Dietary phenylalanine requirement and tyrosine replacement value for phenylalanine of juvenile blunt snout bream, Megalobrama amblycephala

Two 9-week experiments were conducted to estimate the dietary phenylalanine requirement and the tyrosine replacement value for phenylalanine of juvenile blunt snout bream. In experiment I, six isonitrogenous and isoenergetic experimental diets were formulated to contain graded levels of phenylalanine ranging from 0.51% to 2.04% of the diet at a constant dietary tyrosine level (1.07% of dry diet). Weight gain (WG), specific growth rate (SGR), feed efficiency ratio (FER) and protein efficiency ratio (PER) increased with increasing dietary phenylalanine levels up to 1.11%, and thereafter declined. The whole body moisture, crude protein and lipid contents were significantly affected by dietary phenylalanine levels, while no significant difference was found in ash among dietary treatments. Plasma phenylalanine level significantly increased with increasing dietary phenylalanine level, whereas alkaline phosphatase (AKP) activity showed a reversed trend. Dietary phenylalanine level significantly affected plasma glucose concentration. Using quadratic regression analysis of SGR and FER, the dietary phenylalanine requirement was estimated to be 0.93% of diet (2.74% of dietary protein) and 1.01% of diet (2.97% of dietary protein), respectively. In experiment II, six diets with graded levels of l-tyrosine ranging from 0.45% to 1.34% of diet (with fixed phenylalanine level, 0.82%) were formulated to investigate the tyrosine replacement value for phenylalanine. Based on SGR and FER, the tyrosine replacement value for phenylalanine in juvenile blunt snout bream was estimated to be 50.0% and 50.6% on a weight basis or 45.6% and 46.1% on a molar basis, respectively. Above all, the total aromatic amino acid requirement was calculated to be 1.86% and 2.04% of diet (5.47% and 6.00% of dietary protein) on the basis of SGR and FER, respectively. Considering excess dietary phenylalanine resulted in reduced growth performance, the dietary total aromatic amino acid should not exceed 3.44% of diet (10.1% of dietary protein). (C) 2015 Elsevier B.V. All rights reserved.