Benefits touted for adding pulses to rotation

Research conducted at Agriculture Canada has found pulse crops are good for fields, the environment and the bottom line

While a dry fall made for an easy harvest in 2020, it also highlighted the crucial role of moisture and the need to protect available cropping options.

“Most important for pulse crops, a very important, essential role is diversifying wheat grower cropping systems,” said Kui Liu, a research scientist at Agriculture Canada’s Swift Current research centre.

“This is particularly true for the semi-arid Canadian prairie regions.”

Liu was speaking at the Prairie Certified Crop Advisor Board online agronomy research update in December. His work focuses on crop rotation, both from the perspective of a healthy “managed ecology” and that of returns at the farmgate.

The benefits of pulse crops are well known. Liu cited their nitrogen-fixing ability in improving efficient use of the key nutrient, as well as increased yields in subsequent cereal and oilseed crops.

This is borne out by a multi-year crop rotation study Liu and his colleagues are conducting at seven sites across the Prairies. Preliminary results show that an oilseed and pulse rotation delivers above-average yields.

Pulses also help lower agricultural emissions of the powerful greenhouse gas nitrous oxide (N20), which is a timely consideration as the federal government introduces legislation to limit emissions.

Pulses do have their disadvantages. These include low crop residues, which can lead to degraded soil quality, and fungal diseases such as Aphanomyces root rot, which must be managed carefully through crop rotation.

Liu and his colleagues designed an experiment with 14 different rotation treatments using three pulses: peas, lentils and chickpeas. These were rotated with wheat and mustard.

Not all pulses performed the same. While wheat planted on lentil or pea stubble generally yielded better, the effect with chickpeas was less certain. Liu said a possible explanation is lentils and peas have shallow root systems that conserve deep soil moisture.

“The deep roots of chickpea consume lots of water in the deep soil profile and that leaves less water available for a coming crop,” he said.

Chickpeas also underperformed in the nitrogen efficiency department, as measured by total protein yield. Liu attributed this to lower chickpea yields due to weed and disease pressure. In one test, for example, disease felled the chickpea crop in its third year when planted on its own stubble. Wheat-pea rotations came out on top by this measure, with the highest production.

Liu cautioned that while chickpeas appear most susceptible, the other pulses are not immune to disease — something to keep in mind when prices are high and the temptation to shorten rotations is great. For example, when researchers analyzed the rotations for economic success, the lentil-lentil-lentil-wheat fields came out on top, something Liu attributes to high prices for the pulse crop.

“This was a surprise for me,” he said. “I wouldn’t grow lentil crops three years in a row if I was a farmer. We need to be aware that high-frequency lentils might cause severe disease outbreak.”

He explained that the researchers were more aggressive in their fungicide application than is typical for a commercial farm operation, which may have helped them dodge the disease for the length of the study. More research would be needed to see if such an approach would reliably work long-term. In the meantime, Liu reported that the recommended rotation regime reliably delivered good returns.

All three pulse crops appeared to give similar benefits when it came to lower greenhouse gas emissions compared to wheat monoculture. With the cereal, the only source of nitrogen was fertilizer, which is susceptible to runoff, leaching, and off-gassing into the atmosphere.

“This low N20 emission in the three pulse-based rotations probably is because of higher nitrogen use efficiency,” Liu said. “So this result clearly indicated that the pulse crop-based rotations reduced N20 emissions.”

The same held true for carbon sequestration. While pulse crops produce little residue, they do alter the microbiology of the soil and improve its quality.

“It’s very clear that diversifying rotation with lentil crops results in enhanced carbon sequestration compared to the wheat monoculture,” Liu said.

In all, Liu and his colleagues looked at diversified crop rotation with six criteria: yield, nitrogen use efficiency, water use efficiency, energy productivity, economic returns and carbon footprint.

“The take-home message is we should diversify the crop rotation with pulses,” he said, although what this will look like will differ from farm to farm.

“A producer might have different pulse options to diversify rotation, based on local environment and soil conditions.”

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