Effects of graded levels of threonine on growth performance, biochemical parameters and intestine morphology of juvenile grass carp Ctenopharyngodon idella

This experiment was conducted to investigate the effects of threonine on growth performance, biochemical parameters and intestine morphology of juvenile grass carp Ctenopharyngodon idella. Six isonitrogenous and isoenergetic semi-purified diets containing casein and gelatin with graded level of threonine (0.73, 1.03, 1.33, 1.63, 1.93, 2.23 g 100 g− 1 DM) were formulated. Each diet was randomly assigned to triplicate groups of 30 fishes (4.02 ± 0.004 g, mean ± S.E.M.) each tank for 10 weeks. The highest weight gain (WG, %), final body weight (FBW, g) and specific growth rate (SGR) were recorded when threonine level was 1.33% of the diet. Second-degree polynomial regression analysis of SGR data indicated that the optimum requirement of threonine is to be at 1.37%, corresponding to 3.61% of dietary protein. Whole body protein, muscle protein, aspartate aminotransferase and cholesterol were significantly affected by the threonine level (P < 0.05). Threonine retention was significantly affected by dietary threonine level (P < 0.05). Fish fed threonine deficiency diets (0.73%, 1.03%) exhibited intestinal villus exfoliation, twist, severely fusion, lower anterior intestinal villus height and serosa thickness. The present results indicated that dietary threonine level not only affected grass carp growth, feed intake and body protein synthesis, but also influenced anterior intestinal development and integrity.