Fixing resistant weed patches
Herbicide resistance is reaching epidemic proportions in Manitoba.
Recent research from the University of Manitoba suggests as many as 75 percent of fields in high risk areas of Manitoba could have Group 1 resistant wild oats.
High risk areas have had at least “one year on, one year off” Group 1 herbicide use. Over half of the cultivated acreage in Manitoba is now considered to be at high risk.
The good news is that in most fields, resistance is in an early stage. It is mainly present as small patches, not yet noticed by farmers.
Read Also
VIDEO: Ag in Motion documentary launches second season
The second season of the the Western Producer’s documentary series about Ag in Motion launched Oct. 8.
If producers can catch these patches early, they can take steps to prevent resistant weeds from spreading across their farms.
Characteristics of a suspicious patch
Farmers should not assume all small, irregular shaped weed patches are resistant to herbicides. A resistant weed patch has certain characteristics that qualify it as suspicious:
- Irregular shaped, roughly circular. Does not look like human error (spray misses, tire tracks).
- Patch cannot be explained by environmental conditions such as hot weather during spraying, frost, rainfall shortly after herbicide application or weed or crop staging (too early, too late, second flushes).
- Patch cannot be explained by any other application factors such as wrong application rate, incorrect tank mix, improperly set sprayer.
- Check the herbicide use history for that field. If any one group of herbicides has been used more frequently than one year in two, the patch may be suspicious.
Key to identifying a suspicious patch:
Other non-suspect weeds on the herbicide label will be controlled in and around the patch.
How to deal with suspicious patches:
- Map and mark the location of the patches so they can be found in future years.
- If the patches are caught early, before the majority of seeds are in the milk to soft dough stage, and are relatively small (less than one-half acre), mow or spray the patches with a non-selective product like Roundup to prevent seed return. Preventing resistant seed return this year will make a big difference in
future years.
- If the patches are mature, have the seed tested to verify resistance. Resistant seed testing is available from: Ag-Quest, Inc., Box 144, Minto, Man., R0K 1M0, tel. 1-204-776-2087.
- Patchy areas should not be harvested with the rest of the field. Harvest these areas separately. Clean all harvesting equipment before leaving the area and tarp all grain loads to prevent seed spread.
- Check patches each year to monitor their progress. Managing a small patch is easier than dealing with resistance across an entire field.
- Take steps to manage weed resistance permanently. Herbicide rotation and integrated weed management practices are the keys to dealing with resistance in the long term.
– Manitoba Agriculture
