Effect of dietary L-methionine concentrations on growth performance, serum immune and antioxidative responses of juvenile Nile tilapia, Oreochromis niloticus

An 8-week feeding trial was conducted to determine the effect of dietary L-methionine supplementation on growth performance, serum immune and antioxidative responses of juvenile Nile tilapia Oreochromis niloticus. Six iso-nitrogenous (282 g kg(-1) crude protein) diets were formulated to contain graded levels of methionine (0.32%, 0.54%, 0.75%, 0.93%, 1.14% and 1.33% of dry weight) at a constant dietary cysteine level of 0.83 g kg(-1). Each diet was randomly assigned to triplicate groups of 25 juvenile fish (2.3 +/- 0.0 g), which were fed three times daily (8:30, 12:30 and 17:00 hours). The results showed that growth performance and feed utilization significantly improved when dietary methionine levels increased (P < 0.05). Using quadratic regression analysis of weight gain against dietary methionine levels indicated that, the optimal dietary methionine requirement for maximum growth of juvenile tilapia was 9.12 g kg(-1) of the dry diet in the presence of 0.83 g kg(-1) cystine. In addition, maximum C4 content and lysozyme activity were observed in fish fed 7.50 g kg(-1) methionine diet; maximum C3 content and superoxide dismutase activity were obtained in fish fed 9.30 g kg(-1) dietary methionine level. While there was no significant difference in serum glutathione peroxidase activity among all methionine supplemented treatments (P > 0.05). Methionine supplementation decreased malondialdehyde content in serum significantly (P < 0.05) when compared with the control diet, while there was no significant difference among supplemented treatments. These data suggested that L-methionine affected antioxidant status and promoted serum immune response in juvenile Nile tilapia, and at non-stressed status, the requirement of dietary methionine in maintaining normal immunity and physiology is lower than that for maximum growth.