Effect of protein and protein-free energy intake on myofibrillar protein degradation in preruminant calves of 120 and 200 kilograms of live weight.

We studied the effect of protein and protein-free energy intake on the degradation rate of myofibrillar protein in preruminant calves in two experiments. Both experiments were similar in design but were performed with two live weight ranges: 80 to 160 kg (Exp. 1) and 160 to 240 kg (Exp. 2). In each experiment, calves were allocated to either an initial slaughter group or to one of 12 treatments (three calves per treatment), which consisted of six protein intake levels at each of two protein-free energy intake levels. Calves were slaughtered and analyzed for body composition when they had reached the target weight. A balance trial was performed when calves had reached 120 and 200 kg BW in Exp. 1 and 2, respectively. Myofibrillar protein degradation rate was measured by the urinary excretion of 3-methylhistidine. Correction of 3-methylhistidine excretion for myofibrillar protein mass was performed based either on the urinary creatinine excretion rate or on estimated body composition during the balance trial. In both experiments, fractional rates of myofibrillar protein degradation increased with increasing protein intake (P < .01). Fractional degradation rates increased with increasing protein-free energy intake in Exp. 1 (P < .10) and in Exp. 2 (P < .05). We conclude that the increased protein deposition rates, reported earlier, induced by nutrient intake are accompanied by increased myofibrillar protein degradation rates. Furthermore, we consider urinary creatinine excretion to be unreliable as an estimator of myofibrillar protein mass in experiments in which a large range of nutrient intakes is applied.