Effects of dietary cottonseed meal level on the growth, hematological indices, liver and gonad histology of juvenile common carp (Cyprinus carpio)

Seven isonitrogenous and isoenergetic diets containing cottonseed meal (CM) levels of 0, 9, 18, 27, 36, 45 and 54% (with the free gossypol (FG) levels of 0, 107.8, 215.7, 323.5, 431.3, 539.2 and 647.0 mg kg− 1, respectively) were fed to juvenile common carp (Cyprinus carpio) with initial body weight of 11.3 g for 8 weeks to evaluate the effects of dietary CM on growth performance, hematological indices, liver and gonad histology. When the dietary CM level ranged from 0% to 27%, the weight gain (WG) and feed conversion ratio (FCR) showed no significant difference among groups (P > 0.05), but when the CM level increased from 27% to 54%, the WG and protein efficiency ratio (PER) significantly decreased (P < 0.05), and the FCR significantly increased (P < 0.05). The amount of red blood cells (RBC) and the hematocrit (Hct) value were significantly higher in the groups of 27-54% CM compared to the groups of 0-18% CM (P < 0.05), but the hemoglobin (Hb) level showed relative stability in the groups of 0-45% CM, then significantly decreased when the CM level reached 54%. The activities of serum alanine aminotransferase and hydroxyl radicals were significantly increased for fish fed diets containing 45-54% CM compared with fish fed diets containing 0-36% CM (P < 0.05). The free gossypol concentrations in muscle and liver corresponded to the dietary CM levels. There were no significant differences in the gonad index of both sexes among groups (P > 0.05). Compared with the groups of 0% and 27% CM, the hepatocyte volume decreased, and the number of hepatocytes increased in the group of 54% CM, which also demonstrated the higher number of primary spermatocytes and the fewer sperm cells in the testis. Taken together, these results indicated that a safe level of CM in the juvenile carp diet was 27% (FG of 323.5 mg kg− 1) and broken-line model analysis based on WG showed the upper level of CM was 26.85%. The dietary cottonseed meals higher than 36% (431.3 mg kg− 1 FG) may damage growth performance and hematology. Furthermore, high levels of dietary CM (54% CM with FG of 647.0 mg kg− 1) may slightly impair the liver and decrease the number of sperm cells.