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Digestion of carbohydrates (sugar, starch, cellulose and other fibres) - Soluble sugars are broken down and absorbed in the small intestine. Monosaccharides such as glucose and fructose will be absorbed directly by the intestinal cells. Disaccharide's and starch will be hydrolysed by the enzymes in the small intestine (amylase and various disaccharidases) before absorption. The small intestine has a large capacity for absorption of simple sugars and starch. The cleavage of common dietary carbohydrates leads to monosaccharides which can be absorbed (glucose, fructose and galactose) according to the following general scheme:

Sugar (Sucrose) -> glucose + fructose
Milk sugar (Lactose) -> glucose + galactose
Maltose -> glucose
Starch -> maltose -> glucose

Young foals cannot digest sucrose in larger quantities until about 7 months of age. Lactose, which is the natural sugar in milk, is digested well until weaning, and then the production of the lactase enzyme which is required for the cleavage of lactose is gradually reduced. At 2 years of age the activity is so low that large quantities of milk should not be fed. Small amounts of lactose can nevertheless be fed to adult horses. It will pass unchanged to the hind-gut, where it will provide a source of energy for the microbes. Larger amounts will invariably upset digestion and may lead to diarrhoea.

A major part of the starch will be broken down to glucose by the enzymes in the small intestine and absorbed. Variation exists however, depending on the type of starch and the amounts fed per meal. More than 95% of the starch in oats and barley is absorbed in the small intestine and about 80% of the starch from maize. Potato starch is much less digestible and will be transported in significant quantities to the hind-gut where it is fermented. When large meals of concentrates are fed, significant amounts of starch reach the hind-gut and undergo fermentation. The energy is absorbed as short-chain fatty acids, but only after a substantial part of the energy is lost as gas and as feed for the micro-organisms.

The horse does not produce enzymes which can break down complex, so called structural plant carbohydrates like cellulose, hemicellulose, pectins and pentosans. These compounds pass through the small intestine, and will first be digested by microbes in the hind-gut. The end products of this fermentation are the short-chain fatty acids:

Acetic acid (C2) Propionic acid (C3), Lactic acid (C3, usually formed in smaller quantities) Butyric acid (C4).

When the diet contains little or no concentrate, acetic acid will represent a major part of the energy which is absorbed from the gut. Butyric acid will also be used in energy metabolism and to produce tissue fats. In addition to these uses, propionic acid is metabolised to glucose and provides an important component of the horses' blood glucose.

The concentrations of the short-chain fatty acids vary with the feed types and the time after feeding. The ratio between the concentration of these acids in the caecum varies in such a way that high roughage diets will give a high acetate concentration relative to propionate, while high grain diets will favour production of propionate over acetate. In the colon the ratio between the acids is less influenced by the type of feeds given.

The acids which are produced lower the pH of the contents in the hind-gut. A pH in the range between 6.6 and 7.5 is maintained by the numerous glands in all of the hind-gut walls which secrete a bicarbonate rich (alkaline) fluid. A diet with little roughage and large proportions of concentrates may ferment too rapidly producing lactic acid and causing the pH in the hind-gut to drop below normal (lactic acid is more acidic than propionate, acetate and butyrate). A low pH in the hind-gut can be detrimental to fibre-digesting microbes that are less tolerant to a low pH than the starch digesting species. In such situations, gas formation is increased, resulting in gas accumulation and possibly colic.

Diets containing poor quality roughage with a low nitrogen content can starve the microbes to such an extent that low digestive efficiency may result. Mixed rations with some starchy material therefore also benefits fibre digestion by providing a readily available energy source for the microbes.

Ratio between short-chain fatty acids in caecum and colon of animals fed diets with different roughage to concentrate ratios:

Hay: Concentrate Acetic% Propionic% Butyric%
1:0 76.2 14.8 8.0
3:2 70.4 21.2 7.2
1:4 61.2 26.0 10.2
1:0 69.5 16.2 7.6
3:2 68.2 15.0 8.8
1:4 67.0 17.0 9.0