Qualitative assessment of supplemental amino acid needs for growing lambs fed urea as the sole nitrogen source

TL;DR: Methionine was shown to be the first limiting essential amino acid when non specific amino nitrogen needs were supplied by glutamic acid and may provide an index of amino acid status in the growing lamb.

Abstract: 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 con sistently 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) methio nine, 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 in fluence nitrogen retention when infused with methionine, lysine and threonine. Methionine was also shown to be the first limiting essential amino acid when non specific 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.