Bone growth rather than myofibrillar protein turnover is strongly affected by nutritional restriction at early weaning of calves.

Our previous study revealed that weaning of calves aged 5 wk (early weaning) resulted in transient nutritional restriction with subsequent repletion. The present study was conducted to examine age-related changes in nitrogen balance, bone growth and myofibrillar protein degradation after early weaning in calves. At weaning, calves used in Experiment 2 had more severe nutritional restriction than those used in Experiment 1 due to a shorter duration of the suckling period (3 vs. 4 wk), a lesser amount of daily milk replacer (500 vs. 600 g) and a shorter period when given solid feed (for the last week vs. throughout the suckling period). In both experiments, nitrogen and calcium retentions were lower at weaning and transiently increased at 6 wk after weaning. However, detrimental effects on nitrogen and calcium retention immediately after weaning were more evident, and compensatory increases of nitrogen and calcium retention after weaning were more pronounced and continuous in Experiment 2 than in Experiment 1. The changes in plasma intact osteocalcin concentrations and urinary hydroxyproline excretion after weaning were similar to those for nitrogen and calcium retention in Experiment 2, whereas fractional degradation rate of myofibrillar protein was constant throughout the study in both experiments. These results suggest that the extent of nutritional restriction at weaning affects subsequent enhanced utilization of nitrogen and calcium, and that age-related changes in nitrogen retention after early weaning partly reflect changes in bone growth regulated by both forming and resorbing activities, independent of myofibrillar protein degradation.