Abstract
This study evaluated the effects of Bacillus subtilis and garlic supplementation on growth performance, oxidative stress, and immune response in heat-stressed broilers. A total of 320-day-old Hubbard broiler chicks were randomly allocated to four dietary treatments: a control group (thermoneutral) and three heat-stressed groups supplemented with Bacillus subtilis at 3 × 10¹⁰, 4 × 10¹⁰, and 5 × 10¹⁰ CFU/kg each combined with 1 % garlic powder. Heat stress was induced by maintaining the temperature at 32-35°C for three hours daily from day 22-35. Broilers were evaluated for feed intake (FI), weight gain (WG), feed conversion ratio (FCR), malondialdehyde (MDA) levels, cecal microbiota composition, and differential leukocyte counts (DLC). The results showed that higher levels of Bacillus subtilis (4 × 10¹⁰ and 5 × 10¹⁰ CFU/kg) with 1 % garlic significantly (P < 0.001) improved FI, WG, and FCR during the finisher phase. Supplemented groups exhibited lower MDA levels, indicating reduced oxidative stress (P < 0.001). Moreover, DLC analysis revealed enhanced immune function, with reduced heterophil counts and increased monocyte and eosinophil percentages (P < 0.05). No significant differences were observed in cecal microbiota composition. Overall, 5 × 10¹⁰ CFU/kg Bacillus subtilis + 1 % garlic showed the best performance, demonstrating its potential as an effective strategy to mitigate heat stress in broilers.