The horse is technically referred to as a hind-gut fermenter. Herbivores which are hind-gut fermenters are characterised by the development of one or more large fermentation chambers in the distal parts of their digestive tract. In the horse both the caecum and the colon are used as fermentation chambers.
The caecum is a blind sac starting at the point where the ileum empties into the hind-gut. It is about 1 m long and contains, in an average-sized horse, 30 litres of digesta. The colon has a complex anatomy and passes in a double horseshoe-like fashion from the junction with the caecum towards the liver in the foremost part of the abdominal cavity. Next it continues to the pelvic flexure at the rear of the abdominal cavity, then forward again to empty finally into the rectum.
The fermentation chambers have dense populations of microbes (bacteria and protozoa, Table 5) which digest fibrous carbohydrates such as cellulose, hemicellulose and pectins, none of which are broken down in the small intestines. The horse, like all other plant-eating animals, depends on microbial enzymes to extract the energy from these fibrous carbohydrates. No digestion of cellulose could take place in a sterile hind-gut.
pH, ammonia concentration, and number of microbes in the digesta in different parts of the digestive tract of a horse on a hay ration :
|36||492||363||Bacteria/gram x 10E+6|
|0.9||2.6||21.4||E. Coli/ gram x 10E+6|
The concentration of microbes in the hind-gut of the horse is approximately the same as that found in ruminant stomachs. The activity and number of microbes are strongly dependent on the nutrients which reach the hind-gut with the ileal contents, the passage rate, and the buffering capacity of the glands of the intestinal wall. Efficient microbial growth depends on an adequate supply of nitrogen (as protein, amino acids or ammonium ions). A certain amount of readily fermentable carbohydrates (starch and monosaccharides) will speed up bacterial growth and protein synthesis.
Short-chain fatty acids (acetate, propionate, lactate and butyrate) which are produced by bacteria during fibre digestion are absorbed into the blood stream and utilised as energy sources by the horse. The bacteria also synthesise large quantities of water soluble vitamins of the B-complex (Table 6), but the extent to which these are absorbed is not known. However, horses with a normal workload do not experience vitamin B or C deficiencies. Horses which are used for hard exercise or endurance competitions are often treated with extra vitamins, although no exact requirements for these vitamins has been given in the international literature.
Concentrations of B-vitamins in feeds and intestinal contents in mg/kg of dry matter
|Vitamin||Feed||Caecum||Colon Start||Colon End|
In the second half of the digestive tract the faeces gradually dries due to the absorption of water and ions.