Shift to soybeans changes Manitoba’s weed landscape

The shift to soybeans and corn in Manitoba may be causing a shift in the province’s weed population.

Yellow foxtail was a rarity in 2002 and was around the 30th most common weed in the province.

In 2016, based on a provincial weed survey, it ranked sixth in a list of the most prevalent weeds in Manitoba.

“I was very surprised when I did the survey … how much yellow foxtail there was,” said Hugh Beckie, an Agriculture Canada weed scientist who led the weed survey and spoke at the Manitoba Agronomists Conference in mid-December in Winnipeg.

The weed survey also found that a significant percentage of yellow foxtail plants had resistance to Group 1 and Group 2 herbicides.

Out of 60 fields with yellow foxtail, 19 of them had plants with resistance to a Group 1 herbicide. For Group 2 herbicides, 10 out of 60 fields had resistance.

A Manitoba Agriculture document shows that in 2002 yellow foxtail was mostly found in the Red River Valley and the southeastern corner of the province.

However, by 2016 yellow foxtail had spread across the southern half of Manitoba, all the way to the Saskatchewan border. It just so happens that over the last decade row crops have also spread across Manitoba.

Soybeans, once grown only in the Red River Valley, are now nearly everywhere in the province. Acreage was 215,000 in 2007 and topped 2.2 million in 2017.

North Dakota went through a similar acreage shift toward soybeans and corn, and with that came a shift in weed populations. Species such as waterhemp, commonly found in the U.S. Midwest, have taken hold in the state. Waterhemp was discovered in Manitoba in 2016.

If soybeans are altering the weed population in Manitoba, growers may need to adjust their practices.

“Soybean isn’t a minor crop anymore (on the Prairies),” said Rob Gulden, a University of Manitoba weed scientist.

“We should be starting to think about some of those weed shifts that come with it.”

One thing to think about is crop competitiveness. Yellow foxtail, for instance, is uncompetitive with cool season crops such as cereals and canola.

Manitoba Agriculture says on its website that it would take 50 to 100 foxtail plants per sq. metre to reduce wheat yields by five percent.

In corn, however, a small number of yellow foxtail plants can cause problems.

“If left unmanaged in corn, only five yellow foxtail per sq. metre is enough to reduce yield potential by five percent.”

Herbicide resistance is another issue to think about when it comes to a shift in the weed population.

Soybean and corn growers rely heavily on herbicides to keep weeds in check, which means more spraying throughout the growing season.

“It’s the row crops where the rows are so wide and you’re not covering (the ground) … where we start to select for (herbicide resistance),” Gulden said in 2016.

An example of that phenomenon is glyphosate-resistant kochia. The weed is more common in western Manitoba, but in the fall of 2013 researchers discovered Manitoba’s first case of glyphosate-resistant kochia in corn and soybean fields in the Red River Valley.

Since then, glyphosate-resistant kochia has been confirmed in four Manitoba municipalities, including an RM in the southwest.

Beckie plans to do another kochia survey this year in Manitoba. The survey will also study other broadleaf weeds, such as waterhemp.

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