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How to resist resistance

When Canada’s top weed scientists gathered in Winnipeg for their 
annual meeting, glyphosate resistance was on their minds

Public and private sector weed scientists agree integrated weed management, rather than any magic-bullet chemistry, will be the way forward to maintain viable fields against herbicide-resistant weeds.

Scientists from across Canada gathered in Winnipeg in November to discuss new research at the Canadian Weed Science Society’s 66th annual conference. Much of the research on the agenda this year focused on the evolution of herbicide resistance.

Herbicide resistance

Over the past 70 years of weed control, as new herbicides were introduced and farmers took advantage of new chemical technology, “whether we realized it or not, we were rotating our modes of action,” Dow AgroSciences researcher Len Juras said during the conference plenary session.

But the introduction of herbicide-resistant crops in the mid-1990s changed this behaviour. “It changed the mentality of the farmer,” said Juras, who works for Dow at Saskatoon.

Weed control became simpler. Affordable glyphosate helped enable the adoption of conservation tillage. And adding new farm equipment technology, he said, “made all these things into a perfect storm… For the first time, glyphosate use became continuous.”

With glyphosate often used as a single mode of action and weeds evolving glyphosate resistance, what we should be doing, said Juras, is diversifying our weed management practices. However, what we are actually doing is “waiting for new technology.”

Citing a survey of farmers’ concerns, Juras told researchers farmers are more likely to worry about marketing, profitability and farm succession than herbicide resistance. Farmers must be convinced that proper weed control management “is a risk to their profitability.”

Beginning of the end

Carol Mallory-Smith, a weed science professor with Oregon State University at Corvallis, Ore., echoed Juras’s comments, saying on-farm adoption of Roundup Ready crops “was the beginning of the end of built-in resistance management.”

Once farmers learned about this new technology, she said, “it was the fastest acceptance of any new technology we’ve seen in agriculture.”

While weeds were developing resistance to glyphosate, Mallory-Smith said, researchers “were ignoring 30 years of herbicide-resistant research. We really weren’t paying attention to what was going on in the field.”

What was going on in the field was inevitable. Weeds were developing resistance to the herbicides.

In fact, in the case of Palmer amaranth, what the plants were developing was a type of resistance known as “gene amplification” — where a specific gene, resistant to herbicide, is copied in the plant. Mallory-Smith says, “This is a very different kind of resistance that what we’ve seen before.”

Future of resistance

Researchers agree that integrated weed control — including management practices, rotating chemical modes of action and rotating crops and even crop varieties —is necessary to maintain profitable agricultural production without losing more chemicals to herbicide-resistant weeds.

However, “it’s really difficult to change behaviour. Every grower we talk to is waiting for a new herbicide to come along.”

In the future, when new products are developed, Mallory-Smith says, they “may require mandated deployment to prevent a repeat of the experiment with Roundup Ready crops.”

That, she granted, “is a very unpopular comment… Nobody wants more regulations.”

Neil Harker, a researcher with Agriculture and Agri-Food Canada at Lacombe, Alta., also promoted integrated weed management. While more complex systems tend to be more stable over time, he says, with monoculture, he said, “We’re set up in the opposite direction of a complex, stable environment.”

In a typical field, “a lot of canola communities are probably 99 per cent canola” and this lack of variety leaves a lot of opportunities for weeds to take advantage of available niches.

“Allowing more diverse plant populations in our fields isn’t something we can really do,” he said, “unless we’re willing to intercrop, or go to a much lower level of weed management.”

Harker wondered how many herbicides can become ineffective due to evolved weed resistance before our agriculture sector becomes less profitable.

“I’ve been to a lot of meetings where they’ve said, ‘Well, we’ve lost another herbicide to this weed.’” †

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