Can I just feed my horse hay?
Horses are grazing animals and were designed to eat small amounts of roughage (grass or hay) all day long.  But the short answer to this question is that it isn’t ideal to just feed hay to your horse. Most forages do not have perfectly balanced vitamin to mineral ratios for the horses’ diet.

Horses need roughage and their diet balanced on the basis of protein, vitamin and mineral.  What horses do not need is a lot of excess calories or a high level of simple sugars (simple carbohydrates).  Ideally most horses would be fed hay, possibly pasture and then a ration balancer to make their diet well balanced.  Ration balancers (such as Buckeye Grow N Win, Progressive Ration Balancer or Purina Born to Win) are pelleted feeds primarily composed of protein, vitamin and mineral.  They are not high in calories or starch.  Some horses (BUT NOT MANY) may need extra calories because of their job in life and thus might need “grain”.