Phenylalanine Requirement of the Weanling Pig and its Relationship to Tyrosine

The weanling pig's requirement for total dietary aromatic amino acids (TAAA) and the portion of this requirement that can be efficiently provided by tyrosine were studied. An amino acid-fortified corn diet that was first-limiting in TAAA was used in all feeding trials. Chick and microbiological growth assays established that the corn contained .31% bioavailable phenylalanine and .48% bioavailable tyrosine. The optimum crude protein level for the basal corn diet (adjusted by varying the level of L-glutamic acid) was found to be 12% (i.e., 6.72% supplemental glutamic acid), since pigs gained faster and more efficiently at this protein level than at 8, 16 or 20% crude protein. The TAAA study consisted of adding varied levels of L-phenylalanine and L-tyrosine to the diet containing 12% crude protein, .24% L-tyrosine and .16% L-phenylalanine. Gain and gain/ feed were assessed in young pigs (10 to 20 kg) during a 16-day feeding trial. Optimal gain and gain/feed occurred at .56% TAAA. Furthermore, there was no depression in gain or gain/feed when tyrosine was increased from 43% to 49% of the TAAA in the diet containing .56% TAAA. This indicates that the weanling pig's requirement for TAAA is .56% of the diet at 12% protein and that 49% of this requirement can be supplied as tyrosine.