PORTAGE LA PRAIRIE, Man. — Standing under a white tent with soybean plots behind him, Jonathan Rosset had a question for his audience of growers and agronomists.

Does anyone in the crowd plant soybeans in wide rows? That is, 20 inches or wider?

“I’ve kind of got some bad news for you guys,” said Rosset, who spoke at SMART, a Manitoba Pulse & Soybean Growers agronomy day for soybeans held July 19 at the Canada-Manitoba Crop Diversification Centre in Portage la Prairie.

“Those wide rows of soybeans really lengthened that critical weed free period…. You had to keep your crop weed free for much, much longer.”

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Rosset, a University of Manitoba masters student in plant science, is studying the critical weed free period for soybeans grown in Western Canada.

The critical weed free period is the length of time that weeds must be controlled to maximize crop yield.

“(It’s) the time in thermal time, growing degree days or corn heat units that you need to keep a crop weed free,” said Rosset, who is conducting plot trials in Manitoba looking at agronomic choices that lengthen or shorten the critical weed free period.

Weed pressure can substantially cut into yield if producers don’t keep soybeans clean during that time.

Soybeans need clean rows, even in Canadian production. | File photo

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“If you have a very heavy and thick weed community … you might look at an 80 percent yield reduction. That’s what we saw in Carman last year,” said Rosset, who cautioned that there is only one year of data so far and full results won’t likely be published until 2019.

At other locations, plots with no weed control suffered yield losses of 10 to 40 percent.

Weed scientists and agronomists in Ontario have studied the critical weed free period for soybeans in that region, but little work has been done in Western Canada.

“We need to fine tune whatever (recommendations) we get from elsewhere and learn what works and what doesn’t work,” said Rob Gulden, a U of M weed science professor who is collaborating with Rosset on the research.

The first year of results, from 2016, showed that row spacing definitely affects the critical weed free period in Manitoba.

Soybean plots with 7 1/2 inch rows had to be kept weed free until the V4 development stage.

“On your wide row spacings, your 30 inch row spacings, you had to keep them weed free past the R1 stage,” he said. “The difference is almost 100 growing degree days.”

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Rosset cautioned the data is only one year of research, but so far it seems as if a 15-inch row is the tipping point.

“Anything past that 15 inch row spacing, now you’re getting into wider rows … and there is a lot more space for sunlight, nutrients and water to be taken up by weeds,” he said.

Gulden said row spacing might be more important in Western Canada because the growing season is so short.

“The further north you move the more important that (rapid) row closure is because we don’t have the (longer) season for compensation.”

Rosset and Gulden are also comparing soybean varieties to see if that affects the critical weed free period.

Varieties can differ in height, leaf size and bushiness, and those differences can make the crop more competitive with weeds, depending on if conditions are wet, dry or cold in the spring.

“It actually makes a difference, and (the differences) are regionally specific,” Rosset said.

“One variety that could be very competitive (with weeds) in one area, it ends up being not competitive in another area.”

In some cases, a different variety extended the critical weed free period to the V4 stage.

“Almost a full development stage longer (compared to) the other two varieties.”

Rosset also presented data on seeding rates and stand density and how they effect weed control.

The recommended soybean density in Manitoba is 180,000 to 210,000 plants per acre. In his plots, Rosset looked at extremes of 270,000 plants per acre and 135,000 per acre.

The plots with the highest concentration of plots shortened the critical weed free period, but few producers are going to target 270,000 plants per acre because the seed cost would be exorbitant.

Rosset needs to generate more data before he can provide comprehensive results to growers.

For now, it’s clear to Gulden that western Canadian growers need to think differently about soybeans when it comes to weed control.

The traditional prairie crops, such as wheat, canola and oats, are more competitive with weeds early in the growing season, so producers don’t need to lose sleep about weed pressure in May and June.

But soybeans are not competitive.

“The guys in Saskatchewan learned that with all their pulse crops,” Gulden said. “Early season weed control is very critical.”