A total of 1050 Jian carp, Cyprinus carpio var. Jian (23.39 ± 0.06 g) were randomly divided into seven groups of each three replicates, which were fed respectively with seven semi‐purified diets contained 1.25, 2.71, 4.22, 5.78, 7.23, 8.83 and 11.44 mg riboflavin kg−1 diet for 6 weeks. The results showed that riboflavin significantly improved percent weight gain, specific growth rate, feed efficiency and protein efficiency ratio at the level of 4.22 mg kg−1 diet. Gross protein retention efficiency and lipid production value improved with increasing dietary riboflavin levels from 1.25 to 5.78 mg kg−1. Activities of trypsin, lipase, α‐amylase, Na+,K+‐ATPase and alkaline phosphatase in the intestinal tract were significantly improved with increasing riboflavin levels. Weight and protein content of hepatopancreas, intestine protein and intestine length index were also significantly improved.

This study investigated the effects of phenylalanine on growth, digestive and absorptive ability and antioxidant status of young grass carp (Ctenopharyngodon idella). Young grass carp were fed diets containing 3.4 (basal diet), 6.1, 9.1, 11.5, 14.0 and 16.8 g phenylalanine kg À1 diet with a fixed of 10.7 g tyrosine kg À1 diet for 8 weeks. Percent weight gain (PWG), feed efficiency and feed intake of fish were the lowest in fish fed the basal diet (P < 0.05). Trypsin, lipase and amylase activities in the hepatopancreas, and antioxidants including glutathione contents and glutathione reducase activities in the hepatopancreas and intestine were all the highest in fish fed 11.5 g phenylalanine kg À1 diet (P < 0.05). Trypsin, chymotrypsin and amylase activities in whole intestine, and creatine kinase, Na + , K + -ATPase and alkaline phosphatase activities in the proximal intestine, and superoxide dismutase activities in the hepatopancreas and intestine were all the highest when phenylalanine at level of 9.1 g kg À1 diet (P < 0.05). In conclusion, phenylalanine improved growth, digestive and absorptive ability, and antioxidant capacity of young grass carp. The phenylalanine requirement of young grass carp (256-629 g) based on PWG was 10.4 g kg À1 diet or 3.44 g 100 g À1 protein.

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