**Background:** Plant-based pig diets contain antinutritional factors such as β-mannans, phytate, and xylans that are not digested by endogenous enzymes and compromise nutrient utilization. Dietary supplementation with β-mannanase, xylanase, and phytase can hydrolyze these compounds, potentially improving energy and nutrient availability. However, no prior studies had assessed the combined effects of these three enzymes on fecal microbiome, digesta passage rate, and fecal consistency in finisher pigs. This study tested the hypothesis that β-mannanase supplementation in metabolizable energy (ME)-reduced diets containing xylanase-phytase would improve nutrient digestibility, intestinal health, and growth performance.
**Methods:** Forty entire male hybrid pigs (26.0 ± 0.9 kg) were randomly assigned to four dietary treatments in a randomized complete block design with 10 pen replicates (1 pig/pen) over 52 days (finisher I: d 0–22; finisher II: d 22–52). Treatments were: CD0 (control with phytase and xylanase valued at 40 kcal ME/kg), CD70 (CD0 + β-mannanase valued at 30 kcal ME/kg), CD85 (CD0 + β-mannanase valued at 45 kcal ME/kg), and CD100 (CD0 + β-mannanase valued at 60 kcal ME/kg). Diets were corn- and soybean meal-based, isonutritional except for soybean oil and inert content. Growth performance, fecal consistency score, blood biochemical and immunological profile (including superoxide dismutase [SOD], glutathione S-transferase, immunoglobulins M), apparent total tract digestibility (ATTD) using insoluble acid ash marker, total digesta passage rate using iron oxide marker, fecal microbiome (16S rRNA V3-V4 sequencing), carcass traits, and meat quality were assessed. Statistical analyses used ANCOVA or ANOVA with Tukey or t-Student post-hoc tests (P < 0.05 or P < 0.10 when test power < 80%). Microbiome comparisons used Wilcoxon test (P < 0.05) and PERMANOVA.
**Key Results:** In finisher I phase, pigs fed CD0 had greater ADFI (3,076 g) than CD70 (2,590 g), CD85 (2,598 g), and CD100 (2,753 g) (P = 0.002). However, CD0 pigs had lower G:F (0.42 g:g) than CD70 (0.47) and CD85 (0.46) (P = 0.009). No differences were observed in finisher II or overall period for growth performance. Fecal consistency score was unaffected. Blood analysis showed greater SOD concentration in CD70 (279.10 U/mg protein) vs. CD0 (219.70), CD85 (216.90), and CD100 (221.40) (P < 0.001). No other blood parameters differed. For ATTD, CD85 had greater digestible protein (DP, 9.71%) than CD0 (8.61%) and CD100 (8.99%) (P = 0.002); CD70 showed 11.3% increase in DP over CD0. CD85 had greater digestible energy (DE, 4,035 kcal/kg) than all other treatments (CD0: 3,697; CD70: 3,805; CD100: 3,813; P < 0.001). No differences were found in ATTD coefficients or digesta passage rate. Microbiome analysis showed no differences in alpha or beta diversity. Firmicutes:Bacteroidota ratio (FBR) was greater in CD0 vs. CD85 (P = 0.049) and in CD100 vs. CD85 (P = 0.011). Muribaculaceae family was more abundant in CD70 vs. CD0 (P = 0.030). Prevotella genus was more abundant in CD85 vs. CD100 (P = 0.045). For carcass traits, backfat thickness measured by ultrasound was greater in CD0 (17.90 mm) than CD70 (15.33 mm) and CD85 (15.20 mm) (P = 0.094). pH_24h was higher in CD0 (6.14) and CD85 (5.95) vs. CD100 (5.32) (P = 0.060). No other carcass or meat quality traits differed.
**Clinical Implications:** β-mannanase supplementation in diets already containing xylanase and phytase allows a reduction of 85 kcal ME/kg in finisher pig diets while improving gain-to-feed ratio, protein and energy digestibility, and reducing backfat thickness. These benefits occur without adverse effects on metabolic profile, intestinal health, fecal microbiome diversity, or meat quality. The findings support using β-mannanase as an energy-sparing strategy in commercial pig production, potentially reducing feed costs. The CD85 treatment (45 kcal ME/kg from β-mannanase) appeared most effective for nutrient digestibility and energy utilization. The study also demonstrates that moderate ME reduction alters the fecal microbiome (particularly FBR and Prevotella abundance) without disrupting overall gut ecosystem stability.