The Limiting Amino Acids in Growing Cattle

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Abstract
Responses to abomasally infused amino acid(s) were used to determine the first, second and third-limiting amino acids in rumen synthesized microbial protein in growing steers fed a semipurified diet essentially protein free but presumed to be adequate in total nitrogen (14.2% protein equivalent, N × 6.25). The steers averaging initially 181 kg were kept in metabolism cages and infused continuously with the test amino acid(s). Urinary nitrogen excretion of steers infused with L-methionine was lower than the urinary nitrogen excretion of steers infused with lysine, threonine or tryptophan. In a subsequent study the infusion of a combination of methionine and lysine increased (P<.05) nitrogen retention over the infusion of methionine alone. The combination of methionine, lysine and threonine increased (P<.05) nitrogen retention over the infusion of a combination of methionine and lysine. Plasma concentrations of methionine, lysine, threonine and tryptophan were significantly (P<.05) increased when each of the amino acids was infused singly. The infusion of lysine or a combination of lysine and tryptophan with methionine lowered (P<.05) the plasma methionine concentration below plasma concentration when isomolar concentration of methionine alone was infused. The nitrogen balances and plasma levels of free amino acids responses to postruminal administration of various combinations of amino acids indicate that methionine, lysine and threonine are the first three limiting (and order of limitation) amino acids in growing steers whenever rumen synthesized microbial protein is essentially the sole source of protein. Copyright © 1978. American Society of Animal Science . Copyright 1978 by American Society of Animal Science.