Enterolith colics are more common in the western US, where alfalfa hay is used as a major source of roughage. They are sometimes called protein colics.
Alfalfa contains high levels of magnesium and has a high protein content. As the protein digests in the colon, ammonium is released and binds with the magnesium and phosphorus, the building blocks of enteroliths. At the same time, alfalfa hay creates an alkaline environment in the colon favorable to the formation of enteroliths.
Some water sources, too, can contain high levels of magnesium. Have your water tested
Reduce your horse's alfalfa consumption. At least half of your horse's daily roughage should be grass or grass hay. Be aware that wheat bran contains high levels of phosphorus, too. A cup of apple cider vinegar twice daily can help to acidify the colon.