Your horse is built to graze. To break down the roughage it has evolved to consume, your horse's stomach produces up to 16 gallons of acid every day. If your horse is not grazing all day, filling his belly with high-quality roughage, there is nothing in his stomach to buffer all that acid.
Grazing on high-quality roughage also requires a lot of chewing. Chewing produces saliva, a surprisingly potent acid buffer. Concentrates, on the other hand, do not provide the all-day protection of grazing or stimulate as much acid-buffering saliva production.
Provide the maximum amount of turnout possible. Pasture is the best colic preventative. If your horse is confined to her stall, provide free choice, high-quality hay. Feed mid-maturity alfalfa hay or a grass/alfalfa mix for the biggest buffering bang for your buck. If you do feed grains, use no more than 1/2 pound per 100 pounds of body weight and wait at least 6 hours between grain feedings.