Forage legumes are unique in their ability to produce high-quality forage to enhance animal performance and the ability to utilize atmospheric N that eliminates the legume plants dependence on soil N.
Biological N2 fixation (BNF) rates are dependent on the infection by an effective rhizobia strain on the root hairs for each legume species. Producers planting forage legumes have the potential to utilize both benefits. However, both forage quality and BNF are influenced by numerous factors, including legume species, management practices, and climate, which determine their contribution to a forage-livestock system. Because of soil and climatic differences, the region of the United States dictates which legume species are grown, how productive they are, and how they are utilized in a forage-livestock system.
Major factors influencing the contribution of legumes are reviewed in this paper followed by a discussion of predominant legume species and their utilization in various regions of the United States.
The review found that producers know that forage legumes contribute N to the forage and crop production systems, but their greater forage quality appears to be the driving force behind planting forage legumes. This may be because forage quality is less complicated, easier to measure, and can be observed in animal performance in contrast to BNF. In the dairy industry, it is obvious alfalfa is grown for its greater quality forage. It appears that dairies are concentrated in areas where alfalfa can be grown economically with or without irrigation.
However the N contribution of alfalfa is also recognized because of its use in crop rotations. Clovers and other forage legumes are grown in grass mixtures primarily for grazing and some hay production. They are planted for both high-quality forage and for their N fixing capacity but, as stated earlier; the good animal performance from the greater forage quality is more visible than the N contribution. Cool-season annual legumes are used as cover crops or in row-crop rotations for N and weed control. In the southeast U.S. cool-season legumes mainly compliment warm-season perennial grasses because their growing seasons only overlap in April and early May. The legume precedes the warm-season grass, which enhances N transfer, provides some spring weed control, and the lesser forage quality of warm-season perennial grass increases the value of the greater legume forage quality. Regardless of the reason for planting forage legumes, both high forage quality and BNF benefits occur. However the forage quality benefit is easier to see and better understood by livestock producers.
Forage Legumes: Forage quality, fixed nitrogen, or both
Gerald W. Evers
doi: 10.2135/cropsci2010.06.0380; Published online 30 Dec. 2010 in Crop Science.
Photo of round hay bales of Alfalfa (Medicago sativa) in a Montana field, USA
By Gary D Robson, courtesy of Wikipedia.org.