The efficiency of
protein utilization for growth in preruminant calves is decreasing with increasing
body weight. In contrast to calves weighing less than 100 kg of
body weight, heavy preruminant calves do not respond in
protein retention to an increased intake of indispensable
amino acids in dose-response studies. The marginal efficiency of
protein utilization is low compared with pigs and milk-fed lambs at a similar stage of maturity. A reductionist approach was taken to perceive the potential mechanisms for the low
protein utilization in preruminant calves. Neither an imbalance in the
dietary protein to energy ratio nor a single limiting indispensable
amino acid was responsible for the low efficiency. Also,
amino acids were not specifically used to detoxify
ammonia. Alternative hypotheses to explain the low efficiency are discussed and result in (i) a reduced post-absorptive supply of
amino acids: e.g. by fermentation of milk in the (premature) rumen or preferential
amino acid utilization by specific tissues; or (ii) a reduced post-absorptive
amino acid utilization: e.g. by decreased
insulin sensitivity, utilization of
amino acids for gluconeogenesis or an asynchronous nutrient supply. In conclusion, several mechanisms for the low efficiency of
protein utilization in heavy preruminant calves were excluded. Other physiological processes which are potentially involved remain to be studied, because the large potential for improving
protein utilization in heavy preruminant calves asks for further exploration of their
amino acid metabolism.