Supplementation of a Multienzyme Complex, an Organic Acid-Essential Oil Complex, and Prebiotic Alone or in Combination Affects Growth, Nutrient Utilization, and Immune Function of Rainbow Trout (Oncorhynchus mykiss)

Abstract

Dietary supplementation of functional additives is an effective strategy to improve the nutritive value of low fish meal diets for fishes. The present study is aimed at investigating the dietary effects of a multienzyme complex, an organic acid-essential oil complex and prebiotic on growth, immune function, and intestinal health of rainbow trout (Oncorhynchus mykiss). A positive control diet (PC) and a negative control diet (NC) were formulated to contain 200 g/kg and 100 g/kg of fishmeal, respectively. Seven other diets were formulated by supplementing a multienzyme complex (MC, 0.125 g/kg), organic acid-essential oil complex (OEC, 0.5 g/kg), and prebiotic (P, 2 g/kg) alone or in combination to the NC diet (MC, OEC, P, MC+OEC, MC+P, OEC+P, MC+OEC+P). All diets were formulated to be isoproteic (434.3 g/kg-439.1 g/kg) and isolipidic (119.2 g/kg-125.9 g/kg) and fed to rainbow trout of $7.66\pm 0.05\text{g}$ for 56 days. The weight gain, apparent digestibility coefficient of dry matter, protein efficiency ratio, and protein retention efficiency were significantly increased, and feed conversion ratio was significantly decreased by the three additives alone or in combination ( $P<0.05$ ) compared to the NC. No significant differences in growth performance and feed utilization were observed among the treatment groups. Compared to the NC group, serum alanine transaminase, aspartate transferase activities, and malondialdehyde content reduced significantly, while serum superoxide dismutase increased in all except for the MC group, and alkaline phosphatase and lysozyme activity increased in all except for the MC and MC+P groups ( $P<0.05$ ). The foregut protease activity of the MC+P and OEC+P groups and the amylase activity of the OEC and MC+OEC groups were significantly higher than those of the NC group ( $P<0.05$ ). The villus height of P, MC+OEC, and OEC+P groups, the villus width of MC group, and the muscle thickness of MC, MC+OEC, and MC+OEC+P groups were also significantly higher than those of the NC group ( $P<0.05$ ). Compared to the PC group, the richness and diversity of intestinal microorganisms in the NC group and all the supplemented groups were significantly reduced ( $P<0.05$ ), but no differences among them ( $P>0.05$ ). In conclusion, the individual or combined supplementation of multienzyme complex, organic acid-essential oil complex, and prebiotic in a low fish meal diet (100 g/kg) improved the growth performance, nutrient utilization, and immune function of rainbow trout, but no synergistic effects were observed in the combination of the three supplements.