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Herbicides Can Set Back Stressed Crops

Herbicides are designed to help crops but they can also hurt them when they’re under stress as many are this year.

Herbicide-tolerant groups are no exception, Anastasia Kubinec, Manitoba Agriculture, Food and Rural Initiatives’ (MAFRI) oilseeds specialist told the 16th annual Crop Diagnostic School organized by MAFRI and University of Manitoba July 5.

“If your crop is stressed, herbicides still impact your crop,” she said. “It’s not like you’re spraying water on it. You have to make sure your plants are actively growing.”

In one plot, a stand of Roundup Ready canola died after being sprayed with double the recommended rate of glyphosate, Kubinec said. She stressed it was an anomaly that hasn’t been seen under field conditions, but it underscores how herbicides can set back stressed crops.


The plot was seeded late and received an inch of rain on already saturated soil before being sprayed, Kubinec said.

“It’s called herbicide tolerance, not herbicide immunity,” she said. “You can still get some plants killed. Even in your Liberty, Clearfield and Roundup Ready you’ll probably get five to 10 per cent of your plants killed by the product you’re spraying on it. Not all of it is absolutely immune to the herbicide. So it’s something you need to watch.”

Glyphosate sometimes injures stressed Roundup Ready canola, but Bruce Murray, DEKALB’s agronomist and former MAFRI weed specialist, says he’s never seen it kill RR canola.

“Once in a while you’ll see a yellow flashing – a chlorosis kind of thing, but within a few days it jumps out of it,” he said in an interview later.

Herbicides of all types work best under ideal growing conditions, Murray said. The crop needs to be growing well so it can metabolize the herbicide; weeds need to be growing well so they’ll take up the herbicide and die.

“It’s not ideal this year,” he said. “It has been tough – everything about this year has been tough, but as long as guys are on label… I’m not worried even with double-ups.”

In addition to crops and weeds being stressed, some farmers might have sprayed when their canola was more advanced than is recommended and boosted the application rate to try and take out weeds that are bigger than they would’ve been if sprayed earlier.


Monsanto’s research shows when Roundup Ready canola received double the recommended rate of glyphosate, at first flower, yields dropped by almost 12 per cent but could be lowered by as much as 35 per cent. (This rate was applied for research purposes and should not be applied by farmers, Murray said.)

“Where field overlaps at this rate would occur, the average yield loss would be 16 per cent but could exceed 36 per cent,” according to a Monsanto publication.

“Injury to the crop from excessive rates or late applications is often visual – chlorosis, growth reduction or poor pod formation – but sometimes it is not obvious. It’s important to note that although visual injury may not be observed there could still be a yield penalty at harvest. The amount of yield loss could be as high as one-third.”

The next generation of RR canola Monsanto hopes to introduce in a few years will tolerate higher doses of glyphosate, which can be applied when the crop is more advanced and control harder-to-kill weeds, Murray said. [email protected]


It’scalledherbicidetolerance, notherbicideimmunity.”





About the author


Allan Dawson

Allan Dawson is a reporter with the Manitoba Co-operator based near Miami, Man. Covering agriculture since 1980, Dawson has spent most of his career with the Co-operator except for several years with Farmers’ Independent Weekly and before that a Morden-Winkler area radio station.



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