The weeds plaguing Manitoba fields
By Julienne Isaacs
Agriculture and Agri-Food Canada (AAFC)’s 2016 herbicide resistant weed survey in Manitoba highlighted increased resistance in grass weeds versus broad-leaf weeds in the province, according to AAFC resistance specialist Hugh Beckie.
Sixty-eight per cent of Manitoba fields had herbicide resistant weeds in 2016, compared with 48 per cent in 2008 and 32 per cent in 2002.
Manitoba has always led the Prairie provinces in terms of frequency of herbicide resistant weeds, Beckie says, even though Saskatchewan is close behind. The 2014/2015 survey in Saskatchewan noted 57 per cent of fields contained herbicide resistant weeds – and the difference between the provinces is getting tighter.
“That gap is closing,” Beckie says.
The herbicide resistant weed survey is a subset of the larger AAFC general weed survey, led by Saskatoon research station biologist Julia Leeson, which is conducted in each of the Prairie provinces roughly every five to ten years.
In 2016, a total of 659 randomly chosen fields in all of Manitoba’s major cropping zones were included in the province’s general weed survey, 151 of which were included in the herbicide resistance survey.
Beckie says there were a few surprises when it came to top weed rankings in the general weed survey.
Green foxtail was still in the top weed from 2002, but wild oat, which had been second in 2002, was bumped to fourth place, replaced by wild buckwheat. Barnyard grass, fourth in 2002, took third place in the overall rankings.
Yellow foxtail was the biggest surprise. The weed, a cousin of green foxtail, had the greatest increase in ranked position, from 32nd place in 2002 to 6th place in 2016. “Part of it may be due to resistance because it’s escaping herbicide control,” Beckie says.
In terms of herbicide resistance, this survey was the first to document resistance in yellow foxtail and barnyard grass.
Rob Gulden, a weed scientist in the department of plant science at the University of Manitoba, says there are other possible herbicide resistant weeds on the horizon for Manitoba. Glyphosate resistant kochia has been in Manitoba since 2013, and as of last year it was found in at least five municipalities.
Glyphosate resistant waterhemp and glyphosate resistant Canada fleabane are not yet confirmed in the province but could come north via floodwater from North Dakota. “Not sure how fast things will move, but there’s some evidence that suggests that these weeds have moved with overland flooding of smaller tributaries to the Red River,” he says.
Most concerning of the Manitoba survey results is the incidence of multiple resistance in wild oat, says Beckie. Group 1 plus Group 2 herbicide resistance in wild oat has been found in 42 per cent of fields in Manitoba; in Saskatchewan this number is still only at 25 per cent.
“The incidence of multiple resistance is getting worse,” he says. “This presents the biggest challenge to producers in terms of management, especially in cereal crops.”
In the 2016 survey, Manitoba producers were also asked to rank their top 20 management practices in terms of which they feel are most effective in managing weeds.
Options include scouting multiple times per season, tank mixing herbicides, crop rotation, tillage and growing competitive crops.
The practice that stands out the most, says Beckie, is tillage: Manitoba producers rely on tillage to manage herbicide resistant weeds, in sharp contrast to Saskatchewan producers, who don’t.
Overall, producers with herbicide resistant weeds rely more on herbicide application at all application windows, according to Beckie. But these producers also have greater adoption of scouting prior to herbicide application, tank mixing herbicides, and growing weed competitive crops.
The latter method is a key management strategy. “It’s the foundation of good agronomy and it goes a long way in preserving herbicide longevity,” Gulden says.
Producers should also focus on rotating herbicide groups, tank mixing herbicide active ingredients, and of course rotating crops.
“The other thing we’re learning with controlling wild oats is that we want to reduce seed production, and again, a good herbicide program and a competitive crop reduce weed seed production.”
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