Effect of Meal Type and Size on Cecal pH in Horses

The horse’s digestive tract is designed to process small amounts of grass and other forage on a continuous basis. Modern horse management often includes large grain meals, periodic feeding of hay, and long stretches when the horse has little or nothing to eat. These variations of the natural eating pattern can upset the microbial balance in the horse’s gut, increasing the risk of digestive upsets.

Meal type and size affect starch digestion in horses. One effect of digestion relates to pH balance in the hindgut. For instance, cecal pH will be significantly lower at four, five, and six hours after feeding 13 lb (6 kg) of sweet feed to horses compared to when only hay is fed. When concentrate is fed, lactic acid levels are also higher.

Fecal pH varies between horses as well as by time of day and diet in the same horse. For this reason, it is not practical to say that one specific number is the optimal pH for all horses at all times. In one measurement of fecal pH, grass-fed horses produced feces with a pH of 6.75; racehorses fed 70% oats and 30% forage had an average fecal pH of 6.38; and forage-fed ponies had a value of 6.49. However, it is reasonable to say that extreme changes in the hindgut caused by grain overload are a major cause of laminitis in horses. With grain overload, cecal pH could fall as low as 5.72 within eight hours of the meal. This change may be followed by a significant increase in Lactobacillus microorganisms, the death of favorable organisms, a resulting release of endotoxins, and an accumulation of lactic acid.

In a presentation at the 12th Equine Nutrition and Physiology Symposium, researchers reported on a study1 in which horses were fed 1 to 2, 2 to 3, or 3 to 4 g of starch/kg body weight/meal using either oats or corn as a source of the starch. As expected, at all levels of intake, cecal pH was lower when corn was fed, and the differential between the two cereals increased in proportion to starch intake. Increasing intakes to the levels generally accepted as “safe” by some nutritionists did not significantly reduce cecal pH. However, feeding the same level of corn starch caused a marked reduction in cecal pH to values close to 6.

The researchers considered that a cecal pH of 6 represented subclinical acidosis, and pH values below 6 signified considerable risk for the development of clinical conditions.

1Radicke, S, E. Kienzle, and H. Meyer. 1991. Preilial apparent digestibility of oats and corn starch and consequences for cecal metabolism. Proceedings of the 12th Equine Nutrition and Physiology Symposium, Calgary, Canada, pp. 43-48.