Objective This study aimed to explore the effects of compound probiotics supplemented at different dosages on production performance, immune function and antioxidant capacity of broilers, as well as their intestinal microbiota structure, and further discuss their application potential as functional feed additives.

Methods A total of 480 healthy 1-day-old yellow-feathered broilers were selected and randomly divided into 4 treatment groups with 6 replicates per group and 20 broilers per replicate. The control group (CON) was fed basal diet, while the experimental groups P400, P600 and P800 were supplied with 400, 600 and 800 g/t compound probiotics in basal diet respectively, with a 56-day feeding trial period.

Results At 56 days of age, the final body weight, average daily gain from 29 to 56 days and 1 to 56 days of broilers in groups P600 and P800 were significantly higher than those in the CON group (P<0.01). At 56 days of age, the serum immunoglobulin M (IgM) contents of groups P600 and P800 were significantly elevated compared with the CON group (P<0.05), and the jejunal secretory immunoglobulin A (SIgA) levels of the two groups increased significantly (P<0.01). The serum total superoxide dismutase (T-SOD) activity of group P800 was markedly enhanced relative to the CON group at day 56 (P<0.05), and the serum glutathione peroxidase (GSH-Px) activities of groups P600 and P800 were significantly higher than the control (P<0.05). Hepatic T-SOD activity was significantly increased in group P600 at 28 days of age (P<0.05). The total antioxidant capacity (T-AOC) levels in the liver of all experimental groups rose significantly at 56 days of age (P<0.01), and the hepatic malondialdehyde (MDA) content of group P800 was significantly lower than that of the CON group (P<0.05). Analysis of cecal microbiota at day 56 indicated that compound probiotic treatment dramatically altered the intestinal microbiota structure compared with the CON group. At the phylum level, the relative abundance of Firmicutes was raised while that of Bacteroidota declined. At the genus level, probiotic supplementation generally increased the relative abundance of beneficial bacteria including Faecalibacterium, Lactobacillus and genera related to Oscillospiraceae, and reduced the abundance of opportunistic pathogens such as Bacteroides. The P600 dosage group exerted the most obvious regulatory effect on the composition of cecal microbiota, accompanied by variations in the relative abundance of multiple genera affiliated to Firmicutes.

Conclusion Dietary supplementation of compound probiotics can not only improve the immune function and antioxidant capacity of broilers, but also boost broiler growth performance by modulating intestinal microbiota structure.