Dietary total aromatic amino acid requirement and tyrosine replacement value for phenylalanine for fingerling Oreochromis niloticus (Linnaeus)

Abstract

This study was conducted to determine the dietary total aromatic amino acid requirement and tyrosine replacement value for phenylalanine for fingerling Oreochromis niloticus by conducting two 8‐week experiments. In experiment I, phenylalanine requirement was determined by feeding test diets (350 g kg–1 CP; 16.72 kJ g–1 GE) with graded levels of phenylalanine (4, 6.5, 9, 11.5, 14 and 16.5 g kg–1 dry diet) at a constant level (10 g kg–1) of dietary tyrosine to triplicate groups of fish (1.65 ± 0.09 g). Quadratic analyses of absolute weight gain (AWG), protein deposition (PD%) and phenylalanine retention efficiency (PRE%) indicated the requirement at 12.1 g kg–1 dry diet. In experiment II, six diets with different levels of L‐tyrosine (2.0, 4.0, 6.0, 8.0, 10.0 and 12.0 g kg–1 dry diet) with 12.1 g kg–1 phenylalanine as determined in experiment I fixed in all the test diets were fed to fish (1.67 ± 0.17 g) to determine the tyrosine requirement under same conditions. Quadratic regression analysis of AWG, PD and PRE demonstrated the tyrosine requirement at 8.5 g kg–1 dry diet, corresponding to 24.2 g kg–1 of dietary protein. Based on the above data, the total aromatic amino acid requirement was found to be 20.6 g kg–1 of dry diet, corresponding to 58.8 g kg–1 of dietary protein, and tyrosine replacement value for phenylalanine was found to be 37% on a molar basis.