Studies on the protein requirements of growing cattle. Effects of differing intakes of protein and energy on growth and nitrogen metabolism in young entire males.

Forty-eight Friesian entire male cattle, with an initial live weight (LW) of 135 kg, were used in two experiments to measure the response to increasing levels of dietary protein (9-11 and 7.5-10.5 g nitrogen X 6.25/kg LW0.75) at differing energy levels (800-900 kJ metabolizable energy (ME) kg LW0.75) over 120-d periods. Digestibility and N balance measurements were also made during the experiments. The diets, which were based on barley and soya-bean meal, were individually fed twice daily. In a third experiment, similar diets were given to four similar animals fitted with intestinal cannulas, at constant energy intake but with variations in dietary protein of 7.5-13.5 g N X 6.25/kg LW0.75. Chromic oxide paper was used as a digesta marker. Positive responses in LW gain and N balance to additional protein were found in both experiments but these were significant (P less than 0.05) only in the second experiment and were associated with significant (P less than 0.01) increases in the digestibility of modified acid-detergent fibre and ME intake. Mean values, which were not significantly different between treatments, for the degradability of dietary protein in the rumen and the efficiency of microbial protein synthesis were 0.57 and 31.3 g/kg organic matter apparently digested in the rumen respectively. Corresponding values obtained by regression analysis were 0.56 and 28.2. The results in general support the Agricultural Research Council (1980) proposals and suggest that undegraded dietary protein was not limiting in these experiments but that rumen-degradable protein levels were limiting on some treatments. Regression analysis indicated that the mean response to additional protein (g LW gain/g N X 6.25) per kg LW was 0.52 in Expt 1 and 0.51 in Expt 2. These responses could be largely explained by increases in ME intakes. Measurements of duodenal amino acid flow showed marked increases in essential amino acids (EAA) across the rumen. However, EAA flows, were not significantly increased at higher N intakes suggesting that protein per se was not limiting in these experiments.