Hay is the result of the cutting, drying and storage of long stem plants, such as various grass or legume species. Grass species include timothy, orchardgrass, ryegrass, fescue, brome and bluegrass; while legume species includes alfalfa (lucerne), clover and birdsfoot trefoil. Both types of plants make excellent hay that can be stored and fed to horses when fresh pasture is not available. There are important differences between legume hay and grass hay that directly affect which you might choose to feed your horse.
Legumes are different from grasses and other plants because they form a symbiotic relationship with bacteria, rhizobia, in the nodules of their roots. These bacteria are capable of ‘fixing’ nitrogen in the atmosphere and air into ammonia and then ammonium, which the plant can then use to make protein (note that protein contains nitrogen in its amino acids). As such, legumes tend to be a very good source of dietary protein for animals that eat them (including humans, as beans, lentils, peanuts and soybean are all from legume plants). In contrast, grasses cannot obtain nitrogen as easily from the environment and, therefore, tend to be lower in protein. Nutritionally, legumes also tend to have higher levels of energy (calories) per unit weight and more calcium than grasses.
For example, above are some average nutritional values of legume and grass hays on an as fed basis (source: www.equi-analytical.com).