No matter which crops Saskatchewan farmers grow, the same weeds appear year after year. While the culprits are consistent, how you manage them is not, especially as herbicide resistance cases mount.
For nearly 50 years, Canada thistle, wild oats, wild buckwheat and green foxtail have appeared in the top-five list of problematic weeds in Saskatchewan. The fifth weed in that top-five list varies, often reflecting climactic conditions of the time, said Saskatchewan Agriculture’s provincial weed control specialist, Clark Brenzil.
“In 2003, it was lamb’s-quarters and 2014–15 it was volunteer canola,” he said. “This was the first time that volunteer canola ranked this high in the survey.”
“This may be the result of a fall with high winds that resulted in heavy shattering losses in canola,” he added.
Canada thistle made the news last year because of its impact on shipments to Vietnam. In June of 2018, Vietnam’s Plant Protection Department (PPD) found wheat and pea shipments to be in noncompliance with its phytosanitary requirements. As a result, PPD announced its intention to reject any shipments affected by the weed beginning January 1, 2019.
In a recent interview, Cam Dahl, president of Cereals Canada, said the risk of rejection effectively renders shipments to Vietnam economically unfeasible.
There are two good options for managing Canada thistle, especially in wheat, said Brenzil. Growers can use pre-harvest glyphosate, he said. He does caution that they must keep residue levels in mind and apply when the crop is dead ripe.
Products containing clopyralid will provide some level of control as well. Research conducted by Dow AgroSciences in the 1990s found that a combination program of a clopyralid-based herbicide, plus a pre-harvest treatment resulted in 95 per cent control, whereas each product alone was around 80 per cent. “With Canada Thistle being a creeping-rooted perennial, control measures you take in one year impact the next year,” he said.
“With Canada thistle, you kind of have to think a little forward as to where you’re managing that, unless you’re doing pre-harvest glyphosate on every single crop that you’re growing,” said Brenzil. “Not every producer is willing to do that.”
Glyphosate provides a good tool at harvest, he continued. When applied in the fall it kills the growth of new roots for the following year. Applications can be made post-harvest as well, but keep in mind the tops of weeds will be lopped off at that point, making them more difficult to target. Before implementing post-harvest control, six weeks must pass before spraying and even then, the glyphosate rate must triple compared to pre-harvest.
Because Canada thistle is very competitive, Brenzil suggests spraying low-value crops in preparation for high-value crops. “If you have a patch of Canada thistle, it will nearly completely exclude any crop that’s growing in it,” he said.
Wild oats and others
Wild oats are another weed of concern, said Brenzil. Sixty per cent of Prairie populations have Group 1 resistance, and 30 per cent of populations have Group 2 resistance.
“I suspect if we looked for Group 8 resistance, we’d find that probably 20 to 30 per cent had resistance as well,” he said. “We’re running out of options for wild oat management in Western Canada.”
Volunteers are also pervasive. “Canola and wheat volunteers are as high as they have been,” said Brenzil. “Some of this has to do with pre-harvest shattering [canola] and material running off the sieves of the combine.”
Some of this can be chocked up to harvest speed (going too fast) and combine settings, whether intentional (blowing fusarium diseased kernels out the back) or accidental (non-optimized settings for canola).
Kochia also raises concerns, after two hot dry seasons. Well-adapted to high salinity levels, kochia appears where water tables are high and evaporation brings salts to the surface.
“On its own, it’s not super competitive, but it’s really good at taking advantage of gaps in the stand,” said Brenzil.