Dietary vitamin A requirement of juvenile Wuchang bream (Megalobrama amblycephala) determined by growth and disease resistance

Abstract

The present experiment was conducted to determine the dietary vitamin A (VA) requirement of juvenile Wuchang bream by formulating six isonitrogenous and isoenergetic purified experimental diets containing 0, 1934, 3885, 7924, 15,935 and 47,917IU VA (as retinol acetate) kg−1 diet, respectively. Each experimental diet was fed to triplicate groups of 25 juveniles each with initial average weights of 2.40±0.01g in the circular tank for 90days. Weight gain, protein efficiency ratio, plasma complement 3, plasma complement 4, white blood cell, red blood cell and hemoglobin content significantly increased with increasing dietary VA level, reached a peak with 1934 or 3885IUkg−1 VA, and then decreased. Feed conversion ratio (FCR) was lower in the group fed 3885IUkg−1 dietary VA than those fed the 0, 15,939 and 47,917IUkg−1 VA diets. Hepatosomatic index and plasma alanine aminotransferase activity were higher in fish fed the 0IU VA kg−1 diet than those fed with dietary VA. Similar trend was obtained for aspartate aminotransferase activity and the highest activity was found in the group of 47,917IUkg−1 VA. The cumulative mortality after Aeromonas hydrophila challenge of the group fed the 3885, 7924 and 15,935IUkg−1 VA diets was significantly lower than those fed 0IUkg−1 dietary VA. The overall results of the present study indicate that the dietary VA requirement of juvenile Wuchang bream on the bases of the broken-line regression analysis of FCR was estimated to be 3914IUkg−1 of diet. Statement of relevanceThe requirements and immune responses of vitamin A in aquatic animals have been studied in many fish. Dietary deficiency symptoms of vitamin A include skeletal deformity, eye lesions, hemorrhages of fins and eyes, and reduced immunity and growth. Therefore, it is important to know the vitamin A requirement of targeted species and incorporate into diets to prevent deficiency symptoms. Wuchang bream (M. amblycephala) is a principal species in Chinese freshwater polyculture systems with production in China of approximately 0.72milliontons in 2012. However, the growth and immune responses associated with vitamin A in Wuchang bream have not been studied. Therefore, the aim of this study was to quantify the dietary vitamin A requirement of Wuchang bream using growth and immunity parameters.The present experiment was conducted to determine the dietary vitamin A (VA) requirement of juvenile Wuchang bream by formulating six isonitrogenous and isoenergetic purified experimental diets containing 0, 1934, 3885, 7924, 15,935 and 47,917IU VA (as retinol acetate) kg−1 diet, respectively. Each experimental diet was fed to triplicate groups of 25 juveniles each with initial average weights of 2.40±0.01g in the circular tank for 90days. Weight gain, protein efficiency ratio, plasma complement 3, plasma complement 4, white blood cell, red blood cell and hemoglobin content significantly increased with increasing dietary VA level, reached a peak with 1934 or 3885IUkg−1 VA, and then decreased. Feed conversion ratio (FCR) was lower in the group fed 3885IUkg−1 dietary VA than those fed the 0, 15,939 and 47,917IUkg−1 VA diets. Hepatosomatic index and plasma alanine aminotransferase activity were higher in fish fed the 0IU VA kg−1 diet than those fed with dietary VA. Similar trend was obtained for aspartate aminotransferase activity and the highest activity was found in the group of 47,917IUkg−1 VA. The cumulative mortality after A. hydrophila challenge of the group fed 3885, 7924 and 15,935IUkg−1 VA diets was significantly lower than those fed 0IUkg−1 dietary VA.In a word, an excess or deficiency of VA reduced WBC, RBC, HBG, C3 and C4 contents, increased blood AST and ALT activities and enhanced morality after A. hydrophila infection. Taken together, our results suggest that the optimum requirement of dietary VA for juvenile Wuchang bream is 3914IUkg−1 of diet based on the broken-line regression analysis of FCR versus different dietary VA levels, respectively.