Alfalfa, red clover and lespedeza are examples of legume hays you can feed to horses. Fescue hay infected with endophyte fungus causes reproductive problems in mares if fed during late pregnancy. It is also low in energy and horses don't like it very much. If harvested before it gets too mature, however, it usually works for mature geldings or open mares, providing they have adequate supplementation. Historically, oats have been the first choice of feeds.
Oats are medium in energy, require little or no processing and have more protein than most grains.
However, they are variable in energy content. You should avoid oats with a light weight per bushel because of their low energy and high fiber content. The best oats usually come from the north central states such as Minnesota, North and South Dakota and northern Iowa. Corn is fine for feeding horses, but is highly concentrated in energy. You must take care not to overfeed it.