ANN 111: Principles of Animal Nutrition and Feed Technology (2+1)

CARBON NITROGEN BALANCE TECHNIQUE

Measurement of energy retention by the Carbon and Nitrogen balance technique:

• The main forms in which energy is stored by the growing and fattening animal are protein and fat, for the carbohydrate reserves of the body are small and relatively constant.
• The quantities of protein and fat stored can be estimateed by carrying out a carbon and nitrogen balance trial; that is by measuring the amounts of these elements entering and leaving the body and so, by difference, the amounts retained.
• The energy retained can then be calculated by multiplying the quantities of nutrients stored by their calorific values.
• Both carbon and nitrogen enter the body only in the food, and nitrogen leaves it only in faeces and urine.
• Carbon, however, leaves the body also in methane and carbon dioxide and the balance trial must therefore be carried out in a respiration chamber.
• The procedure for calculating energy retention from carbon and nitrogen balance data is best illustrated by considering an animal in which storage of both fat and protein is taking place.
• In such an animal intakes of carbon and nitrogen will be greater than the quantities excreted, and the animal is said to be in positive balance with respect to these elements.
• The quantity of protein stored is calculated by multiplying the nitrogen balance by 100/16 (=6.25), for body protein is asumed to contain 16% nitrogen. It also contains 51.2% carbon, and the amount of carbon stored as protein can therefore be computed.
• The remaining carbon is stored as fat, which contains 74.6% carbon. Fat storage is therefore calculated by multiplying the carbon balance, less that stored as protein, by 100/74.6.
• The energy present in the protein and fat stored is then calculated by using average calorific values for body tissues.
• These values vary from one species to another, for cattle and sheep those used are commonly 9.37 Kcal per g for fat and 5.32 Kcal per g for protein.