[Determination of the protein quality of food and animal feed]

Invited review: Further progress is needed in procedures for the biological evaluation of dietary protein quality in pig and poultry feeds

Abstract. Recently, biological procedures for feed protein evaluation in pig and poultry diets have been based on the amino acid composition of feed ingredients considering the animal's losses during processes of digestion or total protein utilization in a different manner. Such a development towards individual amino acids (AAs) was inevitable according to the disadvantage of traditional protein quality measures, like biological value (BV) or net protein utilization (NPU), to be non-additive in complex animal diets. In consequence, such measures are generally not suitable for predicting the final protein quality of protein mixtures from the individual protein value of feed ingredients. Otherwise, recent measures of AA disappearance from the small intestine up to the end of the ileum (ileal AA digestibility) also do not provide a true reflection of the biological availability of individual feed AAs independent of the extent of taking into account endogenous AA losses during digestion processes. Sophisticated procedures for protein evaluation are needed considering the AA losses, both during absorption and utilization after absorption. Advantages and limitations of important developments in procedures are discussed. Accordingly, the development of an exponential modelling approach is described (the Göttingen approach), which overcomes some of the traditional disadvantages by measuring the individual AA efficiency. Connecting feed protein evaluation, the modelling of quantitative AA requirements, and improved ideal protein concepts offers different fields of application. In addition, as demonstrated by example, the modelling of nitrogen losses per unit protein deposition and the minimizing of this parameter yields a further interesting tool for lowering the nitrogen burden from protein utilization processes. Finally, it is pointed out that traditional laboratory procedures also need to be updated, adapted to current knowledge, and validated according to the increasing hurdles for animal studies from the viewpoint of animal welfare. Modelling is a procedure with the potential to reduce the number of experimental animals significantly. This development needs more attention, higher acceptance, and wider application in the future of protein evaluation.

The nutritional significance of endogenous N-losses along the gastro-intestinal tract of farm animals

In animal production, endogenous protein losses associated with the digestion process are important losses, but difficult to measure. Measuring methods include feeding N-free diets, regression techniques based on amino acid profiles, and separating feed protein and endogenous protein by markers like homoarginine, hydrolysed casein or stable isotopes like 15N. Endogenous losses arise from saliva, digestive enzymes, bile, shedded epithelial cells and mucins and may be extra stimulated by the presence in feeds of antinutritional factors (ANF) such as lectins, trypsin inhibitors (TI), tannins and fibre. The impact of such factors may differ between non-ruminants and ruminants. The magnitude of the effect of the different factors is quantified and some of the consequences for protein deposition and nitrogen losses to the environment are discussed.