Survey reveals need for integrated approach to weed management

There’s no question weed control is getting more complicated. Warmer winters, drier summers and herbicide resistance are all factors in changing weed patterns and challenges.

And when it comes to tackling the problem, Ontario farmers and the agronomists they turn to for help, are showing increased interest in a more integrated approach to weed control that includes both cultural methods and herbicide application.

The need for integrated weed management strategies is a key finding of a 2021 weed survey conducted by Mike Cowbrough, long-time weed scientist with Ontario Ministry of Agriculture, Food and Rural Affairs. The survey targeted independent and retail agronomists known for their experience in weed management.

Cowbrough asked them to comment on what they see in Ontario fields. More than 30 responded from across the province, with some working together to compile their answers. Taking what Cowbrough describes as a kind of state-ofthe- union approach to the topic, the survey also asked about weed management practices and issues of concern.


There were no surprises on the list of challenging-to-control weeds, with lamb’s quarters, Canada fleabane, foxtails, pigweed, plus common and giant ragweed topping the list of troublemakers.

Cowbrough was intrigued to note that bluegrass made the top 14. With 19 years in the field, Cowbrough noticed rising concern about annual and roughstock bluegrass. “It’s not surprising to me, but the survey shows bluegrass is starting to be a more widespread problem and we don’t have a great amount of intelligence on the best way to control these weeds.”

Bluegrass concerns aside, the survey also focused Cowbrough’s attention on the management practices agronomists and their farmer-clients are talking about. “Everyone knows there are benefits to fall weed control for weeds like perennial sow thistle, but it’s not done on every acre for very practical reasons,” says Cowbrough.

Because corn is grown on more than one-third of Ontario’s agricultural land base, and since that crop is harvested in October, November and sometimes December, he says the window for fall weed control doesn’t always exist on that land. “That is a barrier to fall weed control. It’s pretty tough to manage weeds if there’s snow on the ground.”


Cowbrough, who also serves as project lead on the Ontario Weed Committee, expects his survey results to guide the discussion about weed control research in a new direction.

“Now the hard work can focus on what kind of experimentation do we look at to increase adoption of late-fall weed control. How do we get it done on more acres? How late can we push fall weed control? Agronomists often spend a lot of their time on challenges,” he says. “But we also work with farmers in the province who are top tier in terms of their management practices. It’s good to ask if there’s anything to be learned from what they’re doing and to then ask: ‘Why aren’t more people doing it?'”

The same questions can be applied to cover crops, which the survey recognized as a valuable weed management tool. Ontario farmers know the value of a corn, soybean and winter wheat rotation, with the winter wheat planted after soybeans. Based on typical rotations, that means there is a lot of cropland in Ontario, about two million acres, with nothing on it going into the winter, Cowbrough says.

The survey identified several other management practices that improve onfarm weed control. Residual herbicides, the benefits of using multiple modes-of-action and the two-pass herbicide system made the list, but so did crop rotation, soil fertility and scouting.

Under questions about issues of concern, Cowbrough says he was expecting survey results to focus on things like herbicide resistance or particular weed species. “I was kind of pleasantly surprised that it identified an over-reliance on certain herbicides. That’s positive to me because it’s a proactive recognition that using the same products over and over again is not sustainable.”

The weed scientist was surprised to see that dicamba drift led the list of concerns identified by the survey. While dicamba drift hasn’t caused big issues in Canada, agronomists and farmers are talking about it. “I didn’t expect it to be brought up as often as it was,” says Cowbrough. Canada fleabane (top) and giant ragweed (above) top the list of Ontario’s challenging-to-control weeds.

Other concerns included poor lamb’s quarters control, late-emerging grasses and herbicide resistance. Also on the list were herbicide carry-over and overreliance on group 15 herbicides.

Cowbrough, who took time out from his own spray research to talk to Farm Forum, says his survey shows farmers and agronomists are working together to improve management practices.

“Of the eight terms most frequently identified in the field management part of the survey, only three were herbicide focused,” he says. “For me, the positive is that agronomists are suggesting nonchemical, cultural methods of control. If your goal is to have more integrated weed management strategies, then this is positive news.” FF