Less nitrogen equals less volunteer canola

Tighter row spacing may put the squeeze on late-emerging or slow-growing weeds, but not so for volunteer canola

Volunteer canola grows amid soybean test plots.

Looking to reduce volunteer canola in your soybeans? Hold off on that extra nitrogen, or better yet, find a field that’s been depleted.

Standing amid research plots at the Ian N. Morrison Research Farm near Carman, University of Manitoba PhD student Charles Geddes explained some of the work being done to combat volunteer canola during the Manitoba Pulse & Soybean Growers’ annual SMART Day.

Based on earlier trials, he began to examine the effects of residual nitrogen on both volunteer growth and canola yields.

“What I did was compare the residual soil nitrogen at the site, which was chosen for its low residual soil nitrogen, then I supplemented with 23 lbs. of nitrogen per acre, which is still fairly low,” he explained.

The results showed that lower levels of nitrogen did decrease canola biomass and cut down on the number of canola seeds that returned to the soil, without any statistically significant decrease in soybean yield.

Geddes said that given that soybeans fix their own nitrogen, the results are not a huge surprise.

“What we’re doing is increasing the competition that the volunteer canola is going to have with the soybean, because the soybean will still have enough nitrogen when the canola may not,” he said, adding there is a management angle there that producers can use in their own operations.

Rob Gulden speaks about weed pressure at Manitoba Pulse & Soybean Growers’ annual SMART Day.
Rob Gulden speaks about weed pressure at Manitoba Pulse & Soybean Growers’ annual SMART Day. photo: Shannon VanRaes

Use to plan rotations

“If producers are thinking about nutrient budgeting — and if they’re doing soil testing — they can measure how much nitrogen their crops are going to be taking out from that, and sort of plan their crop rotations to grow soybeans after crops that would be taking out more nitrogen,” Geddes said, noting more research needs to be on exactly which crops rotations produce ideal conditions.

Other ongoing research at the site has shown that some common wisdom for reducing weeds may not work when applied to volunteer canola. While planting rows closer together is an effective method of controlling slower-growing weeds, it doesn’t seem to hold true for rogue canola plants.

“That probably has to do with the speed at which those two plants grow — one is faster, one is slower, of course soybeans being the slower one, and canola being the faster one,” said University of Manitoba associate professor, Rob Gulden. “And so reducing the row spacing is not having that big an impact on soybean yield retention or volunteer canola… with later-emerging weeds however, we would expect to see better competition, and there may still be a some benefit.

Research so far suggests that the better way to control canola and maintain yields is to increase seeding rates.

Herbicides are also an option for controlling volunteer canola, but one that comes with added expense and risk, given the continuing spread of herbicide- resistant weeds.

“My thoughts on that is that we’ve got to be careful, we’ve got to use good, sound agronomy, and if we go to wide-row soybean production, herbicides alone are a pretty dangerous game, I would like to see some inter-row tillage,” Gulden said. “With soybeans and corn moving into our rotations, we’re going to see shifts in the weeds, big shifts.”

About the author


Shannon VanRaes is a journalist and photojournalist at the Manitoba Co-operator. She also writes a weekly urban affairs column for Metro Winnipeg, and has previously reported for the Winnipeg Sun, Outwords Magazine and the Portage Daily Graphic.



Stories from our other publications