The Benefits of Grass–Legume Combinations in Cover Cropping

Growers often plant cover crop mixtures to get more out of a single sowing, and the most common pairing is a legume with a grass. Popular combinations include hairy vetch or red clover with winter rye where winters are cold, and bell beans with oats where winters are mild. Pairing a legume with a grass has several advantages:

Bell beans with winter rye and oats

A fall planting of bell beans with a rye-and-oat nurse crop, sown to compare nitrogen-fixing fall covers. About seven weeks after an early-September sowing, the beans are filling in and the grains have held back some of the annual weeds.

• Together, legumes and grasses even out the carbon-to-nitrogen balance of the residue once it's turned in. Nitrogen-rich legumes decompose fast, while carbon-heavy grasses break down slowly. Mixing them supplies nitrogen to help decompose the grass residue and shortens the wait before you can plant the next crop.
• A quick grower like oats or winter rye acts as a nurse crop, sheltering slower, less vigorous plants such as red or sweet clover. The grass establishes fast and crowds out weeds that would otherwise compete with the legume.
• Mixing short and tall species covers more ground and suppresses erosion and weeds better than either alone. Tall sorghum and low cowpeas, for instance, shade the soil together and keep weed seeds from sprouting.
• Adding legumes raises the nutritional value of hay meant for livestock.
• Mixtures also spread the risk of a failed stand and cope with variation across a field: where the legume struggles, the grass may thrive, and the reverse.

Further Reading

• Cover crop decision tool: 5 steps to choosing what to plant, when and where
• Cover crop species for organic farming systems: an introduction, from eOrganic.
• Cover crops and green manures: an overview, from ATTRA, the Sustainable Agriculture program of NCAT.