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Sweetclover as fallow replacement offers healthier, more productive soil

Date posted: June 20, 2002

An old Prairie crop is showing new benefits to improve farmers’ bottom lines and boost the health of Prairie soils.

New studies at Agriculture and Agri-Food Canada’s Lethbridge Research Centre show that using sweetclover as a fallow replacement can increase soil nitrogen and dramatically reduce weeds, resulting in higher subsequent crop yields and lower fertilizer and herbicide requirements. This adds to the fundamental benefit of cover crops to greatly reduce the risk of soil erosion, ensuring healthier, more productive land over the long-term.

“Farmers are always looking for ways to get more out of their crop rotations and we believe there’s a huge untapped scope for using cover crops,” says Dr. Bob Blackshaw, a weed scientist who led the studies along with Dr. Jim Moyer. “Sweetclover is just one example. This crop has been around a long time on the Canadian prairie, but its advantages as a fallow replacement are only now fully coming to light.”

Fallow is traditionally used to conserve soil moisture, but the trade-off is greater soil erosion and reduced soil quality, says Blackshaw. A major research thrust at Agriculture and Agri-Food Canada over the past 20 years has been to find ways of reducing fallow.

“The shift to low till and direct seeding systems, along with improved rotations, have reduced fallow by roughly 60 percent over the past two decades,” he says. “The next step is to look at systems that still have fallow, and find better ways of managing that fallow year.”

Over three cycles of a three-year experiment, the researchers examined the benefits of yellow sweetclover undersown in field pea, mustard and flax. Sweetclover provided excellent groundcover throughout the 20-month fallow period and compared favorably to straight fallow in several key categories. It boosted soil nitrogen by 16 to 56 kg/ha, which increased wheat yields in the third year. Most impressively, sweetclover had a tremendous impact on weed suppression, reducing overall weed densities by 75 to 97 percent.

“We were amazed at the weed results,” says Blackshaw. “We thought sweetclover would help suppress weeds, but I don’t think we had any idea we could get this kind of powerful, long-lasting weed control. The results suggest this is at least partly due to allelopathic compounds released from decomposing sweetclover.”

Sweetclover’s potential to deplete moisture was a major concern heading into the study, but the effects of the drought-tolerant crop were minimal, he says. “We were pleasantly surprised. We did have less soil moisture in the first summer, but we ended up trapping more snow in the winter, which tended to equalize things out. In years with reasonable moisture, the difference will not be enough to significantly affect the following crop.”

Sweetclover is likely to also work well as a companion with wheat or barley, he says. The crop can slightly hinder weak competitors such as flax, but this is unlikely with hardier cereals.

Sweetclover is sown at the same time as the main crop and, as a biennial legume, farmers can underseed it right away in year one, says Blackshaw. “Sweetclover continues to grow into the fall after the main crop is harvested - so right away you’ve got fantastic ground cover. It survives the winter and traps snow, and resumes growth in early spring.” Producers can terminate growth at the bloom stage, which is usually around the end of June or early July.

Organic producers widely use sweetclover as a green manure crop, says Blackshaw. The crop’s weed suppressing ability will add to their management flexibility. Sweetclover Weevil is a potential pest, but it is rare where sweetclover is grown periodically.

Research on sustainable crop rotations conducted at Agriculture and Agri-Food Canada’s Lethbridge Research Centre illustrates the government’s commitment to support research that minimizes environmental impacts as proposed in the new Agriculture Policy Framework. The framework should help increase and improve the use of farm management systems that enhance efficiency and performance, and secure the long-term sustainability of producers’ operations.