Grasses provide biomass for organic matter and late fall or early spring livestock grazing. While the crop is growing, grasses scavenge nutrients such as N and reduce soil profile leaching. After crop termination, the grass residue provides topsoil protection by forming a mulch which helps retain soil moisture and suppress weeds. Over time, the mulch is slowly broken down by soil microbes into organic matter.
The most popular leguminous cover crops are used to fix atmospheric nitrogen, prevent erosion, and add organic matter to the soil. Legumes are not as effective at removing excess nitrogen as other cover crops. Legumes are generally lower in carbon and higher in nitrogen than grasses, resulting in faster breakdown of any residues in the field. Because of the faster breakdown of legume residue in the soil, weed control from legume residue does not last as long, nor does it increase soil organic matter as much as grasses do. Combining legumes and grasses is the most effective system for using legumes as a cover crop.
Forbs include broadleaved plants which do not fix nitrogen. Within this group, cool season cover crop species are dominated by the Brassicaceae family. These plants grow rapidly after fall planting and suppress weeds by shading the soil. As forbs grow, they scavenge excess soil nutrients left from the previous crop. Also, Brassicas provide wildlife a nutritious, late winter food source. After cover crop termination, the tubers rapidly degrade and create holes in the soil for increased water infiltration.