| A total of540young grass carp (initial weight257.00±2.53g) were fed six semi-purified diets containing14.17(unsupplemented control group),29.15,44.16,59.17,74.18and89.19mg Zn kg-1diet for56days to investigate the effect of dietary zinc on the growth, anti-oxidant capacity and flesh quality. After8weeks feeding trail,15fish with similar weight was radomly selected from each treatment to conduct a challenge trail with copper exposure for4days. The effect of dietary zinc on immune was investigated.The result showed that dietary zinc significantly increased the growth performance of young grass carp, The final weight, percent weight gain, special growth rate and protein efficiency ratio were significantly increased with increasing dietary Zn levels up to the optimum level (P<0.05). A significant increasement was seen in FI and FE with zinc supplementation of young grass carp (P<0.05). Muscle protein and ash content were significant improved by dietary zinc supplementation, whlie moisture content was not significantly different among dietary groups (P>0.05). The concentration of serum zinc and activity of alkaline phosphatase (AKP) significantly enhanced with dietary zinc level up to a point (P<0.05), beyond which it plateaued (P>0.05). Cooking loss and shear force in the muscle significantly reduced with increasing level of zinc up to a point, and thereafter increased (P<0.05). The muscle collagen content and pH value increased with the increasing zinc levels, whereas the trend of protein carbonyl content was opposite. Patterns of AHR, SOD, CAT, GR activities and the content of GSH were the same as that in PWG; whereas, the trend of ASA, GPX and GST activities was significantly increased by zinc level up to a point (P<0.05), then reached a plateau (P>0.05). Correlation analysis showed that shear force have a negative correlation of the OH-scavenging ability (r=-0.863, P<0.05), the O2--scavenging ability (r=-0.910, P<0.05) and the activities of SOD (r=-0.998, P<0.01), CAT (r=-0.833, P<0.05), GPx (r=-0.866, P<0.05) and GR (r=-0.976, P<0.01). Hydroxyproline content was positively related to the activities of the O2--scavenging ability (r=+0.971, P<0.01), SOD (r=+0.814, P<0.05), GPx (r=+0.951, P<0.05), GST (r=+0.901, P<0.05).The weight of head kidney and spleen of young grass carp were significantly increased with increasing level of zinc up to a point, and then decreased (P<0.05). However, the head kidney and spleen index showed an opposite tendency. The PC and MDA content in head kidney and spleen of young grass carp were significant reduced with increasing dietary Zn levels (P<0.05). The AHR, SOD, CAT, GPX, GST and GR activities and the content of GSH of head kidney significantly increased with dietary zinc level up to a point (P<0.05). There was no significant diffeience of head kidney ASA activity among groups (P>0.05). Patterns of CAT, GPX, GST and GR activities and GSH content in the spleen were the same as that in head kidney; whereas, the trend of ASA and SOD was significant enchanced at zinc-unsupplemental group (P<0.05), and no further improvement was seen among other groups (P>0.05). Meanwhile, dietary zinc great significantly (P<0.01) or significantly (P<0.05) improved the T-AOC, lysozyme, complement3and immunoglobulin M content of head kidney, spleen, serum and intestinal in young grass carp.In conclusion, the present results demonstrated that dietary Zn improved the growth performance and flesh quality, and enhanced antioxidant status and immunity in young grass carp. Correlation analysis showed that Zn improved the flesh quality and immunity may be due to the fact that Zn enhanced the antioxidant status in young grass carp. Meanwhile, the dietary Zn requirement for maximum growth of young grass carp (257-670 g) was determined to be56.88mg/kg diet. |
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