The horse has been identified as a non-ruminant herbivore, an animal without a rumen that eats forage. When a horse is out in the wild with thousands of acres of free-roam grazing, and the only external demand is to maintain itself and occasionally run from predators, this configuration serves it quite well, most of the time. However, when a horse is confined to small acreage or total confinement, where the selection of forages is limited, or it is placed under performance demands seen in modern competitions, the non-ruminant herbivore meets with some nutritional challenges. First of all, confinement practices prevent the horse from benefiting from his ability to be a highly selective grazer, grazing only on young, immature forages that his system can digest and utilize efficiently. Second, the modern demands placed on the horse increase his nutrient requirements and horsemen must figure out how to meet those higher nutrient needs in an amount of feed a horse can safely consume and utilize.

An understanding of the horses’ digestive tract, where feedstuffs are digested and how that impacts the end products of digestion, is necessary to help the horse meet these challenges. The digestive tract of the horse is divided into two sections, the foregut and the hindgut. The foregut has similarities to the prececal digestive systems of a monogastric animal such as a dog, man or pig. The hindgut acts more like the rumen of a cow. Both sections contribute to the nutritional status of the horse, but in very different ways. The foregut, more specifically the small intestine, is the primary site of nutrient absorption in the horse. Digestion in the small intestine is accomplished by enzymatic degradation of starches, fats and proteins. The efficiency of this enzymatic digestion is affected by factors including meal size and frequency, particle size, and interactions between diet constituents. These factors affect rate of passage (ROP) of feedstuffs through the small intestine and the surface area of substrate accessible to enzymatic action, both of which impact prececal digestibility coefficients.

Starches

In discussing absorption and storage of starches, one clarification should be made. Many times, the terms starch and carbohydrates are used interchangeably. Is starch and carbohydrate the same thing? Basically, all starches are carbohydrates but not all carbohydrates are starches. Carbohydrates include sugars, starches and fibers. The term carbohydrate arises from the fact that most substances in this class have empirical formulas suggesting they are carbon "hydrates", with the ratio of carbon to hydrogen to oxygen being 1:2:1. For example, the empirical formula of D-glucose, the precursor to all carbohydrates, is C6H12O6.  There are three major classes of carbohydrates: monosaccharides, or simple sugars; oligosaccharides, which are short chains of monosaccharides joined together by covalent bonds and are considered sugars as well; and polysaccharides, which consist of long chains having hundreds or thousands of monosaccharide units and are considered non-sugars. The most abundant polysaccharides, starch and cellulose, consist of recurring units of D-glucose, but differ in how the D-glucose units are linked together. In starch, the monosaccharides are linked together by (-linkages, which are subject to mammalian enzymatic digestion. Cellulose, on the other hand, contains monosaccharides linked together by (-linkages, and can only be broken by cellulase, a microbial enzyme. Often, in the horse’s diet, sugar and starch are referred to as soluble or hydrolysable carbohydrates and cellulose is referred to as an insoluble or fermentable carbohydrate. This is in reference to whether the carbohydrate is hydrolyzed or digested prececally, as with starch, or fermented in the hindgut, as with cellulose. Concentrates and forages consumed by horses contain primarily starch and fiber (cellulose, hemicellulose and lignin), which require breakdown to monosaccharides before absorption. Starch will be digested primarily in the duodenum and proximal jejunum, while the fiber component will be digested in the cecum and large intestine