Sulfonylurea herbicide resistance in Sonchus asper biotypes in Alberta, Canada

Summary Two Sonchus asper (spiny annual sow‐thistle) biotypes, suspected of being resistant to the sulfonylurea herbicide metsulfuron‐methyl, were collected in 1996 from two barley (Hordeum vulgare) fields in central Alberta, Canada. Both fields had received at least six applications of acetolactate synthase (ALS)‐inhibiting herbicide(s). The responses of the two resistant (R) biotypes and two susceptible (S) biotypes to several sulfonylurea herbicides, and to herbicides and herbicide mixtures with other mechanisms of action, were compared. Both R biotypes were highly resistant to all sulfonylurea herbicides, but their control with other herbicides and mixtures was effective and comparable to that of the S biotypes. ALS extracted from an R biotype was about 440 times more resistant to metsulfuron‐methyl than that of an S biotype, indicating that resistance was conferred by an ALS enzyme that was less sensitive to inhibition by the herbicide. Competitiveness and seed production of S. asper varied among biotypes, but the differences were probably the result of ecotype differences rather than resistance or susceptibility to sulfonylurea herbicides. This is the first reported occurrence of target site‐based S. asper resistance to ALS‐inhibiting herbicides.