Tryptophan Requirement of Growing Swine as Determined by the Oxidation of an Indicator Amino Acid2

The tryptophan requirement of growing swine was determined using the oxidation of L-[1-14C]-phenylalanine as an indicator of the adequacy of the dietary tryptophan level. Forty crossbred boars (30 to 45 kg) were fed a basal diet containing 16% protein supplied by corn and gelatin. A series of experimental diets containing .05, .08, .10, .15, .20 and .25% L-tryptophan were prepared. The diets were supplemented with crystalline amino acids to provide 135% of the recommended levels. Release of 14CO2 was measured for 1 h following a meal of the experimental diet containing 20 microCi 14C phenylalanine. Increasing dietary tryptophan from .05 to .13% decreased release of 14CO2. Further increases in dietary tryptophan concentration did not significantly influence 14CO2 production. Regression analysis using a two-phase linear regression crossover model indicated that phenylalanine oxidation was minimized by a dietary tryptophan concentration of .13%. It was concluded that at a concentration of .13%, tryptophan was no longer limiting the retention of the other amino acids, thus this is the requirement for maximum protein retention by the young, growing boar.