Effects of Varying Levels of Fermented Camellia oleifera Seed Meal on Growth Performance, Biochemical Indices, and Intestinal Health in Nursery Pigs

Camellia oleifera seed meal (CSM) is a protein-rich by-product from camellia oil production, but its utilization in animal diets is restricted by antinutritional compounds, especially tea saponins. This study evaluated the effects of solid-state fermented CSM, produced using Bacillus subtilis, Aspergillus niger, and Trichoderma longibrachiatum, on growth performance, antioxidant capacity, immune status, and intestinal health in weaned piglets, and identified the optimal dietary inclusion level. A total of 84 piglets (15.46 ± 0.10 kg) were randomly allocated to four treatments: a basal diet (control) or the basal diet supplemented with 2.5%, 5.0%, or 7.5% fermented CSM for 48 d. Compared with the control, 2.5% fermented CSM maintained normal growth performance while improving neutral detergent fiber digestibility, antioxidant activity, immune-related indices, jejunal morphology, and expression of tight junction proteins and MUC-2. The relative abundance of butyrate-producing Collinsella was altered, and colonic butyric acid showed a significant quadratic response to graded FCSM supplementation; meanwhile, isocaproic acid concentration was reduced. In contrast, supplementation at 5.0% or above negatively affected growth, nutrient utilization, and intestinal health. Overall, piglets exhibited an inverted U-shaped response to fermented CSM supplementation, with 2.5% identified as the most suitable inclusion level. This study provides a theoretical basis for the application of fermented CSM as a potential partial replacer of conventional feed ingredients in nursery pig diets.