If your horse ingests sand, he is at risk for sand colic. Most horses who eat sand don't eat it because they like it -- sand just mixes in with their forage or hay. From there, it piles up like, well...sand. It irritates the intestinal wall and may cause colic. Learn how to prevent sand colic here.
Horses develop sand colic in several ways. If your pastureland is sandy, your horse may ingest sand during normal grazing. Usually, this small amount of sand mixes with food and simply passes from the body in manure but it can irritate the intestinal lining as it makes its scratchy way through your horse. It can cause diarrhea and painful colic symptoms.
Horses fed on sandy ground are more at risk for an impaction-type sand colic. Even if you elevate the food, they will consume large amounts of sand as they nibble on spilled hay and grain. Sand can accumulate in the intestine and hamper digestion. The intestine begins to work less efficiently, and more sand builds up. In time, this load of sand can cause a severe sand colic.
According to the College of Veterinary Medicine at Washington State University, sand colic is more prevalent in the south and southwestern regions of the US. Speak with your veterinarian if you have concerns about your horse's sand intake; there are tests he can conduct to determine your horse's risk.