The Marginal Efficiency of Utilization of All Ileal Digestible Indispensable Amino Acids for Protein Gain Is Lower than 30% in Preruminant Calves between 80 and 240 kg Live Weight

Abstract

A previous study showed that the marginal efficiency of utilization of digestible nitrogen for deposition in the body in preruminant calves is only ∼30%. The study consisted of two similar experiments that were performed in two live weight ranges: 80–160 and 160–240 kg. In each experiment, 36 calves were allotted to one of twelve dietary treatments, consisting of six protein intake levels at each of two protein-free energy intake levels. This paper presents amino acid analyses of dietary and body proteins of these experiments with the following goals: 1) to identify possible limiting indispensable amino acids, and 2) to quantify the effect of protein and energy intake on the amino acid composition of deposited body proteins. The marginal efficiency of utilization of ileal digestible amino acids for deposition in the body did not exceed 30% for any of the indispensable amino acids. Increasing protein intake increased the ratio of indispensable to dispensable amino acids in deposited body proteins, likely caused by an increase in muscle-to-bone ratio in the carcass with a concomitant decrease in the proportion of collagen protein. It was concluded that the low marginal efficiency of utilization of digestible milk proteins for growth in preruminant calves of this weight range was not caused primarily by a severe limitation of a single indispensable amino acid in the diet.