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).
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