Requirement for phenylalanine and replacement value of tyrosine for phenylalanine in rainbow trout ( Oncorhynchus mykiss)

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The requirement for phenylalanine (Phe) and replacement value of tyrosine (Tyr) for Phe in fingerling rainbow trout were determined by feeding diets containing various levels of Phe and Tyr. Four tanks of 30 trout each were assigned per diet and fed 3 times daily to satiation at 15°C. With diets containing excess Tyr (1.33%) and 35% crude protein, weight gain of 13-g trout over a 6-week period increased linearly with increasing levels of Phe up to 0.75% of dry diet but was not different ( P>0.05) at 0.75% and 1.75% Phe. Feed efficiency and nitrogen retention improved as Phe level in the diet increased up to 0.55%, but no significant effects were found at levels above 0.55%. Carcass protein content increased with Phe level up to 0.65% and fat content decreased concurrently. Increasing Tyr level up to 0.7% in a diet containing 0.6% Phe increased weight gain of 14-g trout over a 6-week period, but a further increase in Tyr level to 0.8% had no effect ( P>0.05). Analysis of data using a broken-line model showed that the Phe requirement of fingerling rainbow trout is 0.7±0.05% (mean±s.e.) of dry diet or 2.0% of dietary protein, when a diet contains excess Tyr and 35% protein, and that the replacement value of Tyr for Phe is approximately 53% on a weight basis or 48% on a molar basis. Thus the total aromatic amino acid requirement is 1.5% of dry diet or 4.3% of dietary protein.