Tight rotations may not harm canola yields — but soil health suffers

A long-term study on diversity in rotations produced some surprises, but the benefits were also clear


practicing crop rotation in a field
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Brian Beres wasn’t surprised to find more diverse rotations increase cereal grain yields, improve soil health, and increase microbial biomass.

What surprised him was how canola fared under tight rotations.

Quite well, as a matter of fact.

“It was surprising to see that canola didn’t respond to diversity if you looked at crop response variables like protein and yield — there was no difference between two-year rotations and three-year rotations with canola,” said the Agriculture and Agri-Food Canada biologist.

man with glasses wearing a suit
photo: Brian Beres

“You begin to understand why these guys are getting away with these really tight rotations of canola. It really isn’t impacting their bottom line of grain yield at all.”

Beres’ research began in Lethbridge in 2008 as a way to study how cereal crops might respond to continuous cropping in the event of an issue that requires field isolation. Using triticale “as a proxy” for other cereals, Beres studied rotations that had low diversity (triticale and soft white spring wheat), medium diversity (triticale with peas or canola), and high diversity (canola, triticale, and peas).

The first thing he looked at was yield.

“The highest-diversity rotation of canola, triticale, and peas produced the highest yield, followed closely by just triticale and canola, then by triticale and peas, and finally just the cereal-based system,” he said, adding that growing cereals back to back, year after year, could cut yields by up to 17 per cent.

“Cereals are definitely responding to diversity.”

But while canola yields may not suffer from lack of diversity in tight rotations, soil health does.

“If you start to look below ground, things look a lot different,” said Beres, who will be giving a detailed presentation on his study at Agronomy Update 2015 in Lethbridge on Jan. 20.

“The highest-diversity rotation was creating the most microbial biomass.”

The rotations including peas produced the most microbial biomass, followed — “interestingly” — by a rotation of strictly cereals. Microbial biomass suffered the most in the triticale-canola rotation.

“Even though there’s no difference between grain yield, what’s going on below ground is quite different, where one that has more diversity is ranked first and the other is ranked fifth,” he said, adding that it took more than five years to see these effects on soil health.

“A farmer’s not going to see that with what he’s doing.”

And while peas didn’t yield as well in the early years of the project, rotations with peas in them saw long-term benefits, possibly as a result of nitrogen fixation in the soil.

“It seems like it’s taking a long time, but the importance of those peas over time for both grain yield and some of the variables below ground are starting to have an impact later on.”

Overall, his team found that soil is “definitely not as healthy as it could be” in tight rotations.

“The message that’s missing for canola producers is that diversity equals soil health, even though you’re not seeing reductions in grain yield with tight rotations,” he said.

“I don’t know if you could say there’s a degradation of soil health with tight canola rotations, but certainly, you’re not deriving the kinds of benefits we observe with a high-diversity rotation.”

Besides improving soil health, diversity is important for “optimum production” in other ways, he said.

“You can get away with a tight canola rotation, but eventually that might catch up to you with respect to blackleg and clubroot,” he said. “Diversity should offer less incidence of disease and higher attainable yields, and there should be reduced incidences of other pests, like insects and weeds.”

And with clean durum outpricing canola this past growing season, rotation diversity is “sort of a no-brainer” at this point, said Beres.

“With the new marketing era, I don’t see how there could be a really poor business case with having more diversity in some of these rotations,” he said.

“Today, there’s good markets for canola, but for cereals as well and even peas, so I think there’s a business case to be made.”

Beres will also be giving a presentation on fusarium head blight management at Agronomy Update 2015 on Jan. 21.

About the author

Reporter

Jennifer Blair

Jennifer Blair is a Red Deer-based reporter with a post-secondary education in professional writing and nearly 10 years of experience in corporate communications, policy development, and journalism. She's spent half of her career telling stories about an industry she loves for an audience she admires--the farmers who work every day to build a better agriculture industry in Alberta.

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