An unusual type of colic in horses is due to a large stone called an enterolith that forms in the intestine and can partially or completely block it. Colic is common in horses; Kentucky Equine Research indicates part of the reason it's common is that horses have over 100 feet of intestine coiled up in the abdomen in a fairly small space that doesn't allow much expansion. Any significant enlargement of the intestine is likely to be painful, which causes colic symptoms. Nutrition plays an important role in colic and it is especially involved in horses that develop enterolith stones in the intestine.
Enteroliths are stones made up of chemicals in the intestine under certain circumstances and although the cause is not completely known, we know there are several factors that increase the risk of enteroliths forming in the intestine and one of those is feeding alfalfa hay. We don't see a lot of enteroliths in Texas and this may be because we don't feed as much alfalfa as other states.
Other factors that contribute to the formation of enteroliths include the fact that certain breeds like Arabians are more prone to develop the condition. Plus, spending less than 50 percent of their time outdoors, and lack of pasture grazing, also increases the chance of developing enteroliths. Increased levels of magnesium, nitrogen and calcium as well as a high pH of the colon contribute to enterolith formation, and alfalfa contributes to all of these factors.
The stones form around any object the horse may have ingested, including a small rock, nail, coin, metal fragment, cloth or piece of a rope. So if you have a horse that has a history of developing enteroliths, feeding grass hay may be a better option than alfalfa.