Modelling the growth of preruminant calves

Abstract

The emphasis in meat production has shifted from maximizing production volume to the efficient production of lean meat. Body composition can to a large extent be manipulated by nutritional means. It does, however, require integrated knowledge of protein and energy metabolism. Two experiments were performed with male, preruminant calves, to quantify the relationship between nutrient intake (protein, energy) and the rate of gain of live weight, protein and fat in the live weight range 80 to 240 kg. These experiments showed that calves in this weight range respond to increased protein intake by increasing their protein and fat deposition rates. The utilization of digested protein decreased from 60% at low protein intake levels to 35% at high protein intake levels. Extra protein-free energy results mainly in extra fat deposition, but also increases protein deposition rate, even at low protein intakes. Subsequently, the results of these experiments were used for the construction of a dynamic, mechanistic growth simulation model. The model predicts rates of gain of body weight, body fat and body protein (in several tissues) from nutrient intake. It can also be used to predict amino acid requirements. Protein and fat accretion rates obtained in independent experiments could be simulated satisfactorily. Additional measurements, performed in the experiments showed that the rate of muscle protein degradation, measured by the urinary excretion of 3-methylhistidine, increased with increasing protein deposition rates. Furthermore, it was shown that plasma levels of triiodothyronine strongly respond to increased intake of protein-free energy but hardly to increased protein intakes. In contrast, plasma levels of thyroxine and insulin-like growth factor-1 were increased by increasing protein intake, but unaffected by increased protein-free energy intake levels