A comparison of various methods for the determination of the digestibility of crude protein in chickens was made. The chemical methods of dividing faeces and urine according to HARTFIEL and EKMAN were tried out, and a new method based on the determination of alpha-NH2-N was developed and tested. Colostomy of hens served as the basis of the comparison for the correctness of the chemical methods. The best approximation to the digestibility quotient of the crude protein according to colostomy was achieved with the alpha-NH2-N-method, which can to a large extent be well automated and can do without N-determination. Faeces-urine-N-separation according to EKMAN also resulted in a good approximation to the values according to colostomy.

The investigations were carried out in order to develop foundations for a complex growth model including the effect of N-efficiency mechanism first of all for the rat. Therefore N-balance trials with rats of different live weights were made using diets of equal protein quality. The curve of the regressively calculated N-efficiency function courses relatively linear in its lower part, i.e. for low N-intakes. The protein quality value "A.b" was proved to be constant for all investigated live weight ranges. In similar trials with different protein qualities was also found a linear course for low N-intakes. The regressively estimated N-maintenance requirement (for N-Intake = 0) related to metabolic live weight (NMR/LWkg0.67) amounts to 180 mg/d. In addition endogenous N-excretions were determined in 15N tracer experiments on animals of different live weight fed graduated amounts of a quality constant protein. The NMR-values (sum of endogenous excretions from faeces and urea) depend on the level of N-intake, i.e. they can not be considered as constant. The endogenous faecal N-loss related to metabolic live weight is relatively constant indeed, whereas the amount of endogenous urea N-loss is dependent on the level of N-intake. Therefore calculations based on the N-efficiency curve, e.g. amino acid requirement dependent on performance and prediction of N-deposition respectively, should not be carried out on the basis of "N-retention" (N-balance + NMR) but only on the basis of N-balance.

11 different feedstuffs (barley, wheat, rye, oat, wheat bran, soybean meal, peanut meal, cottonseed meal, rapeseed meal, horse bean, fish meal) were tested with adult colostomized broiler hens in digestion experiments; the digestibility of the crude protein and the amino acid (AA) balance of the gastro-intestinal tract were ascertained. The results achieved can be summarised as follows: The AA-composition in faecal protein is relatively independent of the AA-composition of the feedstuffs, it is not constant, however. The NPN-quota in faecal crude protein rises with the increasing crude fibre content in the feed. AA-digestibility calculated from the balance of the gastro-intestinal tract is, with the exception of some AA, in particularly the limiting ones, higher than the digestibility of the crude protein. As the AA-balance of the gastro-intestinal tract does not give a realistic picture of the digestibility of the AA, we recommend to ascertain the AA-balance up to the end of the small intestines in order to gain more profound knowledge with the help of such basic investigations with selected feedstuffs.

Male albino rats of a weight of 90 g received a test ration with 15N-labelled wheat as protein source over a period of 1 to 8 days. The 15N-excess in urine, feces, some organs, the remaining carcass and in the blood was determined in dependence on time. On the basis of two different pool models the N-metabolism values could be calculated. Protein synthesis and disintegration rates calculated by means of the N- and 15N-balances of the pool in the steady state of the pool labelling amounted to 252.9 mg N per day resp 203.0 mg N per day. The calculation based on the time-dependent curve of the daily 15N-excretion in urine resulted in corresponding synthesis and disintegration rates of 197.0 mg N per day and 134.6 mg N per day. Labelling curve and results are discussed.

The final product method introduced here is based on the use of 15N L-Lysine as tracer amino acid for the determination of the protein synthesis and decomposition quotas of the total body with the flux of lysine. For this purpose it is necessary to complete the N flow and N compartments of the 3-pool model by the values of lysine content. The ascertained values of protein synthesis--8.4 g/d/kg--and protein decomposition--6.9 g/d/kg--agree very well with those determined with 15N glycine as tracer amino acid.