This trial was conducted to study the ability of dietary propylene glycol to mitigate winter stress of Nile tilapia under biofloc system. Nile tilapia (average initial weight = 28.50 ± 0.25 g) were fed three isonitrogenous (257.75 g kg−1 crude protein) diets for 47 days in winter season. The first diet was free added with propylene glycol (PG) control. The other two diets were supplemented with 5 mL and 7.5 mL PG kg−1 diet, respectively. At the end of feeding trial, the highest survival rate (P < 0.05) was observed in tilapia fed the diet supplemented with 7.5 ml PG kg−1 diet. A linear response in weight gain (WG; P = 0.034), specific growth rate (SGR; P = 0.041) and protein efficiency ratio (PER; P = 0.038) of tilapia were found by the increase in the PG levels in diets. The supplemental diets with PG did not induce any significant differences (P > 0.05) on feed conversion ratio (FCR) and feed intake (FI) of fish. The activities of alanine aminotransferase (ALT), aspartate aminotransferase (AST), alkaline phosphatase (ALP), cholesterol, triglyceride and low density of lipoprotein cholesterol (LDL-C) were linearly decreased in response to the increased the PG level. Also, linear increase in serum total protein, albumin, globulin and lipoprotein cholesterol (HDLsingle bondC) were found in response to supplementation of PG. Significant linear increased in the concentration of serum ions; sodium, calcium and chloride were observed with increasing the PG level. Significant linear increased were found in superoxide dismutase (SOD; P = 0.035), catalase (CAT; P = 0.001), glutathione (GSH; P = 0.056) and glutathione peroxidase (GPx; P = 0.048) for fish fed 5- or 7.5-ml PG kg−1 under cold stress, with the highest values in fish fed 7.5 ml PG kg−1. While, the activity of malondialdehyde (MDA; P = 0.023) was linearly reduced with increasing of the PG levels in fish diets, with the lowest value in group fed supplemental diet with 7.5 ml kg−1 PG. The response of glucose (P = 0.026) and cortisol (P = 0.193) of fish in cold stress for 47 days were linearly with increasing PG supplementation. The transcription of Δ 9D gene of fish reared under cold stress was linearly up regulated (linear, P = 0.001) with increasing dietary PG level. In conclusion, diet supplemented with 7.5 ml kg−1 PG could decrease the mortality, and enhance the physiological status as well as transcription of Δ 9D gene of fish reared under cold stress. |
