Intestinal Stones
Dr. Robert E. Bray, Star Milling’s Consulting Equine Nutritionist

Intestinal stones can form in the large intestine of the horse and are also referred to as enteroliths. 

Points of Interest

1. A Quarter is shown for scale in front of the middle intestinal stone

   Under certain conditions, the elements magnesium, ammonium, & phosphate crystallize in the hindgut to form intestinal stones; more common shapes are spherical & tetrahedral
   
   
2. The average number of intestinal stones found in horses may vary, from one to several or many; the average weight ranges from a few grams to several pounds
   
   
3. Dietary factors that contribute to the formation of intestinal stones include: high levels of protein (nitrogen) intake; high levels of magnesium intake; more alkaline pH in the hindgut; presence of a nidus (matrix for the stone to form)
   
   
4. West Coast horses have been more likely to develop intestinal stones compared to other regions in the United States
   
   
5. As stones increase in size, they can cause colic, intestinal blockage and even death
   
   
6. Alfalfa hay grown in the southwestern U.S. contributes a dietary excess of magnesium, nitrogen & calcium to the diet; phosphorus content in alfalfa is modest; the excess calcium acts as a buffer thus contributes to higher pH levels in the hindgut
   
   
7. Alfalfa is a major contributor of the elements that form intestinal stones; nevertheless most horses fed alfalfa do not have problems with intestinal stones
   
   
8. Replacing the forage portion of a diet that is 100% alfalfa hay with a portion or all grass hay will reduce the intakes of nitrogen, calcium & magnesium
   
   
9. Horses that had intestinal stone surgery or diagnosed with intestinal stones should not be feed alfalfa hay, cubes or pellets
   
   
10. Studies indicate that grains or grain containing feed mixes will reduce the hindgut's pH; studies have also indicated that feeding apple cider vinegar will reduce the pH of the hindgut; however there are no studies that indicate feeding apple cider vinegar will dissolve stones in the gut or eliminated the formation of stones with horses being fed a predominately alfalfa forage
    
    
11. Fieldwork has suggested that the pH of drinking water may also be a contributing factor in increasing alkalinity of the hindgut; certain regions in California with elevated levels of water pH (7.6 and higher) have been associated with higher frequencies of horses diagnosed with intestinal stones
    
    
12. Dietary considerations relative to intestinal stone formation should include: eliminating the excess magnesium; eliminating the excess nitrogen by not feeding excess protein; reducing the pH of the digestive tract, balancing the calcium intake; and balancing the dietary ratio of calcium & phosphorus; eliminating the nidi in feeds is not a practical solution
    
    
13. If intestinal stones are a concern then one needs to revisit their use of alfalfa as a primary forage source
    
    
14. Hard drinking water is not a source of excessive magnesium intake; the influence of hard water is more likely through the alfalfa plant's ability to concentrate magnesium in the plant during the growing stage
    
    
15. Wheat bran will not be a source of excessive magnesium intake; bran is usually fed in insufficient quantities to be considered as a major factor