Effects of Partial Replacement of Wheat Bran with Poplar Wood Composite Fiber on Growth Performance, Nutrient Apparent Digestibility, Immune Function, and Gut Microbiota in Growing Pigs

The objective of this study was to evaluate the effects of partially replacing wheat bran with poplar wood composite fiber (PWCF) on growth performance, immune status, apparent total tract digestibility (ATTD), and gut microbial composition in growing pigs. A total of 140 healthy crossbred (Duroc × Landrace × Yorkshire) growing pigs with an initial body weight of 47.25 ± 0.49 kg were randomly assigned to two dietary treatments, with five replicates per treatment and fourteen pigs per replicate. The control (CT) group was fed a corn–soybean meal-based diet containing wheat bran and rice bran meal, whereas the experimental group received the same diet in which 2% wheat bran was replaced by PWCF. The experiment lasted for 60 days. Compared with the CT group, replacing wheat bran with PWCF did not affect body weight, average daily feed intake, feed conversion ratio, or average daily gain on days 30 or 60 (p > 0.05). In addition, no negative effects were observed on ATTD of nutrients and serum immunoglobulin A (IgA), IgG, and IgM levels at either time point, indicating that PWCF can serve as a suitable partial substitute for wheat bran in growing pig diets. However, it could regulate nitrogen metabolism by reducing blood urea nitrogen (BUN) concentration and the BUN/creatinine ratio, as well as decreasing total free amino acids in serum (p < 0.05). In addition, the antioxidant capacity can be transiently improved by increasing catalase activity. Gut microbiota analysis showed that the replacement significantly increased the relative abundances of Treponema, the Lachnospiraceae_XPB1014_group and Prevotellaceae_UCG-001 (p < 0.05). These changes suggest that PWCF modulates gut microbiota and enriches fiber-degrading bacterial populations. Overall, substituting wheat bran with PWCF did not impair growth performance, immunity, or digestibility, while altering microbial community composition. These findings support the potential application of PWCF as an alternative fiber source, contributing to greater diversity in feed formulation.