Proteins

The digestion of proteins starts in the stomach, and is continued in the intestine by the pancreatic enzymes. The proteins need to be broken down to their building blocks, the amino acids, before they can be absorbed and utilised.

When the digesta reaches the end of the small intestine approximately 2/3 of the digestible protein from grains and protein supplements will have been absorbed compared with only 1/3 of that in roughage. In addition to undigested feed nitrogen, the contents of the ileum also contain nitrogen of endogenous origin, i.e. nitrogen which is derived from blood urea, secreted in to the digestive tract, and from sloughed epithelial cells.

Bacteria inhabiting the hind gut have a short life, just a few hours. All bacteria and protozoa are continuously transported with the digesta towards the anus. Only bacteria which have a high rate of multiplication (or mechanisms for attachment to the gut wall) will therefore be able to live in the large intestines for extended periods of time. The bacteria require nitrogenous compounds for the production of amino acids which are integral components of their enzymes and structural proteins. Nitrogen-containing compounds which enter the hind-gut will be broken down by microbial enzymes to amino acids or further degraded to ammonia. Both amino acids and ammonia are utilised by the same types of bacteria in the production of bacterial proteins. In contrast to ruminants, horses are not able to utilise bacterial proteins, since the absorption of amino acids in the hind-gut, where the microbial amino acids are located, is negligible. Ammonia formed from microbial proteins is nevertheless recycled to the gut as urea, and may provide a significant nitrogen source to horses that are fed diets low in nitrogen. As a consequence of their ability to synthesise amino acids from ammonia, bacteria significantly change the amino acid composition of the digesta (table 8)

Content of amino acids in feed, caecal content and in microbes from the caecum of horses.

Concentrations of each amino acid given in % of total amino acids (molar concentrations):

Amino acid

Feed

Contents

Bacteria

Lysine

3.13

5.82

7.06

Treonine

4.28

5.37

6.70

Isoleucine

3.84

4.61

5.49

Methionine

1.37

1.43

1.72

Alanine

8.47

13.60

10.70