A study to explore the effects of low dietary protein levels on the growth performance and nutritional metabolism of grass carp (Ctenopharyngodon idella) fry

An 8-week feeding trial was conducted to investigate the effects of low dietary protein levels (DPLs) on the growth performance and nutritional metabolism of grass carp (Ctenopharyngodon idella) fry. The results showed that the average final weight (AFW), weight gain rate (WGR) and specific growth rate (SGR) reached the maximum values at 43.30% DPL, and then became constant (P < 0.05). The feed conversion ratio (FCR) showed an opposite trend (P < 0.05). Low DPLs (18.20% and 24.30%) suppressed the survival rate (SR) (P < 0.05). The protein efficiency ratio (PER) reached the maximum value at 36.40% DPL (P < 0.05). The moisture content was lowered by at 36.40% DPL (P < 0.05). The crude protein content was improved by 43.30% DPL (P < 0.05). Low DPLs (18.20% and 24.30%) significantly increased the fat composition compared to other DPLs (P < 0.05). 43.30% DPL significantly promoted the rapamycin target protein (TOR) and ribosomal protein S6 kinasepolypeptide 1 (S6K1) mRNA levels, but inhibited the 4E binding protein 1 (4EBP1) and adenosine 5'-monophosphate (AMP)-activated protein kinase (AMPK) mRNA levels (P < 0.05). Low DPLs (18.20% and 24.30%) increased fat synthesis-related genes, the mRNA levels of lipolysis-related genes were significantly higher at 43.30% DPL (P < 0.05). The glucokinase (GK) mRNA levels at 18.20%, 24.30% and 30.60% DPLs were higher (P < 0.05). The glucose 6 phosphatase (G6Pase) mRNA level was higher at 43.30% DPL (P < 0.05). Hepatopancreas transcriptome results showed that compared with the low DPL (18.20%), the appropriate DPL (43.30%) significantly affected the amino acid metabolism and lipid metabolism. This study therefore, demonstrated that low DPLs adversely affected the growth and nutrient metabolism in grass carp fry.