Horses have evolved over millions of years as grazers, with specialized digestive tracts adapted to digest and utilize diets containing high levels of plant fiber. They are capable of processing large quantities of forage to meet their nutrient demands. In an attempt to maximize growth or productivity, horses are often fed diets that also contain high levels of grains and supplements. Unfortunately, this type of grain supplementation often overshadows the significant contribution that forages make in satisfying the horse’s nutrient demands and can lead to serious gastrointestinal disturbances.

Digestive Function

Horses are classified anatomically as nonruminant herbivores or hindgut fermenters. The large intestine of the horse holds about 21 to 24 gallons (80-90 liters) of liquid and houses billions of bacteria and protozoa that produce enzymes which break down (ferment) plant fiber. These microbes are absolutely essential to the horse, because the horse cannot produce these enzymes without them. The by-products of this microbial fermentation provide the horse with a source of energy and micronutrients.

The equine digestive tract is designed in this way to allow the horse to ingest large quantities of forage in a continuous fashion. The small capacity of the upper part of the tract is not well-suited for large single meals, a fact that is often ignored by horsemen. Large single meals of grain overwhelm the digestive capacity of the stomach and small intestine, resulting in rapid fermentation of the grain carbohydrates by the microflora in the hindgut. This fermentation may result in a wide range of problems including colic and laminitis