The foundation diet for almost all horses is a forage-based low sugar/low starch mineral-balanced diet. This can be as simple as a single type of hay or pasture plus a supplement that complements the forage component, or as complicated as forages from several sources combined with pre-mixed bagged feeds, hard feeds (grain) and a supplement that balances both the forage and feed components.
I like to build a diet using three components:
There are many types of grass hays that can be used for the base forage. If a horse has any metabolic issues, hay with low sugar/starch levels should be used - less than 10% if a horse has active laminitis or difficult to control IR. Warm season grasses such as Bermuda tend to have lower sugar/starch levels, cool season grasses (Timothy, Orchard grass) may be higher. Small grain hay (oat hay, wheat hay) can be reasonable or may get as high as 30% sugar plus starch.
Forage should be fed at 1.5% to 2% or more of the horse's body weight (BW) to maintain good gut function; if the calorie level of a hay is too high, it might not be possible to feed enough without causing excessive weight gain. The high calorie, protein and calcium levels of alfalfa can keep you from feeding adequate amounts of fiber.
Beet pulp is a high fiber forage that can be equivalent or higher than alfalfa for energy, and higher protein and calcium levels than grass hay.