Features Agronomy Fertility and Nutrients
Canola thrives on pulse stubble

New research is showing that a previously discouraged rotation may actually outperform canola on cereal stubble. Just one year into the study, the work is filling the research gap on growing pulses before canola.

Fababean green manure produced the highest canola yields. photo courtesy of J. O’Donovan, AAFC.

 “In 2009, we started a study at seven locations across the Prairies to look at the agronomic and economic benefits of growing legume crops in the year preceding canola,” explains Dr. John O’Donovan, research scientist with Agriculture and Agri-Food Canada (AAFC) at Lacombe, Alberta. “We also wanted to see if we could reduce inorganic N requirements for the next canola crop.” The seven locations represented a range of soil zones and growing conditions, all under zero tillage systems.

With rising and volatile prices of N over the past few years, the researchers wanted to investigate if there were any alternative ways of providing nutrients to canola. Research has shown that legume or pulse crops are very good at fixing their own N from the atmosphere and provide a lot of benefits in rotation with cereals but little was known about rotational benefits for canola.

The study included six cropping treatments, pea, lentil and fababean grown for grain, fababean grown as a green manure, and wheat and canola grown for grain. Although fababean is not widely grown due to market and price considerations, it is known to be a good fixer of N. No nitrogen was applied to the legumes, and soil test recommendations were followed for wheat and canola treatments.

In 2010, a hybrid canola variety was grown on all six crop residues. Fertilizer N was applied at five different rates across the treatments, including 0, 30, 60, 90 and 120 kg/ha (kg/ha x 0.89 = lbs/ac). “We saw some very interesting results with some variation at the different sites in terms of the effects of the treatments,” explains O’Donovan. “At virtually all of the locations, the highest canola yields occurred when grown on fababean green manure residue. Although green manure may not be that practical for most farmers, it certainly was an interesting finding.”

Yield on pulse stubble better than cereals
On average, canola yields were 13 bushels per acre higher on fababean green manure stubble as compared to wheat stubble. However, when fababean was grown for seed, the canola yields were similar to wheat stubble. This shows that growing fababean for seed doesn’t really contribute that much to the N economy of canola.

The results of growing canola on pea and lentil stubble were not as high as fababean green manure, but still often outyielded growing canola on wheat stubble. On average, compared to wheat stubble, canola yield increased by four bushels per acre on pea stubble and by about six bushels per acre on lentil stubble. “At some of the sites, the advantage of growing canola on pea or lentil stubble was greatest at the lowest N rates,” explains O’Donovan. “For example, at the 0 and 30 kg/ha N rate, we saw an advantage more than we did at the 60 or 90 kg/ha, which was expected. When the rates of inorganic N were reduced, the advantage of pea or lentil stubble kicked in.”

Canola on canola the worst
One other interesting finding was the result of growing canola on canola stubble. Although this is not recommended due to disease concerns and other risks, the results of the study confirm that the practice can result in reduced yields even over the short term. “Growing canola on canola, and only two years in a row, almost always resulted in the poorest yields across the study,” says O’Donovan.

Beaverlodge Source: AAFC.

“On average, yields of canola grown on canola stubble were four bushels per acre lower than when grown on wheat stubble and about eight bushels per acre lower than when grown on field pea.” At some sites, the yield losses were considerably greater, e.g., at Beaverlodge and Lethbridge.

O’Donovan notes that although they aren’t sure why canola yields were lower when grown on canola, they suspect it may be related to nutrients rather than diseases over the relatively short time period. Disease assessments conducted by Dr. Kelly Turkington at the Lacombe site indicated limited development of plant diseases, including sclerotinia stem rot, which was their main concern. In the current study, two years of canola in a row likely was not enough time to allow for the buildup of diseases. However, other studies have shown that diseases such as blackleg would increase over the long term with back-to-back canola. Growers should therefore avoid growing canola too often in rotation, since there can be short-term yield losses (possibly due to nutrient depletion) and more serious long-term losses due to disease buildup.

“Overall, we found that growing some pulse crops prior to canola can improve yields, especially at low inorganic N rates, and there may be potential for reducing dependence on inorganic N for hybrid canola,” says O’Donovan. “Although likely not practical for most farmers, the best residue was fababean grown for green manure by a big margin. The other big finding is that growing canola sequentially, even two years in a row, will likely reduce canola yield compared to growing canola on other crop residues.”

The results overall were very similar in Beaverlodge and in Lethbridge, especially in terms of the dramatic canola yield increase with fababean green manure, and yield loss when canola was grown on canola residue. This is significant because Lethbridge (and most other sites) received considerably higher than normal rainfall amounts in 2010. Beaverlodge was the exception, and rainfall there was below average. This suggests that the results and conclusions of the study may not be influenced by rainfall amounts.

In 2011, researchers seeded malting barley across all of the treatments using the same five fertilizer rates. “We wanted to find out if seeding malt barley on the different residues at different inorganic N treatments would have an impact on malting barley quality,” says O’Donovan. “One of the issues with malting barley is the low protein requirement, so there has been some reluctance to grow malting barley on pulses. Previous research conducted by Turkington found that growing malting barley the year following pea did increase yields but did not have a major impact on protein levels. In this study, malting barley is grown two years after pulses, so we will see if there is any impact on protein levels.” No study results are yet available. In 2012 researchers plan to return to the legume rotation, pending adequate funding.