Fermented corn stover <i>(Zea mays L.)</i> enhances growth, immune response, histology, gut microbiome, and gene expression in <i>Cyprinus carpio</i> var. <i>koi</i> in biofloc system

This study evaluated the effects of dietary fermented corn stover (FCS) supplementation on growth performance, antioxidant capacity, immune responses, and gut microbiota composition in koi carp (Cyprinus carpio var. koi) cultured in a biofloc system. A total of 300 fish (35.10 ± 0.08 g) were fed diets containing 0, 5, 10, 20, or 30 g/kg FCS for eight weeks. At the end of the feeding trials, blood serum, intestine of fish in each treatment was collected to evaluate the antioxidant activities, histology, gut microbiome, and gene expression. Fish receiving 10 g/kg FCS showed the highest final weight (FW), weight gain (WG), daily weight gain (DWG) and specific growth rate (SGR), with quadratic regression estimating an optimal inclusion level of ∼16.7 g/kg. Pearson correlation results showed that the feed conversion ratio was negatively correlated with FCS, while FW, SGR, and WG were positively correlated. Antioxidant activity was enhanced in FCS-fed groups, evidenced by elevated 2,2′-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radical scavenging and reduced malondialdehyde levels. FCS-10 groups improved intestinal mucosal integrity by increasing villus height, width, and crypt depth. Gene expression analysis revealed significant upregulation of antioxidant-related genes (GST, GPX1, CAT) and immune markers (IL-8, IL-10, TNF-α, TLR4) in the FCS-10 group. Microbiota profiling showed increased Bacillus and Lactobacillus and reduced Aeromonas abundance. These findings highlight FCS as a promising functional feed additive for enhancing growth, immune function, and gut microbial balance in koi carp, contributing to sustainable aquaculture practices.