Effects of Eucommia ulmoides Leaf-Astragalus membranaceus Complex on Antioxidant Capacity, Immune Function, Gut Microbiota and Serum Metabolome of Yellow-Feather Broilers

Background: Phytogenic feed additives may provide sustainable alternatives for improving poultry productivity and health. This study evaluated the chemical composition, antioxidant activity, and in vivo effects of a 1:1 Eucommia ulmoides leaf–Astragalus membranaceus complex (EG) in yellow-feathered broilers. Methods: The nutritional composition, bioactive compounds, and antioxidant activity of EG were characterized. A total of 200 one-day-old yellow-feathered broilers were randomly allocated into four dietary treatments with five replicates per group and 10 birds per replicate: the control group (CG) receiving a basal diet and three EG supplementation groups receiving diets supplemented with 0.5% (EG (0.5%)), 1.0% (EG (1.0%)), 2.0% (EG (2.0%)) Eucommia ulmoides leaf-Astragalus membranaceus complex for 42 days. Growth performance, serum antioxidant and immune indicators, intestinal microbiota composition, and serum metabolomic profiles were analyzed. Results: EG contained 46.53% polysaccharides and exhibited concentration-dependent radical-scavenging activity. Supplementation with EG (0.5%) consistently increased body weight and average daily gain and reduced the feed conversion ratio without affecting feed intake. EG increased serum antioxidant enzyme activities and immunoglobulin concentrations but did not affect routine biochemical parameters. The EG (0.5%) group was therefore selected for microbiome and metabolome analyses. EG altered jejunal and cecal microbial composition without significantly affecting α-diversity and increased cecal Faecalibacterium and Blautia at day 21 and Faecalibacterium and Parabacteroides at day 42. Metabolomics identified 103 differential metabolites associated mainly with amino acid, glycerophospholipid, and energy metabolism. Spearman’s analysis further associated Bacteroidota, Bacillota, Enterococcus, and Blautia with amino acid, carbohydrate, and lipid-related metabolites. Conclusions: These findings indicate that EG (0.5%) may improve growth and feed efficiency while modulating antioxidant status, humoral immune indices, intestinal microbial composition, and systemic metabolism in yellow-feathered broilers.