A SURVEY OF ACCASE INHIBITOR RESISTANT WILD OAT IN A HIGH-RISK TOWNSHIP IN MANITOBA

A survey was conducted in a township near Treherne, Manitoba to determ ine the frequency of Group 1 resistant wild oat in 30 randomly selecte d cereal fields. On average, 61% of the 30 fields were sprayed annuall y with ACCase inhibitor (Group 1) herbicides from 1983 to 1993. Wild o at were sampled at 80-m intervals on a predefined grid pattern across whole fields. Wild oat densities were recorded and seeds were collecte d from 0.25 m(-2) quadrats. Seeds were also collected from conspicuous wild oat patches occurring outside the spaced quadrats. Plants were d etermined to be susceptible or resistant to fenoxaprop-P and/or sethox ydim using a seed bioassay procedure. Results from the structured surv ey indicated that resistant wild oat occurred in nine fields. Densitie s in quadrats containing resistant wild oat were generally higher than in quadrats with susceptible wild oat. By combining the results of th e structured survey with the patch collection, resistance was detected in 20 out of the 30 fields. While resistant weeds generally occurred in small patches, in two of the fields, resistant plants occurred over much larger areas. The evidence suggests that as many as two fields i n three may harbour Group 1 resistant wild oat in high risk townships in Manitoba.