Keeping canola out of your soybean fields

Volunteer canola: 
Western Canada’s fourth 
most abundant weed

Keeping canola out of your soybean fields

Volunteer canola is thriving in Western Canada. In the 1970s, volunteer canola was the 18th most abundant weed. Today it is the fourth most common in Western Canada. Volunteer canola is especially problematic where herbicide-resistant crops, like soybeans, are added to the rotation.

Volunteer canola is a unique weed because it is derived from growing canola crops, said Agriculture and Agri-Food Canada research scientist Charles Geddes, who is based in Lethbridge, Alberta. Geddes wrote his PhD thesis on managing volunteer canola in Western Canada, so there are few experts who know more on the subject.

Canola loses a lot of seed during harvest, mostly due to a large potential for pod drop and pod shatter. “When you harvest canola in Western Canada, you lose about 4,300 seeds per square metre on average, which is quite a bit,” said Geddes. “Seed loss can range from 2,000 to 14,000 per metre squared, so that’s where the problem starts.”

Likely because of its recent domestication, canola has a lot of weedy characteristics. “When seed enters the soil seed bank it can actually be induced into seed dormancy, called secondary dormancy,” Geddes explained. “That’s onset by environmental conditions (warmer temperatures, dry conditions, darkness and lower oxygen concentrations) in the seed bank.”

Every time we grow canola, we’re adding seed back into the seed bank. photo: Charles Geddes, AAFC

In Western Canada, that seed bank can persist for anywhere between three to four years. In springtime, conditions can alleviate dormancy and promote germination. Another problem is that crop rotations generally range between two to three years, which means that seed bank persists throughout the rotation. “Every time we grow canola, we’re adding seed back into the seed bank, which creates an ample supply of seed for emergence in subsequent years,” said Geddes.

Herbicide options

A fairly wide range of herbicides work on volunteer canola. The problem is that most of the canola has been modified to be resistant to specific herbicide modes of action.

“Those herbicide-resistance systems work quite well for weed management in your canola crop, but unfortunately, once those seeds enter the seed bank then you have essentially a herbicide-resistant volunteer canola population that can emerge as a weed in subsequent crops,” said Geddes. “That limits the number of effective modes of action that you have for managing volunteer canola as a weed.”

Farmers, especially in Manitoba, are having trouble managing volunteer canola in soybeans because both canola and soybeans are resistant to glyphosate.

A recently graduated University of Manitoba master’s student, Paul Gregoire, conducted his thesis research on evaluating the economic action threshold for volunteer canola in soybeans. They wanted to figure out what density of volunteer canola was required to cause five per cent yield loss in soybean. “In Manitoba, they grew soybean in narrow (19 cm/7.5 in.) and wide (76 cm/30 in.) rows, and measured soybean yield over a gradient of plots with different densities of volunteer canola,” said Geddes. “Overall, they found that a volunteer canola density of about three plants/m2 resulted in five per cent yield loss of soybean when both volunteer canola and soybean emerged near the same time.”

Researchers have been looking at herbicide options in soybean production. Christian Willenborg, associate professor in the College of Agriculture and Bioresources at the University of Saskatchewan, led herbicide research looking at pre- and post-emergent herbicide treatment strategies. Later studies combined both pre- and post-emergent strategies, which Willenborg said provided the best results.

“If only a pre-emergent treatment is used, in many cases the volunteer canola control is good early on, but because canola tends to emerge throughout the growing season, later-emerging weeds are missed,” he said. “In trials with post-emergent treatments only, very often depending on the product, there wasn’t good enough control of volunteers that emerged early in the growing season.

“It became clear that a two-pass approach was going to be the best to manage glyphosate-resistant canola volunteers in glyphosate-resistant soybeans,” he said.

Beyond herbicides

It’s not just herbicides that play a role in controlling volunteer canola. Most control comes from agronomic management practices, which is what Geddes’ research focused on. In particular, Geddes looked at seed bank management of volunteer canola, looking closely at the role of soil disturbance following canola harvest.

“We found that the best thing we can do is actually disturb the soil as soon as we can after canola harvest,” said Geddes. “What that actually does is it promotes seed-to-soil contact and increases the number of volunteer canola seeds that germinate in the fall after canola harvest.”

Geddes advises that growers till as soon as possible after canola harvest to be sure that environmental conditions are still conducive to germination. photo: Charles Geddes, AAFC

The harsh winter conditions in Western Canada are subject to winterkill, so the weed can’t establish. “Anything you can get to emerge in the fall will actually die over winter,” said Geddes.

Geddes advises that growers till as soon as possible after canola harvest to be sure that environmental conditions are still conducive to germination. Using this technique, Geddes found that they were able to half the amount of viable seed in the volunteer canola seed bank over the first winter following canola harvest.

Another, perhaps obvious, method to decrease the amount of volunteer canola in the seed bank is to reduce harvest seed loss. To better understand harvest loss, Rob Gulden at the University of Manitoba assessed grower management and harvest practices in over 300 fields in Western Canada. In general, he found that harvesting more slowly reduced losses.

“If you’re in a rush and going too quickly through the canola field you’re going to be putting more seed through the back of your combine, contributing to greater harvest losses,” said Geddes.

Geddes also recommends focusing on canola agronomy during production. The more evenly the crop matures, the less harvest seed loss occurs. Adequate nitrogen fertilization also leads to more even maturation, he said. Other factors include using a high enough seeding rate, which also contributes to more even maturation, and making sure that combines are adjusted based on the number of canola seeds blown out of the back of the combine. Varieties with pod shatter reduction traits are a big help too.

“I think the solution to the volunteer canola story is going to be a combination of both breeding and agronomy,” Geddes concluded.

“I think the biggest thing is for farmers to adopt both chemical and non-chemical weed management practices in glyphosate-resistant crops, like soybean,” said Geddes. “And that will contribute to a reduction in the intensity of this problem going into the future, as glyphosate-resistant crops increase in seeded production in Western Canada and move their way from Manitoba into Saskatchewan and Alberta.”

About the author


Melanie Epp

Melanie Epp is a freelance farm writer.



Stories from our other publications