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Is a soybean-canola rotation worth rolling the dice?

Most farmers aren’t jumping on a soybean-canola rotation, but explosive growth of soybean acres and their westward spread into canola country have some asking the question

Farmers better study up on the hurdles of a soybean-canola rotation before trying it in the field, Manitoba Agriculture specialists say.

Soybeans have been a growing story in Manitoba, rising over the last decade to become one of the province’s main crops with almost 2.3 million acres planted in 2017.

Combined with canola, another high-value crop, prices might be enough to tempt some producers into growing the two in isolation. Manitoba Agriculture pegged target soybean price at $10.50 a bushel, behind canola at $11.25, according to 2018 production cost estimates.

Anastasia Kubinec, manager of crop industry development with the province, says very few farmers have bought into the soybean-canola rotation so far. In fact, she says, farmers have been more likely to add soybeans into their existing wheat-canola rotation.

“All three of those crops are quite easy to grow,” Kubinec said. “You don’t necessarily have to have any new equipment. I mean, (you need) a flex header, but you might have a flex cut header for your canola anyways.”

The longer rotation may bring its own benefits, Kubinec said. Agronomists and researchers have been pushing farmers to adopt longer rotations, both for agronomic reasons and to lower resistance pressures.

The three-year rotation between soybeans, canola and cereal might be a better bet in terms of yield bump, according to yield assumptions by Manitoba Agriculture. MASC numbers show canola yields and spring wheat yields get a boost if planted into soybean stubble. Canola also jumps over expected yields if planted into wheat stubble and spring wheat tends to meet its expected yield when following canola.

Soybean onto canola stubble, however, showed no advantage, something that might be of note for farmers looking at soybean-canola.

Building data

There has been little research done on the soybean-canola rotation so far, said Holly Derksen, field crop pathologist with Manitoba Agriculture.

“We’re pretty unique in Manitoba, moving into Saskatchewan and then maybe North Dakota and Minnesota where we have these two crops in common,” she said. “Other areas, it’s canola that’s the new crop, whereas here it’s soybeans that are the new crop, so we still have a steep learning curve to try and figure out what is the best option.”

Holly Derksen, Manitoba Agriculture field crop pathologist, runs down disease considerations for a soybean-canola rotation during Ag Days 2018.
photo: Alexis Stockford

The researcher was able to find only two studies exploring the rotation, one in North Dakota and one out of the University of Manitoba.

The North Dakota study found few significant differences when they compared yields between canola-soybean, wheat-canola and wheat-soybean rotations.

In 2015, one site returned 7.5 bushels an acre more canola on plots grown after soybeans compared to those grown after wheat. The next year, the same site reported higher soybean yields after canola (55.4 bushels an acre) than wheat (50.6 bushels an acre).

Other factors, like the amount of canola oil, flipped results between sites, while one site noted that soybeans were taller after wheat.

Nutrient considerations

At 33-40 pounds of phosphorus removed per acre in canola and 28-30 pounds an acre in soybeans, both crops have a reputation for eating up phosphorus reserves, something that Kubinec worries might be compounded if they are grown year after year and the farmer is not monitoring.

“There has been the assumption by some new soybean growers that they don’t need to be putting phosphorus down with their soybeans and maybe haven’t been doing that or maybe they haven’t been keeping track of where their phosphorus levels are and they’re not topping up in some of those other crops, like a cereal,” she said.

Farmers may also hit a wall on organic matter long term, she cautioned, since neither soybean nor canola leaves much residue in the field to decompose.

Weeds and pests

Anyone using Group 2 herbicides on their soybeans might get a nasty surprise if they don’t thoroughly wash their sprayer before moving on to canola, but Kubinec warns that those issues may also carry through the soil in a soybean-canola rotation.

“You do need to start looking for some of the herbicide residue issues in the soil or keep that in consideration, depending on your rainfall, where you live, what the recrop period is,” she said.

Volunteer canola may become another serious problem, both in soybean-canola rotations or other, longer rotations with both crops. Kubinec warned that the problem might be particularly pervasive if growers plant Roundup Ready varieties of both crops.

“It can cause quite a few challenges because the canola seed can stick around in the seedbed for many, many years,” she said. “Especially if it’s a wet year and you start getting multiple flushes, you can start getting really high levels of the volunteer canola.”

Breaking up the two crops with a different grain, or choosing a different variety of canola, might help address the problem, she added.

Pathogens

Derksen put soybean-canola rotation, and its disease pressures, under the microscope during this year’s Manitoba Ag Days.

Sclerotinia is one of the canola’s main pathogens, but a soybean-canola rotation is not likely to make that problem worse, Derksen said. The pathogen may survive in the soil for more than five years, too long to be effectively controlled by most rotations, she said. At the same time, taking a break from a sclerotinia-susceptible crop after a particularly bad year might help lower pressure.

Fungicides will be critical to controlling the pathogen in canola years, and, to a lesser extent, in soybeans, she said.

Root rots throw up a different challenge. While both canola and soybeans can contract the pathogen, so may a host of other plants, and the pathogens are general enough that adding a different crop will not necessarily starve out the problem, Derksen noted.

At the same time, the University of Manitoba study on soybean-canola found that continuous soybeans and soybean-canola had more severe root rot compared to soybean-corn or a four-year wheat-canola-corn-soybeans rotation.

Varieties will play a large role in disease control, she said. A taller soybean variety may leave a more open canopy and make the field less friendly to fungus, she said. Row spacing can have a similar effect in soybeans, with wider rows giving more airflow. Sclerotinia-resistant canola might also help.

Fusarium problems

Fusarium head blight is no longer just a cereal problem, according to data presented by Derksen, although she noted the risk to pulses is still very low.

Recent research has found the same pathogen that causes head blight in wheat can also show up as root rot in soybeans. However, only three per cent of fusarium found in the roots was the same pathogen causing trouble in cereals, according to Derksen.

“(It’s) just something to be on the lookout for and to see if maybe it increases now that soybeans are moving into an area of more wheat than they have been previously,” she said.

That topic is just one of many questions remaining when it comes to soybean-canola rotations. In particular, she noted that the province is missing long-term rotation data. Derksen expects the two-year soybean-canola rotation to attract more research in the future, given the popularity of both crops.

About the author

Reporter

Alexis Stockford is a journalist and photographer with the Manitoba Co-operator. She previously reported with the Morden Times and was news editor of  campus newspaper, The Omega, at Thompson Rivers University in Kamloops, BC. She grew up on a mixed farm near Miami, Man.

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