A 60-day feeding trial was conducted to determine the effect of dietary fulvic acid supplements on intestinal digestive activity (enzymatic analysis), antioxidant activity, immune enzyme activity and microflora composition of juvenile loach (initial weight of 6.2 ± 0.1 g) reared in experimental aquaria. Five test diets containing 0, 0.5, 1.0, 1.5, and 2% fulvic acid were randomly assigned to three aquaria, respectively. Elevated growth performance including final weight, weight gain (WG), specific growth rate (SGR) and feed conversion ratio (FCR) was observed in loaches that were fed fulvic acid. Maximal weight gain rates and specific growth rates occurred at the 1.5% additive level. The optimal dietary fulvic requirement for maximal growth of juvenile loach is 16.4 g per kg of the diet based on the quadratic regression analysis of specific growth rate against dietary fulvic acid levels. Furthermore, intestinal protease activity, antioxidant activity, lysozyme activity (LZM), complement 3 (C3) content, immunoglobulin M (IgM) content, acid phosphatase activity (ACP) and alkaline phosphatase activity (AKP) were significantly elevated with concomitant increasing levels of dietary fulvic acid. Following a deep sequencing analysis, a total of 42,058 valid reads and 609 OTUs (operational taxonomic units) obtained from the control group and the group displaying the most optimal growth rate were analyzed. Fulvic acid supplementation resulted in an abundance of Firmicute and Actinobacteria sequences, with a concomitant reduction in the abundance of Proteobacteria. Results indicated that fulvic acid supplementation resulted in a reduction in the relative abundance of Serratia, Acinetobacter, Aeromonas and Edwardsiella, and a relative increase in the abundance of Lactobacillus in the intestine. In conclusion, these results suggest that fulvic acid improves growth performance and intestinal health condition of loach, indicates that fulvic acid could be used as an immunoenhancer in loach culture.
Keywords: Antioxidant; Fulvic acid; Immunity; Intestinal microflora; Paramisgurnus dabryanus.
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