Determination Of The Dietary Threonine Requirement For Juvenile Grass Carp, Ctenopharyngodon Idella

Threonine is one of the ten essential amino acids for fish,which usually plays the second or third restrictive amino acid role in formulating fish feed used by most of botanic ingredients.Afew studies,those are concerning lysine,methionine,and arginine requirements,are available on dietary essential amino acid requirement for grass carp,one of the most important economic aquaculture species in China with a yearly production of about 4 million tons.This experiment was conducted to determine the dietary threonine requirement for juvenile grass carp by formulating six isonitrogenous diets(35%protein) containing increasing levels of threonine(0.72%,1.02%,1.32%, 1.62%,1.92%and 2.22%,respectively).Casein-gelatin-zein meal and crystalline amino acid mixture was used as dietary protein source and the dietary amino acid pattern,except for threonine, was simulated the amino acid pattern found in the whole body protein of juvenile grass carp.Each experimental diet was fed to triplicate groups of 30 juvenile grass carp(initial body weight 8.35±0.06g) in freshwater floating net cages(1.0m×1.0m×1.0m) for 60 days.The result showed that with the increases of dietary threonine levels,the weight gains(WG),specific growth rates(SGR), protein efficiency ratios(PER),protein retention efficiencies(PRE),RNA/DNA ratios in muscle, and blood ammonia contents of the fish increased significantly(P＜0.05),which reached their peaks at the 1.62%group,whereas the feed conversion ratios(FCR) and glutamate dehydrogenase(GDH) activities in liver decreased significantly(P＜0.05),which reached their troughs at the same group, and then all of these indices became stable thereatfer(P＞0.05).The serum total protein contents of the fish increased significantly with the increases of dietary threonine levels(P＜0.05),which of the three groups fed the diets with 1.32%,1.62%,and 1.92%dietary threonine were significantly higher than those fed the diets with 0.72%and 1.02%dietary threonine(P＜0.05),and of the group fed the diet with 2.22%dietary threonine was significantly higher than those fed all the other diets(P＜0.05).The serum triglyceride and cholesterol contents of the four groups fed the diets with 1.32%-2.22%dietary threonine were significantly higher than those fed the diets with 0.72%and 1.02%dietary threonine(P＜0.05),but among the former,there was no significant difference found (P＞0.05).The fish fed the diet with 1.62%dietary threonine had significantly higher threonine content and gross amino acid content in muscle than those fed the other diets(P＜0.05).Dietary threonine levels had no significant effect on the alanine amino transferase(ALT) and aspartate amino transferase(AST) activities in liver of the fish(P＞0.05).Dietary proper threonine levels could reduce the moisture content and improve the protein content in whole-body and muscle of the fish, improve the lipid and ash content in whole-body,but not affect lipid content in muscle.The broken-line regression analysis of SGRs,FCRs,PREs,RNA/DNA ratios and GDHs against dietary threonine levels indicated that the 95%optimal requirement of dietary threonine for juvenile grass carp was 1.42%-1.61%(based on diet with 35%protein) viz.4.07%-4.60%(based on protein).