imazamethabenz, flamprop, and fenoxaprop-P were identified from the northwest agricultural region of Manitoba, Canada. These populations were identified after producer reports of failure of imazamethabenz to provide satisfactory control in the field. Although these A. fatua populations had previously been exposed to other herbicides, primarily ACCase inhibitors, imazamethabenz had never before been applied. In growth room experiments, resistant (R) plants were 7.2 and 8.7 times more resistant to imazamethabenz and flamprop, respectively, than susceptible (S) plants, as measured by the ratio of dosages required to inhibit shoot dry matter accumulation by 50% (GR50 R/S). The three populations did not differ significantly (P < 0.05) in levels of resistance to imazamethabenz. Similarly, the populations did not differ in levels of resistance to flamprop. The populations differed in their response to fenoxaprop-P; levels of resistance for two populations were 2.0-fold, while the remaining population was 2.9-fold. An experiment conducted in 1995 in one of the infested fields confirmed multiple herbicide resistance, with A. fatua panicle numbers in August being 36, 128, and 44% of untreated controls at recommended dosages of imazamethabenz, flamprop, and fenoxaprop-P, respectively. Three additional populations of A. fatua with multiple herbicide resistance from other areas of Manitoba were identified in a 1996 field experiment. For the six A. fatua populations in the 1996 experiment with multiple herbicide resistance, panicle numbers expressed as a percentage of the untreated controls varied from 44 to 77% for imazamethabenz, 57 to 83% for flamprop, and 43 to 88% for fenoxaprop-P (commercially recommended dosage of each herbicide). Multiple herbicide resistance in A. fatua is not rare; screening of A. fatua seed samples from across Manitoba and Saskatchewan has identified a number of additional R populations. The evolution of herbicide resistance in the absence of direct selection is a very serious development as producers with multiple herbicide resistance in A. fatua are left with a very limited number of herbicide options for selective control in crops commonly grown in western Canada.
Nomenclature: Fenchlorazole-ethyl, ethyl-1-(2,4-dichlorophenyl)-5-trichloromethyl-1H-1,2,4-triazole-3-carboxylate; fenoxaprop-P-ethyl; flamprop-methyl; imazamethabenz-methyl; Avena fatua L. AVEFA, wild oat; Triticum aestivum L. ‘Teal’, ‘Domain’, spring wheat.