Managing Resistant Weeds Requires a Proactive Approach

It’s a fact — herbicide resistant weeds are present in all Canadian growing regions, across all soil zones, and in different crops and varying climates. Tackling them has become increasingly challenging, with some developing resistance to multiple modes of action. Kochia and Canada fleabane have resistance to three herbicide groups, waterhemp now has documented resistance to five groups and volunteer canola was created herbicide resistant.

Despite all of this, most farmers are able to manage their weed concerns with cultural strategies, crop traits and herbicide solutions. Still, effective weed management is certainly more complicated than it was even 10 years ago, and it’s bound to get more complicated as weed resistance develops going forward.

“Herbicide resistance is a farm-specific and even a field-specific issue,” says Peter Sikkema, professor of weed management with the University of Guelph. “A large percentage of farmers don’t have any resistant weeds at all, and with a varied rotation they may never have to deal with it. But for others who have herbicide resistance on their farm, there are a number of ways they can keep resistance from becoming an expensive issue to deal with.”

THE DOLLARS AND CENTS OF RESISTANCE

Sikkema recently completed research quantifying the actual monetary loss from glyphosate-resistant weeds in Ontario. For example, he says that left unchecked, glyphosate-resistant weeds could cause potential yield losses of $290 million a year in the province.

Fortunately, farmers have adjusted their weed management practices to control glyphosate-resistant weeds. Yield losses are significantly decreased, but there is a cost. In Ontario alone, the added herbicide cost of managing for glyphosate resistance comes in at approximately $28 million a year, and since weed control is rarely perfect, there is still an anticipated yield loss of $15 million from glyphosateresistant weed interference. It adds up to an annual farm-gate loss of $43 million from glyphosate-resistant weeds alone. Sobering numbers to be sure, particularly given the fact that resistance to more than one herbicide is a reality today.

“For those who are struggling with resistance, the issue is daunting … but with good planning we have the tools to manage it” PETER SIKKEMA, PROFESSOR OF WEED MANAGEMENT UNIVERSITY OF GUELPH

LONG-TERM CROP DIVERSIFICATION IS KEY

Sikkema says that when advising growers about developing a long-term weed management plan, he recommends a diverse crop rotation containing at least three different types of crops to reduce the selection pressure for resistant weeds. Ideally, that would include different summer annual crops such as corn and soybean, a winter annual such as winter wheat, and even a perennial crop to change the weed species present in each crop on their farm operation.

He says Eastern growers should use tillage at strategic points in their long-term diversified crop rotation.
“In Ontario, growers frequently use no-till in soybean and winter wheat, reduced tillage in corn and conventional tillage to establish forage crops,” says Sikkema. “Adding cover crops into your rotation is an excellent management option that helps prevent weeds from returning weed seeds to the soil. We have the tools to manage herbicide-resistant weeds, and the really successful farmers know that it takes planning several years in advance to make sure they are managing weeds — resistant or not.”

NO BLINDERS — UNDERSTAND THE PROBLEM

“Knowledge is power when it comes to managing resistant weeds” ERIC PAGE, RESEARCH SCIENTIST AGRICULTURE AND AGRI-FOOD CANADA

“Knowledge is power when it comes to managing resistant weeds,” says Eric Page, research scientist with Agriculture and Agri-Food Canada in Harrow, ON. “When you think you have a weed resistance issue, your first step should be to source independent confirmation, and there are plenty of places now that can do that within a couple of weeks. Once you know you have resistance, the issue now is that some weeds are resistant to multiple modes of action and that makes managing them even more challenging.”

Common management tactics rarely work as standalone solutions, he says. For instance, in addition to seeding a winter crop and a cover crop, he also recommends the use of a pre-emergent herbicide to clean up the field, increasing seeding rates and decreasing row spacing to give the crop a competitive advantage in its early stages.

Given all the potential challenges from resistant weeds, Page thinks Canadian farmers have done well in the way they have managed weed resistance issues so far. “I would like to see the integration of traited solutions and diversifying crops continue, as I think they allow growers to do a lot of wonderful things while maintaining their weed control,” says Page. “I think if these practices continue, with the added use of cover crops, we will go a long way to keeping our herbicide options working for the long term.”

PROACTIVE VS. REACTIVE PLANS

Long-term thinking and land value is something that Adam Pfeffer, agronomic systems manager with Bayer, thinks growers need to remember when it comes to weed management. Pfeffer says once resistance is in a field it will always be there, so it makes sense to be as proactive as possible. He says that fields with resistance issues could have reduced value in the long-term.

“Once we have herbicide resistance, everything we do to manage weeds costs money,” says Pfeffer. “Whether it’s increased input costs, time management, reduced yield — it all adds up. Growers need to be willing to be proactive with their planning and understand the challenges of managing resistant weeds so it doesn’t become a big problem for them five years down the road.”

Pfeffer acknowledges weed resistance has been an issue in Canada for decades and while there is no simple solution, he says growers should be more proactive instead of reactive. If they see weed escapes in year one, they shouldn’t keep doing the same thing until it is truly a problem four years down the road.

“Seeding crops with resistant traits make these issues easier to manage and offers more options for growers. But for some issues, like resistant waterhemp, the weeds are resistant to many different modes of action so growers need to do more,” he says. “In 2021 in fields that were seeded to winter wheat we didn’t see any waterhemp, which shows that crop rotation can help change the weed emergence profile.”

For Sikkema, it all comes down to making the commitment to act, to accept there will be costs and to look ahead. “Herbicide resistance impacts all farming regions so this isn’t a message we can target to just one area, even though not all farms are affected,” he says. “It is difficult to get a farmer to spend money on a problem they may never have, so we need to sell them on diversification, and on the best ways to maximize farm profitability while reducing the selection intensity for the evolution of herbicide-resistant weeds. For those who are struggling with resistance, the issue is daunting. It increases complexity but with good planning we have the tools to manage it.”