Effects of dietary corn silk (Zea mays L.) on growth, immune and antioxidant pathways, histological morphology, gut microbiome, and sensitivity to acute ammonia exposure in the koi carp (Cyprinus carpio var. koi).

Corn silk is a promising aquafeed additive due to its antioxidant, antibacterial, and anti-inflammatory properties. This study evaluated the impact of graded dietary levels of powdered corn silk (PCS) on growth, immunity, antioxidant enzyme activities, histology, gut microbiota composition, gene expression profiles, and sensitivity to acute ammonia exposure in koi carp (Cyprinus carpio var. koi). A total of 300 fish were assigned to five dietary treatments (0, 5, 10, 20, and 30 g/kg PCS) for eight weeks. Fish fed the PCS-30 diet showed significant improvements in growth parameters, including weight gain (WG) and specific growth rate (SGR) (p < 0.05). Antioxidant capacity (2,2'-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid); ABTS) and superoxide dismutase (SOD) activity, increased dose-dependently, while malondialdehyde (MDA) levels remained unchanged (p > 0.05). Expression of the immune and antioxidant genes interleukin 8 (il-8), lysozyme (lyz), catalase (cat), nuclear factor-κB (Nf-κB), and toll-like receptor 4 (tlr4) was significantly upregulated in the PCS-20 and PCS-30 groups (p < 0.05). Histological analysis revealed progressive improvements in anterior intestinal morphology, with the PCS-30 group exhibiting the greatest villus height and width. Gut microbiota analysis showed increased relative abundance of beneficial taxa (Gammaproteobacteria, Lactobacillales, Prevotella), which positively correlated with growth and antioxidant markers, and negatively with MDA levels. Survival under acute ammonia exposure was highest in the PCS-20 and PCS-30 groups. These findings demonstrate the efficacy of PCS, particularly at 30 g/kg, in enhancing growth, immunity, and stress resilience.