The horse differs from other herbivores in that it is a “hindgut” fermenter. Compared to cattle, who primarily digest forage in their stomachs, horses digest forage in their hindgut, or more specifically, in the large intestine.

It is here, in the hindgut, where beneficial microbes break down both fiber and undigested starch into useful byproducts called volatile fatty acids (VFAs). These VFAs are then absorbed by the horse as useful energy. It is estimated that VFAs produced by the horse’s hindgut microbes provide at least 30% of the energy needed by the horse each day.

A healthy hindgut is obviously important to overall horse health. The many billions of microbes that live within the horse’s digestive system live in a mutualistically beneficial arrangement. While the horse provides the microbes with food, the microbes, in turn, provide the horse with VFAs. However, this beneficial arrangement can be disrupted or even severely altered, which can lead to many negative consequences to your animal.

One issue commonly seen in an unhealthy hindgut is called “hindgut acidosis.” The pH of a healthy hindgut of the horse is between the 6.5 to 7 range, which is near neutral. This is a pH where the beneficial fiber-digesting microbes prosper. When a horse is fed too much grain or starch in a meal, the small intestine cannot digest it all. Thus, this undigested excess is passed on to the hindgut. Here, the microbes quickly digest the surplus starch and, as a byproduct, release lactic acid. Under normal circumstances, some starch digestion in the hindgut is tolerated. However, when too much starch reaches the hindgut, more lactic acid is produced, which lowers the pH. If this is severe enough, it will then alter the types of microbes that can live and survive in the hindgut. Simply put, the beneficial microbes begin to die off due to this new acidic environment. To complicate matters, the less beneficial microbes thrive, increase in population, and begin to lower the pH of the hindgut even more by producing more lactic acid.  

Another major concern with hindgut acidosis is that as the beneficial microbes die off, they release endotoxins. These endotoxins are then absorbed by the horse’s digestive tract and are passed into circulation. Then, the endotoxins cause inflammation and, if severe enough, can lead to laminitis as well as other complications, such as colic. These are the major reasons why hindgut health for a horse is an area of concern for equine nutritionists and owners alike.  

The most obvious management tip to lessen the chance of your horse experiencing hindgut acidosis or starch overload is to limit the amount of grain or concentrate fed to a horse at any one meal. In our Feeding Your Horse Often article, we highlight the importance of carefully weighing out your horse’s feed and spacing their meals out throughout the day. For concentrates (or feed), specifically, the general rule is to never feed more than 0.5% of the horse’s body weight during any one meal. For a 1,000 lb horse, this would be no more than 5 lb of concentrate. It is also worth noting that providing high quality forage throughout the day will help support the hindgut and the beneficial forage fermentation microbes.

Another important aspect of hindgut care is feeding prebiotics and probiotics. First, probiotics are also called direct-fed microbials and are actual live microorganisms that can be fed to horses. Probiotics help aid in maintaining the healthy gut microbes in the hindgut of the horse. Prebiotics are not live organisms, but rather nondigestible (to the horse) nutrients that help “feed” the microbes in the gut of the horse. Together, prebiotics and probiotics work synergistically to support the gut microbes. While research in the use of probiotics and prebiotics in horses is still limited, studies have shown benefits. For example, studies have shown prebiotics help with fiber digestion. Other research has shown probiotics help support a horse’s healthy immune system, as well as supporting overall gut health.

To support a healthy hindgut, all Tribute® feeds are fortified with both prebiotics and probiotics. Specifically, our feeds are formulated with Equi-Ferm XL®, which is a microencapsulated active dry yeast. Additionally, select organic minerals and direct-fed microbials are added, which all support gut health. If you are concerned about hindgut health and your current feeding program, please reach out to us directly so we can assist in developing and personalizing a feeding plan for your horse.

Chris J. Mortensen, Ph.D.