Effects of dietary Zn on growth performance, antioxidant responses, and sperm motility of adult blunt snout bream, Megalobrama amblycephala

A growth experiment was conducted to estimate the effects of dietary Zn on growth, antioxidant responses, and sperm motility of adult blunt snout bream, Megalobrama amblycephala. Six experimental diets were formulated, each containing graded levels of Zn (0, 20, 40, 80, 160, and 320mg/kg, respectively) supplied as ZnSO4·7H2O, providing the actual dietary Zn values of 7.8, 32.7, 50.3, 87.2, 165.4, and 328.5mg/kg diet, respectively. Each diet was assigned to three replicate groups of 15 fish (mean initial body weight: 128.6±0.7g) for 12weeks. Results showed that weight gain (WG) and whole body Zn content increased linearly with increasing dietary Zn levels, then remained nearly unchanged when Zn levels reached the 50.3 and 87.2mg/kg diets, respectively. Fish fed with the control diet had a significantly higher feed conversion rate than those fed with Zn supplemented diets (P<0.05). Dietary Zn had no significant effects on crude protein of the fish body (P>0.05), but had a significant effect on moisture, lipid and ash content (P<0.05). Dietary Zn had significant effects on hepatic malondialdehyde content, superoxide dismutase, catalase, total antioxidant capacity, and glutathione peroxidase activity, but had no effect on serum glucose, high-density lipoprotein cholesterol, or total cholesterol content. Serum alkaline phosphatase activity increased significantly (P<0.05), whereas triglyceride content decreased significantly with the increase in dietary Zn levels (P>0.05). Testis histological sections showed few leydig cells in the control group. Sperm motility parameters were also affected by dietary Zn. A broken-line regression analysis showed that the optimum dietary Zn requirement of blunt snout bream (approx.128g) was 52.1mg/kg for maximum WG and 86.2mg/kg for maximum whole body Zn content. Insufficient Zn inhibits the growth and testicular development of adult blunt snout bream. However, superfluous Zn (328.5mg/kg) in the diet decreases the antioxidant function and sperm motility.