High fiber forages such as long stem hay and grasses should provide the majority of an equine diet. Sometimes using an alternative fiber source becomes necessary. Tooth loss or injury to the mouth, face or jaw can make it temporarily or permanently difficult for horse to chew and thus digest hay. Course hay can be especially difficult to chew. Periods of drought or flooding and high fuel prices can make the cost of hay increase significantly. The following link will send you to a fantastic article about alternatives to hay. It discusses feeds that can be used to entirely replace hay such as hay cubes, hay pellets and complete feed. It also covers feeds that are partial replacements for hay such as beet pulp and rice bran. It has a handy chart which will help you figure out how much to feed. I will be keeping this article myself to use as a handy reference when helping you figure out how much of these replacements to feed:)
https://extension.colostate.edu/docs/pubs/livestk/01625.pdf