Rotations and the Cost of Production

Pulses add multiple benefits to your cropping system

by Megan Madden

We all know canola-snow-canola is not a recommended rotation, but what is the ideal way to integrate pulses into a sustainable rotation? The trend of plant-based proteins, combined with the benefits of pulse crops in a rotation, has encouraged rowers to increase pulse acres across the Prairies. Researchers in Western Canada, in association with commodity groups such as Saskatchewan Pulse Growers (SPG), are taking efforts to understand the impact this change has on factors like soil water and nutrients, and implications of including pulses in rotations on soil biology and health.

Perhaps the most well-known benefit of growing pulses is the positive effect on nitrogen in the soil. Alberta Agriculture and Forestry states that pulses “provide a nitrogen benefit that can replace 10 to 15 kilograms of nitrogen per hectare.” Not only do pulse plants fix nitrogen from the atmosphere for their own requirement for optimal growth, but also leave some nitrogen in the soil. The nitrogen from the decaying roots and plant material of a harvested crop release nitrogen into the soil. The following crops benefit from the soil residual nitrogen through the growing season. This is important for a crop like wheat, which requires nitrogen available during the seed filling period to increase protein content and seed quality.

“Pulses provide significant benefits to crops that require nitrogen, which could be almost any crop,” says Sherrilyn Phelps, Agronomy Manager with SPG. “There are also non nitrogen benefits of including pulses in rotation.”

Manitoba Agricultural Services Corporation’s crop insurance data shows that flax does very well on pea stubble. Phelps suggests this is because flax forms associations with arbuscular mychorrhizal fungi in the soil, which helps the crop access nutrients. Pulses are also mycorrhizae compatible, so they stimulate these fungal organisms in the soil, making them available to the flax. Cereals are intermediate while canola does not form these associations, limiting that specific benefit, but total soil microbial population and biodiversity improve when pulses are included in crop rotation.

Research led by Dr. Yantai Gan and his team at the Swift Current Research and Development Centre, Agriculture and Agri- Food Canada (AAFC), has been studying various rotation systems. “In a 2017 publication, we showed that the inclusion of two or more pulses in four-year crop rotations caused a significant shift in the composition of the soil fungal community, a decrease in fungal diversity, and an increase in the proportion of fungal pathotrophs compared to continuous wheat or rotations with only one pulse crop,” says Gan.

However, depending on the pulse type, for example faba beans or chickpeas, which are partially resistant to Aphanomyces, two or more pulses in a rotation may offer a rotation benefit.

As the project leader, Gan indicated that microorganisms often interact with each other in the soil, and some beneficial microbes can inhibit disease-causing microbes. “Microbial communities in the soil, on the rhizosphere (for example, the soil attached on plant roots), and those within the surface of a root (called endophytic microorganisms) can function differently and the outcomes of the beneficial/pathogenic microbial interactions will have a real impact on the health of the soil,” Gan said. In designing a rotation, canola and pulses are both affected by sclerotinia. This should be considered if growing canola on pulse, or pulse on canola stubble.

“Cereals are ideal prior to pulses,” says Phelps. She recommends growing cereals prior to pulses offers the best weed control for the following year’s pulses. “Pulses can perform well when grown on canola stubble with no-till management” says Gan.
However, volunteer canola management is important if grown prior to a pulse, whereas volunteer cereal control is well established in most pulse crops. Dodsland area farmer Jeff Bennett incorporates lentils into a cereal and oilseed rotation for exactly that reason. “Lentils are a great fit to allow greater flexibility of herbicides,” he says. “It is also a benefit not to rely solely on two crops for income. The year break from buying nitrogen is nice too.” While currently known benefits are primarily nitrogen and fertility related, pulses like peas and lentils are good in a rotation due to their short growing seasons, which leaves more residual soil water in the ground for the subsequent year’s crop. This is a significant advantage in dry soil areas. Conversely — faba beans can help draw down moisture in areas where it is excessive as they are longer in maturity.

Early harvest with some pulses also opens the option of adding winter cereals to a rotation, but the lack of stubble with pulse crops could be a concern for maintaining snow cover and assisting with winter survival.

An AAFC Swift Current research project studied 14 different rotation systems with variable levels of pulse intensity from 2010 to 2016. The four-year rotations included field peas, lentils, and chickpeas from zero to three times with wheat. Peas and lentils before wheat, or the rotation systems with peas or lentils included more than once in the rotations, had the highest residual soil water and nitrogen in the 30-90 centimetre depths. Peas and lentils before the wheat increased the grain yield of the wheat by 26 per cent (peas) and 18 per cent (lentils), compared to continuous wheat.

Regardless of the reason for incorporating a pulse into your rotation, you will gain benefit both in the cropping season, and the subsequent ones, especially when other factors such as disease and weed control are managed properly.