Weed control can handle the cold

What effect will extreme cold temperatures have on early-season weed control?

That’s a question University of Guelph, Ridgetown College weed scientist Dr. Peter Sikkema has been hearing a lot lately. With farm fields across Ontario being punished by extreme cold temperatures that have dropped to nighttime lows of minus 5 degrees C, many growers are wondering if burndowns will be effective and whether soil-applied herbicide will still pack a weed-control punch.

During the Exeter-Mt. Forest agribusiness breakfast meeting on Tuesday, May 12, Sikkema told those attending that residual weed control programs should be up to the challenge.

“To the best of my knowledge, under cold temperatures, there is increased possibilities for crop injury. However, I think the length of residual weed control will not be compromised by these cold temperatures,” says Sikkema. He explains that residual weed control is correlated with how quickly a herbicide breaks down in the soil. Typically, soil-applied herbicides are broken down more quickly under warm, moist conditions.

Cold temperatures will slow down bacteria and fungi activity in the soil that break down the herbicide so the length of residual control should not be compromised, says Peter Sikkema.

“These cold temperatures will actually slow down bacteria and fungi activity in the soil that break down the herbicide so the length of residual should not be compromised,” Sikkema says.

When it comes to the efficacy of burndowns, it’s important to consider the active ingredient. “If you have a systemic herbicide like glyphosate or the Group 4s, my experience has been if you have really cold temperatures, the speed of activity decreases really rapidly,” says Sikkema. “However, if you go back four to six weeks after application, final weed control still seems to be similar — it’s just much slower acting under cold temperatures.”

The story is different, however, for burndowns that rely on contact herbicides, said Sikkema. “The contact herbicides under cold temperatures could compromise weed control in those burndowns so you need to be careful when making a general statement on the impact of cold temperatures on weed control.”

During the breakfast meeting RealAgriculture agronomist Peter Johnson noted that growers also need to be wary of herbicide antagonism when using tank mixes at cold temperatures. “If ever there was a time where antagonism would play, it’s under these kind of conditions,” he said.

Johnson added that many agronomists recommend “splitting out the metribuzins from the glyphosates during these weather conditions if you want good efficacy.” That point was further emphasized by Sikkema. He noted growers will see antagonism with the “clay-based” herbicides atrazine and metribuzin.

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