Annual legumes could play an important role on farms with both grain and livestock, say Montana State University (MSU) researchers.

Rotating wheat with an annual legume grown for forage or grazing can increase wheat yield. The legume would provide forage for livestock and help improve soil fertility and pest management, both of which benefit crop yields, say the researchers.

Studies of legume adaptability and rotations were conducted on conventional and organic fields in eastern and central Montana as part of a USDA-Western Sustainable Agriculture Research & Education-funded study. In those trials, spring-germinating black medic did not provide enough biomass for a good green manure or forage crop. Fall-seeded lentils and winter peas appeared better adapted to the region than medic.

“Winter peas can provide adequate forage for livestock grazing in late spring, or be harvested as hay and fed to livestock with wheat straw,” reports Chengci Chen, researcher at MSU's Central Agricultural Research Center, Moccasin.

On conventionally managed fields at the center, winter wheat grain yield following the winter peas grown for hay or lentil green manure compared favorably to winter wheat following summer fallow. However, lentils grown for grain followed by winter wheat produced the highest net return – three times that of winter wheat-fallow, primarily due to exceptionally high prices for lentils.

On the organic farm near Stanford, the winter wheat grain yield was the same when the crop was grown after winter peas grazed by cattle or lentil green manure. It averaged 8 bu/acre more than winter wheat grown after oats or after winter peas grown for seed.

Although the winter wheat grain yield was the same following grazed winter peas or lentil green manure, the net return was higher from the rotation with grazed winter peas. Chen explains that “The forage value of winter peas for cattle grazing was higher than the value of the nitrogen credit from the lentil green manure.” However, he adds that “The long-term benefits of green manure to soil quality improvement and subsequent returns were not estimated.” Net returns were lowest from the oat-wheat rotation, being 2.8 times lower than the grazed winter pea-wheat rotation.

The compatibility or competition of peas and lentils with wheat was also evaluated for conditions when wheat would grow in a mixture with annual legume volunteers, such as in organic production. On sites without additional nitrogen fertilizer, winter peas and lentils inter-planted with winter wheat had yields half of that when they were planted alone. However, the presence of the legumes did not decrease the winter wheat yield.

In contrast, when 60 lbs nitrogen per acre were applied, wheat yields were around 25% higher when the crop was grown alone than when grown with the legumes. The added nitrogen gave the wheat an advantage over the legumes, causing the legumes to yield up to 25% less than when grown alone.

“The bottom line, though, is total yield per acre,” says Chen.

The total yields of intercrops were greater than the sole wheat crop without nitrogen, which could be attractive to organic farmers. In years of high pea or lentil prices, the additional crop could add to net return per acre, depending on the feasibility of harvesting a mixed crop. When legume prices are low, the volunteer legume can be incorporated into the soil as a nitrogen source to benefit future crops, at no cost to the intercropped wheat harvest. The benefit of intercropping is less pronounced in fields receiving additional nitrogen in the form of commercial fertilizer or manure. Crop rotation with legumes is likely a better option than intercropping on conventional farms.

“In conventional fields, winter pea and lentil volunteers can serve as a cover crop to provide ground cover in the winter and be sprayed out in the spring with a broadleaf herbicide to avoid the crop competition with wheat,” Chen adds.