Effect of graded levels of dietary methionine on growth performance, carcass composition and physio-metabolic responses of genetically improved farmed tilapia (GIFT) juveniles reared in inland saline water of 10 ppt

Abstract

The current study aimed to optimize dietary methionine level for improving growth performance, carcass composition and physio-metabolic responses of genetically improved farmed tilapia (GIFT) juveniles reared in inland saline water of 10 ppt. Seven isonitrogenous (∿370 g crude protein/kg), isolipidic (∿80 g lipid/kg) and isocaloric (∿19 MJ gross energy/kg) purified diets were made with graded levels of methionine viz., 4.9 (M4.9), 7.2 (M7.2), 9.9 (M9.9), 12.3 (M12.3), 15.1 (M15.1), 17.4 (M17.4) and 19.8 g/kg (M19.8) with a constant level of cystine (3.2 g/kg). Completely randomized designs were followed to constitute seven distinct experimental groups and each group comprised of three experimental units. Fifteen juveniles (3.16 ± 0.01 g) were randomly stocked in each experimental unit. During the 60-day feeding trial, juveniles were fed with their respective experimental diets thrice daily at an apparent satiation level. The results revealed that growth (weight gain percentage, thermal growth coefficient and specific growth rate) and feed and nutrient utilization (feed conversion ratio, protein efficiency ratio and apparent net protein retention) were improved significantly with an increment in dietary methionine levels up to 12.3 g/kg, and methionine supplementation beyond that did not give extra benefit in relation to growth performance. Dietary methionine supplementation at various levels had no effect (P > 0.05) on the body indices, survival, carcass moisture and ash content and carcass essential amino acid composition of GIFT juveniles. The carcass protein level increased with an increment in dietary methionine levels up to 12.3 g/kg and then declined with further increment of dietary methionine level. However, carcass lipid content displayed the inverse trend of carcass protein content. GIFT juveniles fed a diet containing 12.3 and 15.1 g methionine/kg displayed elevated activities of liver and muscle aminotransferases activities, while the activity of digestive enzymes was unchanged among dietary groups. An increment of methionine up to 12.3 g/kg of diet resulted in a significant rise in serum protein and globulin concentration. Based on broken-line linear and second-order polynomial regression analysis in relation to weight gain percentage and feed conversion ratio, the optimum methionine requirements for GIFT juveniles reared in inland saline water of 10 ppt ranged from 12.1 to 14.1 g/kg of diet (32.7–38.1 g/kg of dietary protein) with supplementation of cystine at 3.2 g/kg.