Abstract The aim of this study was to investigate the effects of dietary selenium (nanoparticles, organic, and inorganic forms), curcumin (CUR), and their combination on survival, growth performance, body composition,innate immune responses, and glutathione peroxidase activity of rainbow trout (Oncorhynchus mykiss). CUR at level of 400 mg/kg dry diet and each of selenium nanoparticles (Se-NPs), organic selenium (Sel-Plax®), and sodium selenite at level of 1 mg/kg Se dry diet were added to basal diet. A total of 240 rainbow trout with mean initial weight of 14.65 ± 0.86 g were fed eight diets including control (basal diet), CUR, Se-NPs, Se-NPs + CUR, organic Se, organic Se + CUR, sodium Se, and sodium Se + CUR for 8 weeks. No significant increase in survival rate, growth performance, feed utilization, and body composition was observed in fish-fed CUR and Se included diets compared to control (P > 0.05). The highest lysozyme and alternative hemolytic complement activity was observed in fish-fed CUR and organic Se + CUR-supplemented diets (P < 0.05). Fish-fed Se-NPs and Se-NPs + CUR-supplemented diets had the highest glutathione peroxidase activity (P < 0.05). The results of the present study indicated that the combination of CUR and Se in nanoparticles and organic forms was more effective in promoting innate immune responses of rainbow trout compared to the other combined or separated Se and CUR forms.

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