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The future of herbicide-resistant weed control

It’s easy to oversimplify when it comes to thinking about the spread of herbicide-resistant (HR) weeds. Overuse of herbicide resistant crops or individual modes of action is part of the reason why, in some places, HR weeds are becoming tougher to tackle. But it’s not the whole story, says Breanne Tidemann, a research scientist with Agriculture and Agri-Food Canada specializing in weed science.

“Herbicide-resistant crops are not black and white, they’re not bad or good, they’re not a solution or a problem,” says Tidemann. “Demonizing HR crops is not accurate. You can use them incorrectly and get bad results. But you can also use them in a good and beneficial way and they can be used to help manage resistance,” she adds.

Tidemann is a co-author on a new research review paper in the journal Weed Science that looks at whether HR crops are a long-term solution to HR weeds in the western United States and Canada.

The paper is based on a symposium, held at the Western Society of Weed Science annual meeting in 2021, where Tidemann presented alongside Canadian and U.S. weed researchers on topics in integrated weed management.

Caio Brunharo is an assistant professor in applied weed physiology at Pennsylvania State University and the lead author on the paper.

There are two main ways herbicide-resistant populations can be introduced to a field, Brunharo says – via “gene flow,” when resistance spreads through the movement of weed seed or other propagules from farm to farm, and locally with the repeated re-use of a single mode of action over many years.

“The main idea of the paper was to discuss whether releasing new HR crops is the solution for HR weeds. After a lot of discussion, we concluded that HR crops have been and are important–they are part of the solution, but they’re also part of the problem. Every time we release a new variety, growers go ahead and use it and sometimes overuse it, and we keep getting HR weeds,” says Brunharo.

Integrated weed management
New HR cultivars are developed using conventional breeding and genetic engineering, the authors write. 71.5 million hectares in the U.S. and 12.5 million hectares in Canada are planted to crops containing HR traits. Broad adoption of HR corn, cotton and soybean cultivars in the early 2000s led to strong herbicide selection pressure on weed populations. 

Around the world, over 500 different cases of HR weed species have been documented. The U.S. and Canada rank first and third, respectively, in the number of reported HR biotypes.

In the Canadian Prairies, the most common HR crops are canola in Alberta, canola and lentil in Saskatchewan and soybean and canola in Manitoba. So far, HR weed evolution to specific herbicides used in HR crops hasn’t been a significant problem across the Prairies, the authors write, probably due to the variety of HR traits available. 

Canola is an example of how HR crops can provide excellent contributions to integrated weed management, the authors write, due to a combination of factors, including the variety of HR canola traits used in rotation, cultural practices such as conservation tillage and early seeding, and investment by the private sector.

But in other crops, such as cotton and soybean, this doesn’t hold true. Ever-increasing combinations of stacked traits are required “to keep up with increasing incidence and complexity of cross- and multiple HR weeds,” the authors write.

Tidemann says simply stacking traits can create problems down the line. For example, if a canola variety that combines resistance traits to glyphosate and dicamba is planted in a field with weed populations resistant to glyphosate, “all the selection pressure is on the dicamba,” she says. “I don’t see that as being beneficial in the long term. [A] beneficial [approach] would be looking at new modes of action, modes of action that don’t have resistance yet, or combining those two.”

Stewardship
Herbicide stewardship is crucial to keeping herbicide resistance traits in play. But this may mean more than just adding new single resistance traits to existing stacks, the researchers write in the paper. Because growers already have an “urgent need” to control the HR weed populations in their fields, new herbicides will swiftly face severe selection pressure in some contexts. This fact necessitates both new weed control technology and greater education about how herbicides are used.

“Overall, the primary goal should be to reduce herbicide selection pressure in weed populations wherever and whenever possible,” they argue.

“Therefore, we need to reduce the frequency of herbicides always doing the heavy lifting and use effective combinations of non-herbicidal practices that aid both herbicide performance and crop competition to suppress weed growth and fecundity.”

Brunharo says the researchers’ main conclusion is that HR crops should be viewed in a wider context–as one tool among several. “Stewardship is the number one conclusion we have from this paper,” he says.

But this responsibility falls on industry and the research community as well as individual growers. “Registration requirements and industry stewardship plans need teeth,” the authors of the paper write. 

“To avoid a ‘tragedy of the commons,’ recommendations for maximum herbicide-use intensity (within and across growing seasons) and HR crop rotation frequency are needed,” they emphasize. “Comprehensive training for growers in proper stewardship practices is critical for optimizing and prolonging the benefits and minimizing risks of HR crops as they are repeatedly and widely deployed across millions of hectares of cropland annually.”

Crop rotation is a critical tool at growers’ disposal, adds Tidemann. But the more resources producers can turn to in the battle with herbicide resistance, the better. “When we utilize physical, chemical, cultural and biological ‘modes of action’–the more ‘modes of action’ you’re using, the longer it takes weeds to develop resistance, and the more sustainable your control over the long term,” she says.

This includes harvest weed seed control, a set of cultural practices that focus on destroying or minimizing weed seed returning to the seed bank.