Quantitative Assessment of Supplemental Amino Acid Needs for Growing Lambs Fed Urea as the Sole Nitrogen Source

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Abstract
The supplemental amino acid requirements of growing lambs fed urea as the sole nitrogen source were estimated by nitrogen retention responses to continuous, isonitrogenous abomasal infusions of limiting amino acids. Nitrogen retentions were maximal at equivalents of 0.40, 0.10, 0.16 and 0.10% of the diet for supplementary glutamic acid, methionine, lysine·HCl and threonine, respectively. The response curves confirmed earlier qualitative experiments in which the limiting order of essential amino acids was methionine, lysine and threonine. A relationship existed among nitrogen retention, free plasma methionine concentration and level of supplementary methionine. Plasma methionine remained low and relatively constant as nitrogen retention increased with the addition of supplementary methionine. When nitrogen retention was maximized, further increases in supplementary methionine resulted in an elevation of plasma methionine levels. The same relationship appeared to exist with graded levels of abomasally infused threonine.