The horse is technically called 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 horseshoe-like fashion from the junction with the caecum towards the liver in the front of the horse, to the pelvic flexure on the left, forwards again, and finally empties in the rectum.
The fermentation chambers contain a very dense population of microbes (bacteria and protozoa, Table 5) which can digest fibrous carbohydrates like cellulose, hemicellulose and pectines which are not broken down in the small intestines. The horse is, like all other plant eating animals, dependent on microbial enzymes to extract the energy from these fibrous carbohydrates; no digestion of cellulose would take place in a sterile hind-gut.
pH, ammonia concentration, and number of microbes indigesta in different parts of the digestive tract of a horse fed hay :
Small intestine
|
Caecum |
Colon |
pH |
| 7.4 |
6.6 |
6.6 |
pH |
| 52 |
29 |
54 |
Ammonia-N, mg/l
|
| 36 |
492 |
363 |
Bacteria/gram x 10E+6
|
| 0.9 |
2.6 |
21.4 |
E. Coli/ gram x 10E+6
|
| 0 |
5600 |
0 |
Protozoa/ gram |
The concentration of microbes in the hind-gut of the horse is approximately the same as that found in the ruminant stomachs. The activity and number of microbes are strongly dependant 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. An efficient microbial growth is dependant upon a sufficient nitrogen supply (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 (mainly acetate, propionate, lactate and butyrate) which are produced by the bacteria during fibre digestion are absorbed into the blood stream and utilised as energy sources by the horse. The bacteria also synthesis large quantities of water soluble vitamins of the B-complex (Table 6), but the extent to which these are absorbed is not known. However, horse 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 advised.
Concentrations of B-vitamins in feeds and intestinal contents in mg/kg of dry matter:
| Vitamin |
Feed |
Caecum |
Colon Start |
Colon End |
| B1 |
1.1 |
7.1 |
7.8 |
7.8 |
| B2 |
0.4 |
7.0 |
9.2 |
12.2 |
| B6 |
0.2 |
2.4 |
6.1 |
6.2 |
| Nicotinic acid |
3.0 |
121.0 |
96.0 |
119.0 |
| Pantothenic acid |
0.8 |
39.2 |
34.4 |
20.5 |
| Biotin |
0.01 |
0.2 |
3.8 |
2.3 |
| Folic acid |
0.1 |
3.0 |
4.7 |
2.7
|
In the second half of the digestive tract the faeces gradually dries due to the absorption of water and different ions.
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