Quantitative Evaluation of the Tryptophan, Methionine and Lysine Needs of Adult Swine for Maintenance

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Abstract
Nongravid gilts averaging 145 kg were used to determine maintenance (1 g nitrogen retention/day) and nitrogen equilibrium requirements for tryptophan, methionine and lysine. Nitrogen balance increased linearly with levels of each of these amino acids. Urinary nitrogen derived from urea and ammonia decreased, but that derived from creatinine remained constant, as the amino acid level increased from zero to adequacy. From 80 to 90% of the total urinary nitrogen could be accounted for in these 3 components. Expressed as mg/day/kg3/4 the requirements for maintenance were: tryptophan, 5; methionine, 26 and lysine, 25. The adequacy of the determined maintenance requirements was tested by feeding diets containing the 7 essential amino acids at their calculated requirement levels, with graded levels of glutamic acid (nonessential amino nitrogen). A linear response to glutamic acid resulted, and the observed maximal nitrogen balance was only slightly short of the predicted balance of 1 g/day. Cystine could supply 94% of the maintenance requirement for total sulfur-bearing amino acids.