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 determine the frequency of Group 1 resistant wild oat in 30 randomly selected cereal fields. On average, 61% of the 30 fields were sprayed annually with ACCase inhibitor (Group 1) herbicides from 1983 to 1993. Wild oat were sampled at 80-m intervals on a predefined grid pattern across whole fields. Wild oat densities were recorded and seeds were collected from 0.25 m(-2) quadrats. Seeds were also collected from conspicuous wild oat patches occurring outside the spaced quadrats. Plants were determined to be susceptible or resistant to fenoxaprop-P and/or sethoxydim using a seed bioassay procedure. Results from the structured survey indicated that resistant wild oat occurred in nine fields. Densities in quadrats containing resistant wild oat were generally higher than in quadrats with susceptible wild oat. By combining the results of the 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 in three may harbour Group 1 resistant wild oat in high risk townships in Manitoba.