Soybean School: Why tackle herbicide resistance with multiple modes of action?

Leaf browning and burn after glyphosate application could be an indication of herbicide resistance when the weeds do not die, says OMAFRA's Mike Cowbrough.

Many agronomists talk about the importance of multiple modes of action when planning a weed control program. In this episode of RealAgriculture’s Soybean School, Ontario Ministry of Agriculture, Food and Rural Affairs weed specialist Mike Cowbrough shows you why.

We catch up with Cowbrough as he surveys a patch of Canada fleabane that has survived a glyphosate burndown. Soybeans have been planted in the field and are about to emerge and Cowbrough is reviewing his few remaining control options for glyphosate resistant fleabane as the soybeans are ready to break ground.


  • Ideally, the Canada fleabane in this field should have been controlled three to four weeks earlier, but Mother Nature refused to cooperate.
  • It’s 10 days after glyphosate application and Cowbrough is not very optimistic. “We’re seeing yellowing in the growing point – hopefully this dies, but I also see a lot of browning and leaf burn and that sometimes can be an indication that we are dealing with a herbicide-resistant population.”
  • The 10-acre field poses a common dilemma that many growers struggle with related to herbicide-resistant weeds, says Cowbrough. The 20-metre patch of resistant fleabane probably makes up about 1% of the field – it’s a small patch. “So the dilemma becomes… I’m going to use glyphosate in my burndown to control the majority of weeds. But I don’t know if this fleabane is resistant or not. Do I tank mix another mode of action that will cost me another $5, $10, $15 per acre or do I just hope it dies with glyphosate?” he asks.
  • Growers who opt to add another herbicide with another useful mode of action to their tank have a much better chance of controlling the herbicide resistant patch of fleabane. The only option for growers who stick with straight glyphosate is to hand-pull the offending weed patches. “If we let this all go to seed, we’re looking at hundreds of thousands of seeds. This 20-square-metre patch is going to be much bigger next year,” warns Cowbrough.

Click here for more Soybean School episodes.


Bernard Tobin

Bernard Tobin is Real Agriculture's Ontario Field Editor. AgBern was raised on a dairy farm near St. John's, Newfoundland. For the past two decades, he has specialized in agricultural communications. A Ryerson University journalism grad, he kicked off his career with a seven-year stint as Managing Editor and Field Editor for Farm and Country magazine. He has received six Canadian Farm Writers' Federation awards for journalism excellence. He's also worked for two of Canada's leading agricultural communications firms, providing public relations, branding and strategic marketing. Bern also works for Guelph-based Synthesis Agri-Food Network and talks the Real Dirt on Farming.


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