Effects Of Dietary Protein Levels On Growth, Feed Utilization And Nitrogen Metabolism Of Grass Carp(Ctenopharyngodon Idellus) In Three Growth Stages

The present study was to investigate the effect of dietary protein levels on growth, feed utilization and nitrogen metabolism of grass carp(Ctenopharyngodon idellus).1. Six isoenergetic diets were formulated with20%,25%,30%,35%,40%and45%protein level. Fish (12.24±0.34g) were randomly distributed into six groups each with three replicates in tanks (300L). Fish were fed thrice daily (08:00,12:00and16:00) to apparent satiation for8weeks. The results showed that weight gain and feed utilization were significantly affected by dietary protein levels (P<0.05). Fish fed the diet with30%protein had the highest weight gain, specific growth ratio, feed efficiency ratio and feed intake than other groups. Protein retention efficiency, protein efficiency ratio and serum glucose inversely correlated with dietary protein levels (P<0.05).25%protein group had the highest hepatosomatic index and intraperitoneal fat ratio, liver crude lipid in35%protein group was significantly higher than other protein group(P<0.05).The highest protein content and lipid content of whole body were found in40%and30%protein group. The contents of serum total protein, muscle crude lipid and blood urea nitrogen were significantly increased with the increasing of dietary protein levels (P<0.05). There were no significant differences on viscerosomatic index, activities of serum alanine aminotransferase and serum aspartate aminotransferase (P>0.05).The dietary optimal protein for grass carp from weight gain and feed efficiency ratio estimated using the second order polynomial regression was ranged from31.29%to33.17%.2. Five isoenergetic diets were formulated with15%,20%,25%,30%and35%protein level. Fish (209±10g) were randomly distributed into five groups each with three replicates in net cages (2m×2m×2m). Fish were fed twice daily (08:00and16:00) to apparent satiation for8weeks. The results showed that weight gain and feed utilization were significantly affected by dietary protein levels (P<0.05). Fish fed the diet with25%protein had the highest weight gain, specific growth ratio and feed efficiency ratio than other groups. Protein retention efficiency, protein efficiency ratio, viscerosomatic index and lipid content of whole body inversely correlated with dietary protein levels (P<0.05). There were no significantly differences on survival, hepatosomatic index, intraperitoneal fat ratio, activities of serum alanine aminotransferase and serum aspartate aminotransferase (.P>0.05).30%protein group had the highest crude protein contents of whole body. Protein content of whole body, serum total protein, blood urea nitrogen, the activities of aspartate aminotransferase, glutamate dehydrogenase in liver and adenosine5’-monophosphate deaminase in muscle were significantly increased with the increasing of dietary protein levels (P<0.05). This denoted increased catabolism of protein to supply energy with the increasing of dietary protein levels. The dietary optimal protein for grass carp from specific growth ratio and feed efficiency ratio estimated using the second order polynomial regression was ranged from26.5%to27.2%.3. Five isoenergetic diets were formulated with15%,20%,25%,30%and35%protein level. Fish (454±8g) were randomly distributed into five groups each with three replicates in net cages (2m×2m×2m). Fish were fed twice daily (08:00and16:00) to apparent satiation for8weeks. The results showed that weight gain and feed utilization were significantly affected by dietary protein levels (P<0.05). Fish fed the diet with25%protein had the highest weight gain, specific growth ratio and feed efficiency ratio than other groups. Protein retention efficiency, protein efficiency ratio, viscerosomatic index, hepatosomatic index and intraperitoneal fat ratio inversely correlated with dietary protein levels (P<0.05).30%protein group had the highest crude lipid contents of whole body. The composition of liver and muscle were no significantly differences (P>0.05). There were no significantly differences on survival, serum total protein, serum glucose, activities of serum alanine aminotransferase, activities of serum aspartate aminotransferase and activities of liver alanine aminotransferase (P>0.05). The content of blood urea nitrogen, the activities of aspartate aminotransferase, glutamate dehydrogenase in liver and adenosine5’-monophosphate deaminase in muscle were significantly increased with the increasing of dietary protein levels (P<0.05). The dietary optimal protein for grass carp from specific growth ratio and feed efficiency ratio estimated using the second order polynomial regression was ranged from25.82%to26.24%.