Digestive Function of Horses
Horses and their relatives utilize cellulose and other fermentable substrates in much the same way as ruminants, but, lacking forestomachs, perform fermentation in their large intestine. Certain other herbivores have also adopted this "caudal fermentation" lifestyle, most notably rabbits and rodents.
Anatomy of the Equine Gastrointestinal Tract
Horses have a simple stomach and, for our purposes here, the stomach and small intestine are unremarkable and similar to other monogastric species. However, the equine large intestine is massive and anatomically complex in comparison to most other animals. The small intestine empties ingesta into the cecum through the ileocecal orifice. The cecum also houses the cecocolic orifice, from which its contents flow into the colon. The colon itself is huge and, by folding on itself several times, is divided into segments and subsegments as follows:
ascending colon (the "great colon" - by far the largest segment)
descending colon (small colon)
The cecum and ascending colon have bands of smooth muscle (teniae) which cause these organs to form pouches called haustrae. The descending colon becomes the rectum at the inlet of the pelvis. Finally, some anatomists simplify the segments of the equine large gut into cecum, ventral colon, dorsal colon and small colon.
Large Intestinal Motility
Motility in the equine hindgut provides the same fundamental services as in the large intestine of other animals: mixing, retention and propulsion of ingesta. Motility in the cecum consists of mixing contractions in which the haustra alternately contract and expand. Additionally, every few minutes the a strong, mass movement-type contraction occurs that forces some of the cecal contents through the cecocolic orifice into the ascending colon.
Within the ascending colon occurs segmentation and haustral contractions that efficiently mix ingesta and expose it to the mucosa for absorption of water, electrolytes and volatile fatty acids produced through fermentation. There are peristaltic contractions that "fight" with antiperistaltic contractions, leading to additional mixing and an overall transit rate that is rather slow (it takes 2-3 days to traverse to the colon). Within the small colon, the predominant patterns of motility are peristalsis and segmentation - segmentation contractions assist in formation of the fecal balls characteristically observed in horses.
Fermentation and Physiology of the Equine Hindgut
Digestive function in the stomach and small intestine of horses occurs pretty much as in any other monogastric animal. Dietary protein is digested and absorbed as amino acids and much of the soluble carbohydrate is hydrolyzed and absorbed as monosaccharides in the small gut. Cellulose and related molecules pass through the small gut intact, although such plant material may be softened and swollen prior to entry into the cecum.
Structural carbohydates like cellulose and hemicellulose, along with starch and other soluble carbohydrates that escape digestion in the small intestine, flow into the large intestine where it is subjected to fermentation.
The large intestine of horses and other hindgut fermenters is a fermentation system analagous to the rumen. The process of fermentation that occurs in the hindgut is essentially identical to that which occurs in the forestomachs of ruminants. Most importantly, horses survive as herbivores because volatile fatty acids are produced in large quantities, absorbed through the cecal and colonic epithelium, and distributed for use throughout the body. One significant difference from the ruminant strategy is that that large quantity of microbial protein generated in the equine large gut is wasted because there is no opportunity there for significant absorption of amino acids.
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