Utilisation of feed protein

Abstract

Animal and food characteristics, which exercise an influence on protein utilisation, are discussed in the first part of our paper. Maintenance requirements have been related to the metabolic body weight and amount to 3.0 g DCP/kg WI-11 resp. 3.25 g PDI/kg W 0. 15. Protein requirements for growth depend on the amount of protein deposited by the animal. Protein deposition is higher in bulls than in steers, and higher in late-maturing than in early-maturing breeds. Moreover, protein accretion has been shown to be age-dependent, The information on the efficiency of utilisation of absorbed amino acids for tissue synthesis is still insufficient. Based on the data of INRA, a value of 0.63 has been adopted for our calculations. Amino acids absorbed from the small intestine of ruminants are supplied by undegraded food proteins and by microbial proteins synthesised in the rumen. The available data on protein degradation indicate that as much as 85 per cent and as little as 30 per cent may be degraded in the rumen. The synthesis of microbial protein is essentially governed by the energy supply and by the availability of nutrients, especially nitrogen. To simplify matters, the quantity of microbial protein has been related to the DOM intake, the relevant figure being 135 g microbial crude protein per kg DOM. The true digestibility of (undegraded) food protein is obtained from a regression equation. The values resulting from this equation are in the range of 60 95 per cent. With regard to the true digestibility of microbial protein, a value of 70 per cent is adopted. Examples of the co-ordination of protein requirements and protein supply are given in the second part of the paper. A factorial approach has been used for this purpose. Rations are formulated for two breeds (Friesian and Charolais) at the beginning and the end of the fattening period. The basic feedstuffs to be used are grass silage and maize silage ; a cereal and an meal are assumed to be available concentrates. With the grass silage diet, a forage : concentrate ratio of 60 : 40 (Friesians) resp. 40 : 60 (Charolais) was chosen. With the maize silage diets, the corresponding ratio was 80 : 20 and 60 : 40 for the two breeds. The factorial analysis shows the required crude protein content to be highest for the young animals on the grass silage diet (15.8 15.9 per cent CP in DM). The corresponding value for young animals on the maize silage diets is 14.5 per cent resp. 15.1 per cent for Friesian and Charolais bulls. The appropriate CP content for the heavy animals varies only little with diet and breed (12.0 13.0 per cent). The omission of the oil meal mixture in the second half of the fattening period leads to a deficit in rumen degradable nitrogen. In our model, this gap is filled by the addition of urea. In our example rations, the utilisation of feed nitrogen (N deposition divided by N intake) can be assumed to be 0.22 0.26 at the beginning and 0.13 0.16 at the end of the fattening period.