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Carbon – nitrogen balance and comparative slaughter methods

Carbon – Nitrogen Balance

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 estimated 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.

Carbon in CO2 was determined by an open circuit respiration system.

Energy retention (E, kcal) was calculated as E = 12.386 C (g) – 4.631 N (g).

  1. 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.

  2. The quantity of protein stored is calculated by multiplying the nitrogen balance by 100/16 (=6.25), for body protein is assumed to contain 16% nitrogen. It also contains 51.2% carbon, and the amount of carbon stored as protein can therefore be computed.

  3. 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.

Comparative Slaughter Methods

The comparative slaughter technique is a protocol used to estimate changes in the body composition of birds during an experiment. The protocol is based on the assumption that the body composition of the experimental group of birds (EG) at the beginning of an experiment can be predicted accurately and precisely from the carcass compositions of comparable birds from the same population that are slaughtered at the beginning of the experimental period.

The measurement of energy retention in growing and fattening animals by direct or indirect calorimetry is a very expensive and labour-intensive procedure. In addition to the serious technical background large highly qualified professional staffs is also required.

The comparative slaughter technique (CST) is another method for determining energy retention in animals. This technique requires that at the start and at the end of a trial, representative animals of each treatment are slaughtered and analysed for dry matter, protein, fat, and sometimes for energy. The energy content of the body can also be calculated from protein and fat data. The method is useful for chicken and pigs. But CST is also expensive when applied to large animals.

Accurate results are only be obtained when the time interval between the start and end of the trial is long (thus the weight change is large) and the number of animals per treatment is not too small, otherwise the influence of between animal-variation on the results could be large