Farmers won’t be able to spray their way out of this — Canada is now third in the world when it comes to weeds able to survive pesticides that used to kill them.
“We’re quickly being overrun by herbicide-resistant weeds,” said federal research scientist Charles Geddes. “There have been difficult herbicide-resistant weeds in the past, but we’re seeing the number grow now. The problem is only getting worse.
“It’s very serious.”
Canada ranks behind only the U.S. and Australia for herbicide-resistant weed biotypes, a problem that has been growing in scope and severity over the past three decades.
The country’s expanse is partially responsible — the more space, the more weeds and weed types there are likely to be — but “ultimately, herbicides are the cause of herbicide resistance,” said Geddes.
“I don’t know that we’ve been misusing herbicides, but I do think we have perhaps been overrelying on certain herbicides in our cropping systems,” he said. “Weeds are constantly evolving and adapting to whatever selection pressure we impose. It just so happens that the most common type of selection pressure that we impose is herbicide.”
Today, Canada is home to roughly 75 unique herbicide-resistant weeds, the majority of which can be found in Ontario, Alberta, Manitoba, and Saskatchewan.
In Alberta’s most recent weed survey in 2017, populations of both common lamb’s quarters and green foxtail were newly found to be Group 2 resistant, adding to the growing number of weeds in the province that have become resistant in the past 10 years.
“In recent decades, we’ve been blessed with a sort of grace period,” said Geddes. “After the release of a new herbicide mode of action, there’s a period there where we don’t see any resistance showing up — the selection pressure happens over time.
“It does take time to evolve resistance and a certain number of recurrent herbicide applications with a specific herbicide mode of action. That’s what we’re seeing now.”
Increasing glyphosate resistance is particularly troubling, especially in kochia, which has been cutting a swath through the southern Prairies for nearly a decade.
“Some of the most difficult ones are glyphosate-resistant weeds,” said Geddes. “In Western Canada, kochia is quickly becoming a very problematic weed.”
Kochia is considered to be fully resistant to Group 2 herbicides, and over 50 per cent of kochia populations in southern Alberta are also resistant to glyphosate (a Group 9 herbicide). Group 4 — or dicamba — resistance was also found in 18 per cent of kochia populations in 2017.
Alberta farmers are now also starting to see more triple-resistant kochia (resistant to Group 2, Group 4, and Group 9 herbicides). In 2017, 10 per cent of kochia populations were triple resistant, and that number has likely risen in the two years since then. (The next kochia survey in Alberta is scheduled for 2020.)
“Chemicals are easy to use and they provide very good weed control, so it’s always tempting to rely on herbicides as the only management tool against weeds,” said Geddes.
“But we’re at the point now where that’s just not a sustainable way to manage a farming operation.”
And farmers have the most to lose if they can’t get ahead of herbicide-resistant weeds in their fields, said Geddes.
“Weeds ultimately end up resulting in yield loss, and the fact that certain herbicides aren’t managing certain weeds effectively creates an opportunity for those weeds to persist in the environment and cause yield loss in subsequent crops,” he said.
Controlling weeds becomes more expensive — or even impossible, in some cases — when these herbicides are lost, he added.
“In the United States, certain glyphosate-resistant weeds — which they call super-weeds — have caused some growers to actually have to revert back to hand-weeding their field. You can imagine how expensive that would be,” said Geddes.
“So farmers are facing greater input costs as far as a weed control program and reduced efficiency overall because of the time it takes to implement some of these weed management tools.”
Farmers can reduce their risk of finding herbicide-resistant weeds in their field by focusing on diversity — employing a varied crop rotation, rotating herbicide modes of action, and using “a diverse tool box of different weed management tools and techniques.”
“We know that diversity in a weed control system helps prevent any development of herbicide resistance in the weed population, so we need to be including non-chemical weed management tools in the program as well,” said Geddes.
That starts with promoting a crop that will successfully compete against weeds. Increasing seeding rates and reducing row spacing is one way to boost the crop’s ability to compete. Including different crop life cycles — like winter annuals — is another.
“Using tools that reduce weed seed production can limit the number of individual plants from that population,” said Geddes.
These non-chemical weed management tools may be less convenient than herbicides — limiting their widespread adoption thus far — but there is a limit to the number of chemical control options out there, he said.
Canada’s agriculture industry has been pushing different weed control strategies for as long as weeds have been overcoming their resistance, and now it’s up to farmers to put those strategies to good use in their own fields.
“Herbicide resistance is a serious issue, and I think that the industry is taking it seriously. There is a collective effort going on to address it,” said Geddes.
“But the big thing now is the adoption of some of these best management practices.”
For more information about managing resistant weeds, visit www.weedscience.org.