There are a surprising number of different varieties of grass hay, with timothy being the most widely grown across North America. Timothy is an easy crop to establish on most soils, and it tolerates cold well, sprouting early in the spring, weeks before most other hay crops. Bluegrass, fescue, reed canarygrass, ryegrass, and Sudangrass are some of the other varieties of grass hays fed to horses. Grass hays don't harbor blister beetles, and they often are less dusty than legume hays, making them a preferred choice for horses with respiratory problems. Their more modest protein content makes them a better choice than legumes for mature horses.
Mixed hay is a combination of legumes and grasses.
There are considerable advantages to growing these plants together as one crop. First, horses consider legumes tops in palatability, so picky eaters who turn their noses up at straight grass hay might consume a flake if it contains some legumes as well. Second, a lower-protein grass hay might help balance the high protein level of a legume and create bales that are appropriate to feed to mature horses. Third, the addition of nitrogen-producing legumes to a grass hay crop actually helps fertilize the field and increases the yield of the grass hay. In many parts of the continent, mixed hay is the preferred feed for horses, although the mix might be any of a number of combinations of legumes and grasses.