Effects of Bacillus licheniformis Supplementation on Laying Performance, Antioxidant Capacity, and Cecal Microbiota in Post-Peak Laying Hens

Bacillus licheniformis, a multifunctional probiotic, improves performance in peak laying hens, but its effects during post-peak laying—characterized by declining ovarian function, impaired intestinal barrier, and oxidative stress—remain unclear. This study evaluated the effects of dietary B. licheniformis on laying performance, antioxidant and immune parameters, intestinal morphology, cecal microbiota, and fermentation metabolites in post-peak-phase laying hens. A total of 720 forty-three-week-old laying hens were randomly assigned to four treatments: a basal diet or the basal diet supplemented with B. licheniformis at 2.5 × 109, 5 × 109, or 1 × 1010 CFU/kg for 31 days. Supplementation at 5 × 109 CFU/kg consistently improved the laying rate from 78.96 ± 0.47% to 80.40 ± 0.64% (+1.44 percentage points, p = 0.0091) and decreased the feed-to-egg ratio from 2.53 ± 0.02 to 2.47 ± 0.02 (−2.4 percentage points, p = 0.005). It enhanced systemic antioxidant capacity, as indicated by a decrease in serum MDA (35.8% p = 0.002) and increases in GSH-Px (33.2%, p = 0.002) and SOD (56.0%, p = 0.011). Jejunal and ileal villus height and villus-to-crypt ratio were also increased (p < 0.05). Cecal microbiota shifted toward a higher relative abundance of Clostridia_UCG-014 (p = 0.006) and norank_o_RF39 (p = 0.017) and a lower abundance of Megasphaera (p = 0.009), whereas individual and total VFA concentrations did not differ among groups (p > 0.05). In conclusion, 5 × 109 CFU/kg B. licheniformis exerted the most consistent benefits in post-peak-phase laying hens, likely through the modulation of gut microbiota, intestinal barrier integrity, and antioxidant status. These findings support B. licheniformis as a promising probiotic for sustainable laying-hen production.