An Assessment of Essential Amino Acid Requirements of Growing Steers

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Abstract
The total sulfur amino acid, lysine, threonine and tryptophan requirements were studied in Holstein steers (average BW 274 kg). The steers were fed twice daily, at equal intervals, a 9.5% protein-high concentrate ration. Abomasal contents were collected and abomasal nitrogen and amino acid flow were determined from feed and abomasal contents marker to nitrogen ratios. Chromic oxide and lignin were used as indigestible markers. Incremental levels of L-methionine, L-lysine, L-lysine in the presence of 11 g/day methionine, L-threonine and L-tryptophan were infused into the abomasum. Changes in plasma amino acid concentrations were used as response criteria of amino acid adequacy. Only methionine infusion resulted in a two-phase plasma amino acid response curve and the quantity of threonine, lysine and tryptophan available from the digesta met the requirements of these steers. On the basis of the detected abomasal sulfur amino acid passage and the plasma response curve data, the determined total sulfur amino acid (absorbable) requirement was 18.6 g/day for 274 kg steers fed a 9.5% protein ration and gaining .73 kg/day. The plasma amino acid response curve data indicated that the (absorbable) requirement for lysine, threonine and tryptophan was met by the abomasal flow. Thus the lysine, threonine and tryptophan requirements were set at ⩽22.5, ⩽15.1 and ⩽3.5 g/day, respectively. Copyright © 1975. American Society of Animal Science. Copyright 1975 by American Society of Animal Science.

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