Combined addition of essential oils and Bacillus subtilis to the diet improves egg quality, antioxidant capacity, immune response and modulates intestinal microbiota in late-phase laying hens

Context Dietary supplementation with combinations of additives is an emerging trend in poultry production. However, limited research has explored the impact of simultaneous inclusion of essential oils (EO) and Bacillus subtilis (BS) on hens during the late laying stage. Aims This study evaluated the impact of EO and BS supplementation on intestinal microbiota, immune response, antioxidant capacity, egg quality, and production performance of aging hens. Methods A total of 360 Lohmann Brown hens (65 weeks old; average egg production rate of 89.5%) were randomly assigned to three dietary treatments, each with six replicates (n = 20 per replicate). The treatments were conducted for 8 weeks as follows: (i) basal diet; (ii) a basal diet supplemented with 0.50 g EO/kg + 0.25 g BS/kg; and (iii) a basal diet supplemented with 0.50 g EO/kg + 0.50 g BS/kg. Key results Diets supplemented with 0.25 g/kg of BS and 0.50 g/kg of EO resulted in increased eggshell thickness and yolk index on Day 56 of the experiment, elevated plasma concentrations of immunoglobulin G and interleukin-2, and higher activities of total superoxide dismutase and catalase in the liver (P < 0.05), but reduced the proportions of Escherichia coli and Salmonella in the cecum, and Staphylococcus aureus in the ileum of hens (P < 0.05). Additionally, supplementing diets with 0.50 g/kg of EO and 0.50 g/kg of BS resulted in increased eggshell thickness on Days 28 and 56 of the experiment, yolk index on Day 56 of the experiment, immunoglobulin G concentration, total antioxidant capacity in the plasma, and catalase activity in the liver (P < 0.05). Moreover, it modulated gut microbiota by increasing the relative abundances of Lactobacillus, Anaerostipes and Ruminiclostridium_9, and the proportion of Bifidobacterium, as well as by reducing the proportions of E. coli, Salmonella and S. aureus (P < 0.05). Conclusions Overall, the simultaneous dietary supplementation with EO and BS improved eggshell thickness and yolk index, modulated the intestinal microbiota, and partially enhanced antioxidant capacity and immune response in older hens. Implications Combined EO and BS can be successfully used as feed additives for improving the health and egg quality of aging hens.