Utilization of proteins for growth and egg production

The amino acid requirements of poultry and the utilization of proteins in poultry nutrition have been studied intensively in the last 20 years.In the process this species, and above all growing chickens, have become the most thoroughly tested animals with regard to their nutritional requirements.On the other hand, the introduction of new protein sources like the single-cell proteins has created a lot of new problems which have not been solved as yet, although the amino acid patterns and the digestibility of the proteins seem to be fairly good. I . Methods of measuring protein utilization and recent experimental resultsSince experiments with higher animals are expensive and time-consuming, fastgrowing birds which at the same time have a high reproductive rate have become very popular.In recent years Japanese quail have frequently been used.The most important procedures are experiments measuring growth or egg production, and balance techniques.As the protein needs of birds are comparatively high, greater amounts of supplementary protein sources are necessary.Above all, in the nutrition of the growing chicken the amount of protein contributed by cereals is relatively low.Therefore lysine, the limiting amino acid in cereals, does not play as important a part as methionine, which is limiting in the oil-seed proteins (see e.g.Erbersdobler & Gropp, 1971).I n rations for laying hens lysine may become marginal, mainly if the protein content of the diet is reduced to a low level and if greater amounts of proteins poor in lysine are used.In this connexion the use of newly developed lysine-rich cereals can be very helpful, as can be seen from two experiments with 'opaque-2' maize shown in Fig.I (Zucker, Erbersdobler & Gropp, 1968; Chi & Speers, 1973).In practical rations for broiler chickens these new cereal varieties cannot be recommended (see also Erbersdobler & Gropp, 1971).They can, however, be very useful if protein sources poor in lysine or tryptophan are introduced more liberally.Following the introduction of mixtures of crystalline amino acids, the requirements of poultry have been studied exactly, and reference amino acid mixtures have been defined for many purposes.We use a mixture with higher amounts of essential amino acids than is used in Illinois (see e.g.Huston & Scott, 1968; Sasse & Baker, 1973), but similar to the mixture of Lee & Blair (1974).