Long‐Term Effects of a Dietary Humic Substance–Butyrate Blend, and Yeast Cell Wall on Growth Performance, Immune Status and Gut Microbiota of Juvenile Channel Catfish Ictalurus punctatus

This study evaluated the long‐term effects of a dietary humic substance and butyrate blend (HSB) alone or with yeast cell wall (Beta‐MOS) in channel catfish. Around 1440 fish (31.31 ± 0.24 g) were distributed among 12 outdoor tanks in a pond‐supplied flow‐through design. Four dietary treatments were evaluated in triplicate: Diets containing no supplements (control), 2 g kg −1 of HSB (HSB), 0.5 g kg −1 of yeast cell wall (BM) or a combination of 1 g kg −1 HSB and 0.25 g kg −1 BM (HSB+BM) for 90 days. All growth indices increased over time ( p ≤ 0.050); however, diet did not affect growth parameters ( p > 0.05), except for per cent weight gain (PWG) ( p = 0.027). PWG differed among treatments after 90 days ( p = 0.016), with the HSB yielding 470% PWG compared to 400% in the control ( p = 0.011). Serum lysozyme activity was unaffected by dietary treatments ( p = 0.128) but varied over time ( p < 0.001), with higher activity at 30 days compared to 60 or 90 days ( p < 0.001). Target immune genes in the head kidney were not different ( p > 0.050). Meanwhile, splenic il1b and tnfa expression varied over time ( p = 0.036; p = 0.023), with upregulation at 30 days. Gut microbiome analyses showed treatment effects on richness after 90 days ( p = 0.027), while beta diversity was influenced by time rather than treatments. Differential abundance analysis identified opportunistic taxa in the gut after 30 days, followed by commensal colonization at 60 and 90 days. Overall, dietary HSB at 2 g kg −1 showed growth potential in channel catfish. While 0.5 g kg −1 of BM did not affect performance, their combination at half inclusion rates yielded comparable growth. Therefore, HSB has the potential to support long‐term growth performance while maintaining baseline health performance in commercial catfish aquaculture.