Weed-free fields called impractical

A pristine field of soybeans or corn, completely free of weeds, can be a beautiful sight.

However, it doesn’t make agronomic sense to maintain that immaculate look throughout the growing season, says a University of Manitoba weed scientist.

“With those wide, wide rows (in corn and soy), guys drive by and if there’s something green in between those rows, bad news. You’ve got to kill (it),” said Rob Gulden.

“I think there is still a lot of that mentality that any weed is bad.”

Jeanette Gaultier, a Manitoba Agriculture weed specialist, said she’s heard of cases where a producer applied glyphosate five times in a year.

“Anecdotally it sounds like it does happen,” Gaultier said.

“For corn, I’ve even heard of three to four in-crop (applications)…. I think there are some guys … that hate seeing weeds in the field.”



The desire to maintain a perfectly clean field can be risky because extra applications of glyphosate can promote weeds with resistance to the herbicide, Gulden said.

“It’s a bad idea, in my opinion. You drive the resistance to possibly happen sooner.”

In 2014, Agriculture Canada scientists found glyphosate resistant kochia in Manitoba’s Red River Valley, a region known for growing corn and soybeans.

Gulden said there’s a relationship between glyphosate resistance and row crops because they are less competitive against weeds than cool season crops such as canola, wheat and oats. Therefore, growers rely heavily on glyphosate to control weeds that pop up between the rows of corn and soybeans.

Gulden said growers who already have glyphosate resistant weeds should control all the weeds to prevent the spread of resistant seeds.

However, producers who don’t have resistant weeds should focus on weeds during a time called the critical weed-free period.

Ontario Agriculture says the critical period for corn is between the third and eighth leaf on the plant, while for soybeans it’s between the first and third trifoliate leaf stage (V2-V3).

“If the crop is kept weed-free for the critical weed-free period, no yield reduction should occur,” the ministry says on its website.

“Weeds emerging after the critical weed free period will not affect yield.”

Volunteer canola is an example of such a weed. It appears in Western Canadian soybean fields in late June and July. Gulden said yellow flowers in a green soybean fields are unsightly but there’s no need to worry about yield loss.

“(By) the time you see it and it looks real ugly, it’s probably too late from a yield perspective,” he said.

“(It’s best to) look away at the end of the season.”

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