OMAFRA’s Mike Cowbrough gave a presentation at the 2024 Eastern Ontario Crop Conference highlighting the benefits of cover crops.

As weeds increase in resistance, try cover crops, specialist says

Nelson Zandbergen
Farmers Forum

KEMPTVILLE — Weeds are increasingly resistant to the current suite of herbicides used in agriculture, but there are alternative techniques to bolster the dwindling chemical arsenal.

“The reality is, I’ve been around for 22 years, and there are no new herbicides coming into the marketplace that I know of,” OMAFRA weed management specialist Mike Cowbrough told farmers who gathered to hear his weed control advice on Jan. 16 at the Eastern Ontario Crop Conference. “We’re seeing more herbicide resistance, so we just need to utilize other things to try and take the pressure off those herbicides.”

Cover crops, a focus on ensuring efficient canopy closure, a shift to pre-emergence herbicides, and a new weed-zapping technology are all part of the mix. Cowbrough’s advice pertained mostly to weed control in soybeans and corn.

Cowbrough highlighted the benefits of cereal cover crops — especially cereal rye — planted and harvested in advance of soybeans. All cereals exude natural alleopathic chemicals that inhibit weeds, but cereal rye puts off the most, he said. “It’s pretty easy to manage, it’s not overly expensive, it’s pretty easy to kill, and it’s also the one that’s been most researched,” he added.

That said, farmers need to manage their expectations about cereal rye as a weed management tool because the crop has a beneficial impact only about two thirds of the time, according to Cowbrough. Canada fleabane is particularly susceptible. “Aside from herbicide, only cereal rye consistently reduces the amount of population of Canada fleabane,” he said.

While drone-applied herbicide is currently illegal, he pointed out that drones can be used to broadcast cereal rye seed into standing corn.

He also emphasized planting corn at adequate seeding rates to ensure the crop reaches canopy closure as soon as possible to outcompete the weeds. In the case of soybeans, he advised using enough fertilizer and planting narrower rows — 7 ½ or 15 inch rows versus 30 inch — to speed canopy closure and get a jump on weed growth. He described 30-inch rows as the worst for weed control in soybeans. “I think we all inherently know that.”

A move toward pre-emergence or soil-applied herbicides will be a change from the one- or two-pass post-emergent herbicides that predominate now, but the shift will prevent weed escapes from growing too big. One problem is that soil-applied herbicides require timely rain “or it’s a wasted application,” he acknowledged. Last year, only 10 mm of rain fell during the applicable time frame, “which was kind of the worst because it wasn’t enough to fully activate the herbicide, but it was enough to stimulate germination of all the weeds you want to kill.”

Farmers should also know that applying sodium glyphosate to weeds in the fall delivers benefits the following spring, he said. “I’ve rarely seen anyone regret doing anything in the fall to control weeds on the chemical side.”

Using a $100,000 electric discharge weeder is another option. The wide boom functions similar to a chemical weed wicker that kills taller weeds by touching them. But instead of wiping the weed with sodium glyphosate, the metal arm sends a jolt of electricity through the weeds to kill them. The machine also requires a 220-hp tractor to supply adequate current to the boom, according to Cowborough.

On a pessimistic note, herbicide resistance continues to gather steam in Ontario’s most problematic weeds:

• Foxtail populations are now 70 % resistant to Pursuit. One population site in Prescott-Russell is resistant to Assure and Venture.

• Lambs Quarters populations are now 32 % resistant to herbicide, mainly Pursuit and Sencor (or atrazine).

• Canada Fleabane is now 100 % resistant to glyphosate, in Cowbrough’s estimation, with an increasing number of samples also showing resistance to Firstrate. In those cases, growers have to switch to pre-emergence weed control.

• Common Ragweed is 90 % resistant to Pursuit and less than 10 % resistant to Sencor. A population in Prescott-Russell is also resistant to Blazer-Reflex. “That’s a big downer if you’re growing IP soybeans, and you have ragweed,” Cowbrough observed.

• Pigweed is 50 % resistant to Pursuit and Sencor.

• Water Hemp — a newer arrival in Eastern Ontario that has turned up in Stormont, Dundas and Glengarry counties as well as Ottawa and Northumberland — is especially resilient. “One plant can be resistant to Roundup, Sencor, Reflex, Callisto and Pursuit,” he said.

“That takes a lot of tools out of the toolbox. Provincially we’re seeing it resistant to multiple modes of action. If you compare yourself to Essex county … you’re where they were eight years ago.”