If hay is sun-cured, it also has high levels of vitamin D. Hay can be extremely variable in protein content. Equines can't digest fiber on their own. They lack the enzymes needed to break the bonds of the polysaccharide fibers contained in hay, so they can't extract the nutrients from the plant material. They are specially adapted to digest the fiber that horses cannot. The two types most people are familiar with are grass hays and legume hays.
Legumes tend to be more nutrient-dense than grasses.
Because of these qualities, they're the preferred hays for young, growing horses as well as lactating mares. Their high protein content makes them less appropriate for mature horses not being used for breeding. In addition, legumes generally are more costly, and in some areas of North America can be infested with deadly blister beetles. 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.