Scout for Herbicide Resistant Weeds

REGINA - Jul 11/05 - SNS -- Farmers should scout fields for signs of herbicide resistance in weeds and failure after spraying their fields, cautions Saskatchewan's Provincial Weed Control Specialist Clark Brenzil.

"Performing both tasks at the same time makes a lot of sense. If producers find a patch of weeds that looks as though it hasn't been controlled, they should look for certain characteristic patterns."

If the patch's border between the controlled and uncontrolled sections is fairly sharp, has square edges or is rather geometric in shape, this is likely a sign of a missed portion or an equipment failure. They tend to occur on headlands or at the corners of fields, and if you have missed one weed species, you have missed them all.

"These kinds of occurrences are less and less frequent because of the increasing use of GPS technology by producers. Sometimes, the patches are left intentionally as a means of gauging the effectiveness of control, telling producers how well the herbicide is performing."

But if the patch is of a single species where other weeds have been controlled, and the patch is of an irregular shape with less defined edges, then it is more likely that the missed weed is resistant to the herbicide that was applied.

"For example, if the herbicide was a Group 1 (or ACCase inhibitor) type, which may normally control annual grass weeds such as wild oat, green foxtail, barnyard grass, Persian darnel or volunteer cereals, and after application all but one of these weeds was controlled, the odds are high that the missed weed is resistant to the herbicide. If there is still time, the patch could be managed using a herbicide from a different herbicide group. Producers should do everything in their power to prevent the patch from spreading.

"If they find a patch, they should mark its location. GPS technology works very well for this," explains Brenzil. "If the patch can't be controlled with another herbicide, it should be prevented from producing seeds, or treated with another herbicide group. Resistance can be confirmed by sending a sample to the Saskatchewan Agriculture and Food Crop Protection Lab, with a lab submission form, to be analyzed."

Lab forms and instructions on how to submit a sample can be found at: http://www.agr.gov.sk.ca/docs/about_us/department_info/Forms/Crop_Protection_Diagnostic_Form.pdf.

A good way to take a sample is to gather up several seed heads from the patch into a finer mesh bag so the seed can mature on the plant, yet can't drop out. Nylon stockings will work well for this. Immature samples that are sent in green are often mouldy by the time they arrive and can't be used for testing. Samples received by the lab will be stored to allow dormancy to break, and will later be tested for resistance. Results would normally not be known until the following spring.

Brenzil points out that several weeds are known to be resistant to one group of herbicides. Some are resistant to multiple groups. There are other weeds around the world producers should be aware of because of their potential to take root here.

"The most common weeds in western Canada that exhibit resistance are wild oats. They tend to become resistant to Herbicide Groups 1, 2 and 8 or to a combination of those three. Green foxtail often is resistant to Herbicide Groups 1 and 3, or both. In Manitoba, there are reports that green foxtail is resistant to Herbicide Group 2 as well."

Herbicide Group 2-resistant Kochia is quite common in Saskatchewan. Russian thistle plants may be resistant to Group 2 herbicides, as well.

"A population of Group 2-resistant wild mustard and a Group 1-resistant Persian darnel population have recently been discovered in Saskatchewan," Brenzil notes.

Another resistant type producers should beware of is glyphosate resistant (Group 9) Canada fleabane, which has occurred in several U.S. states but has not yet been confirmed here in Canada. Producers should be on the lookout for this weed, as its seeds spread by wind and can move rapidly over great distances.