Quantitative dietary lysine requirement of juvenile grass carp Ctenopharyngodon idella

A 90-day feeding trail was conducted to determine the dietary lysine requirement of juvenile grass carp using six isonitrogenous and isoenergetic diets (38% protein, 14 MJ digestible energy kg− 1) with wheat gluten-casein-gelatin as protein resource supplemented with six graded levels of crystalline lysine (from 0.69% to 3.08% of dry diet). Crystalline amino acid mixtures were supplemented to simulate the amino acid pattern found in the whole-body protein of grass carp except for lysine. Each diet was randomly assigned to triplicate groups of 20 fish each (3.15 ± 0.01 g, mean ± S.E.M.) and fish were fed five times daily at 2.5% body weight per day. No mortality or nutritional deficiency signs were observed except for growth depression in fish fed the diet with low content of lysine. Weight gain (WG), specific growth rate (SGR), feed efficiency (FE), protein efficiency ratio, nutrient retention, proximate body composition, morphometry and hematology were significantly (P < 0.05) affected by the dietary lysine concentrations. WG, SGR and FE were significantly higher in fish fed diet containing lysine 2.18% of the diet. Second-degree polynomial regression analysis of the WG and FE data indicated that the minimum recommended dietary lysine requirement for optimal growth of juvenile grass carp was 2.24% of the dry diet, corresponding to 5.89% of dietary protein. While the lysine availability (92.37%) of the protein sources was considered, the optimal lysine requirement of juvenile grass carp was calculated to be 2.07% of the diet, corresponding to 5.44% of the dietary protein. Additionally, the estimated requirements for the other essential amino acids were calculated from A/E ratios of whole-body amino acid profile based on the lysine requirement determined from the present experiment.