Reconsider those plans to seed back-to-back to canola this spring, recommend Agriculture and Agri-Food Canada (AAFC) scientists. And they have new data to back that up.
Dr. Neil Harker and other study researchers recently published the results of their five-year of the effects of continuous canola crops in The Canadian Journal of Plant Science. The main findings are in the title: “Canola cultivar mixtures and rotations do not mitigate the negative impacts of continuous canola.”
Researchers know that farmers face financial pressure to tighten their canola rotations. Harker and his colleagues wanted to see if, by changing some variables, they could get better agronomic results with continuous canola.
“Because canola rotations are getting tighter on the Prairies — a lot of specialists would recommend growing canola one out of every three years or one in every four — we looked at the extreme case of growing continuous canola to see if we could mitigate some of the risk in doing so by mixing different cultivar types within years and across years,” says Harker.
The researchers mixed canola varieties within or across years hoping to get enough variation and ability to resist disease “to help mitigate the problem of using the same thing over and over.” Part of the problem with this approach, though, says Harker, is that canola seed companies in Canada don’t share as much of their seed information with the public as they do in some other countries. Specific information about seed genetics is seen as “proprietary information,” so mixing varieties for the study was “a bit of a guessing game.” In Australia, for example, where this information is available, there’s at least the option to try different sources of seeds — ones with different kinds of disease resistances — to see if particular combinations or choices work better than others.
The study results were pretty clear: “If we don’t know any of these disease-resistance backgrounds — and even if you did know them — crop rotation is still better than growing continuous canola. All of the data showed that really well.” Out of all of the different rotations researchers looked at, including all sorts of continuous canola rotations (with different variety mixes), as well as a pea-wheat-canola rotation and a wheat-wheat-canola rotation, the best results for canola were the pea-wheat-canola and the wheat-wheat-canola. “Even one year of wheat and two years of canola didn’t give you anything different than continuous canola.” Canola yield was highest when crops were rotated every three years.
Rotations are important
Why are canola crop rotations so important? Plant pathologist Dr. Kelly Turkington, one of the co-authors of the study, says that rotation, “in its simplest form, lets enough time elapse so you have decomposition of any infested crop residues in the fields. Or, if the pathogen survives on its own, that interval between host crops allows for those structures to lose their viability.”
So, if you have blackleg, for example, rotation allows the canola residue harbouring the blackleg pathogen to decompose in the field, hopefully reducing the blackleg survival enough before canola is planted again three years down the road. This way, the fungus doesn’t build up in crop residues, and the pathogen has less potential to adapt to the resistance sources used in the varieties being grown.
“If you don’t allow sufficient time, those infested pathogen residues remain in the field, and the tighter the rotation the greater the potential for them to survive, plus the greater the at pathogen buildup,” says Turkington. In the disease triangle that plant pathologists like to refer to, “disease is the result of the interaction between a susceptible host, a favourable environment, and a virulent pathogen,” says Turkington. Taking out that host plant in that triangle, the canola, is the best bet so far in getting the blackleg pathogen to die off before the next time the crop is planted.
Both Harker and Turkington say that while there is some potential for tighter canola rotations if farmers had more information about varieties (so they could plant varieties from different seed backgrounds with different disease-resistant properties), longer crop rotations are still the best bet. “Continuous canola is just not sustainable,” in the long-term, because there are many other issues beyond disease resistance to take account of, things like weeds and insects, says Harker.
Sometimes farmers see canola’s profit potential and are willing to lose little yield to gain more income. But, rotation is still better for long term sustainability on the farm, says Harker, if you’re willing to take that bit of a financial hit in the short term.
Dilia Narduzzi is a freelance writer in Dundas, Ont.