Effects of different dietary carbohydrate and protein levels on growth, intestinal digestive enzymes and serum indexes in large-size grass carp (Ctenopharyngodon idella)

A 16-week feeding experiment was conducted to study the effects of different dietary carbohydrate and protein levels on growth, body composition, intestinal digestive enzyme activity and serum indexes of large-size grass carp (Ctenopharyngodon idella) initial body weight (398.6±5.9 g). Seven extruded diets with different levels of carbohydrate (C) and protein (P) (C31P30, C34P28, C37P26, C40P24, C43P22, C46P20, C49P18) were made. Experimental results showed that with protein levels decreasing and carbohydrate levels increasing, weight gain rate decreased significantly, protein efficiency significantly increased, hepatosmatic index, viscera index, intestinosomatic index and the fat content of muscle and liver increased significantly. However, the protein content in muscle had no significant difference among dietary treatments. The carbohydrate levels affected significantly the glycogen content in liver and muscle. The glycogen content in liver and muscle increased significantly when the carbohydrate levels were equal to or over 46% and 40%, respectively. The activities of intestinal trypsin and amylase decreased firstly, and then showed a tendency of increase, and lipase activity significantly increased. The content of serum total cholesterol (TC), triglyceride (TG), low density lipoprotein (LDL), high density lipoprotein (HDL), and glutamic-oxalacetic transaminase (GOT)/ glutamic-pyruvic transaminase (GPT)values had a rising trend with dietary carbohydrate levels increasing and protein level decreasing. These results indicated that lower protein (protein levels below 26%) and higher carbohydrate (carbohydrate level above 37%) imposed a negative effect on growth performance, and the hepatosmatic index, viscera index, intestinosomatic index and the fat content of muscle and liver were significantly increased at high level of dietary carbohydrate.