Alfalfa is what is fed here. Sometimes grass hay but is expensive! And oat hay is fed. Mainly alfalfa . If your horses are not used to it, introduce it over time.
You may find you do not need to supplement with grain.
I'm not sure about the alternative side of it, but remember, whatever you feed your horse, make sure he/she has access to the food all the time, otherwise he/she can get very poorly
Good Luck with your search for an alternative
Alfalfa cubes here , you feed about 10lbs twice a day for an average horse. Be sure the horse can chew properly . Cubes and pellets are hay that is compressed and I would use those over say a complete feed. Be sure the horse has access to salt and water at all times. Sometimes with pellets the horses get piggy and gobble down the pellets and choke, if your horse is a gobbler add large rocks into the feeder to slow it down, and large rocks so it cannot accidently eat them. Again with the alfalfa or an alfalfa mix hay cube you may not need to supplement with anything. Most horses do not need additional feed, hay / water and free choice salt/mineral block is closer to how they were made. Horses are made to eat small amounts all day long. Be sure to put the feed into barrels where they have to eat with the head down in a natural manner.