Utilization of amino acids by preruminant lambs. II.* The effects of independent alterations in total nitrogen and total essential amino acid intake

Abstract

Male crossbred lambs weighing 12.5 kg were infusedper abomasum with milk-based diets containing 259, 209 or 175 g crude protein (CP)/kg dry matter. The proportion of crude protein supplied as essential amino acids (EAA) was also varied, to provide diets containing 414, 512 or 613 g EAA/kg CP. This allowed an estimation of the effects of independent alterations in total EAA intake and total nitrogen intake on rates of liveweight gain, and on daily nitrogen balance and metabolizable energy intake. At a given proportion of EAA in crude protein, reductions in the EAA intake reduced the rate of liveweight gain, especially when diets contained 414 or 613 g EAA/kg CP. At constant EAA intake, rates of liveweight gain were reduced when the nitrogen intake was such that the dietary proportion of EAA departed from 512 g/kg CP. Responses in nitrogen balance were similar to those in liveweight gain except that, at constant EAA intake, increases in nitrogen intake improved nitrogen balance. All nitrogen balance responses were less pronounced than liveweight gain responses. Lambs given diets containing 414 or 613 g EAA/kg CP had lower metabolizable energy intakes. This was not due to reduced metabolizability of apparently digested energy, but to differences in apparently digested energy intake. The relative effects of nitrogen intake and the proportion of crude protein supplied as EAA, on liveweight gain and nitrogen balance are assessed by using three-dimensional representations of the responses.

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Part I,Aust. J. Agric. Res., 28: 917 (1977).