Grass hay is really the best choice for horses. It's what they're designed to eat. I wouldn't be looking at any sort of grain for a horse due to the possibility of laminitis or founder ... unless my veterinarian (holy carp I spelled that right on the first try!)
specifically told me to. Horses that eat a lot of grain seem to require a lot more exercise and work to keep their health up.
Alfalfa hay is directly correlated to the incidence of enteroliths
in horses, which can cause colic — or worse, if they aren't smooth. An enterolith that's been broken by contact with another enterolith in the horse's gut can slice into the horse's intestinal wall. I'm not saying I wouldn't use alfalfa, but I'd treat it like I would grain or a nutritional supplement — very little and only as needed. Again, check with your veterinarian.
When I have to buy supplemental feed, I'll drag myself kicking and screaming down to the co-op and get something from the ADM Alliance line of equine feeds
Pasture grass is my first choice, grass hay my second choice. Everything else fills in around the edges.