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Qualitative Assessment of Supplemental Amino Acid Needs for Growing Lambs Fed Urea as the Sole Nitrogen Source¹

Department of Animal Science, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801

The qualitative amino acid adequacy of ruminally synthesized protein for growing lambs was evaluated by nitrogen retention responses to abomasal infusions of amino acids. The experimental animals were fed a controlled level of semipurified diet containing urea as the sole nitrogen source. Methionine alone consistently increased nitrogen retention above isonitrogenous urea-infused controls. Lysine produced a positive nitrogen retention response only after supplemental methionine was provided. Threonine increased nitrogen retention only when infused concurrently with methionine and lysine. The data suggested that the limiting order of essential amino acids for growing lambs fed urea as the sole nitrogen source was: 1) methionine, 2) lysine and 3) threonine. An abomasal supplement of the combination of methionine, lysine and threonine increased nitrogen retention 60% above the urea infusion control values. Tryptophan, histidine or leucine did not significantly influence nitrogen retention when infused with methionine, lysine and threonine. Methionine was also shown to be the first limiting essential amino acid when nonspecific amino nitrogen needs were supplied by glutamic acid. Plasma amino acid patterns appeared responsive to infusions and may provide an index of amino acid status in the growing lamb.

² Present address: Wyzsza Szkola Rolnicza, Krakow, Poland.

Manuscript received 12 June 1970.

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