A horse's digestion is closely tied to his overall health and well-being. Although people may be able to get away with junk food diet for a while, a horse cannot, and food associated diseases such as colic and founder will become an issue if a good diet is absent.
The horse's stomach is very small and only holds four gallons of material. This means a horse must be fed smaller meals to avoid colic. So if you're feeding more than five pounds of grain or .5% of his body weight in grain, split it into two or even three meals.
In a perfect world, horses should have ad lib access to forage, but this can be difficult in certain parts of the country where hay is costly and pastures aren't available.
A horse's diet should include a minimum of 1% of his body weight in forage to stimulate proper gastrointenstinal motility. So if your horse weighs 1,000 pounds, he should have at least 10 pounds of hay each day. However, this may increase according to particular needs or in the cold areas in the winter when horses need fiber to stay warm.
Free choice water should always be available in order to maintain gastrointestinal health.