Abstract
Despite increasing interest in the use of essential oils (EOs) as dietary supplements to alleviate the adverse effects of heat stress in poultry, the underlying biological mechanisms responsible for their protective actions remain incompletely understood. These mechanisms are likely influenced by a complex interplay of physiological responses, the specific bioactive constituents of the oils, and formulation variables such as encapsulation and dosage. This study evaluated the effects of dietary supplementation with a blended EO mixture, comprising cinnamon, thyme, and clove oils in a 50%:25%:25% ratio, and its encapsulated form (CEO) on growth performance, antioxidant status, and the expression of stress-responsive genes in broiler chickens exposed to natural summer heat stress conditions. A total of 350 one-day-old Cobb 500 male chicks were randomly assigned to seven treatment groups: a control group and six groups supplemented with EO or CEO at 50, 100, or 150 mg/kg diet. Each group consisted of five replicates with ten birds per replicate. The heat stress challenge was validated with a temperature-humidity index (THI) of 30.52, indicating moderate to severe thermal stress. Data were analyzed using one-way ANOVA, and treatment means were compared by Duncan's multiple range test at P ≤ 0.05. Dietary supplementation with EO and CEO, particularly 100 mg/kg CEO (T5), significantly improved growth performance. At 42 d, T5 birds achieved the highest body weight (2380 g/bird; P < 0.01) and body weight gain (2336 g/bird; P < 0.01), feed conversion ratio was improved, with T5 showing the best FCR (1.45; P = 0.043) versus 1.56 in the control. Serum total antioxidant capacity (TAC) was highest in T5 (0.80 mM/L; P = 0.003), representing a 29% increase over the control. Hepatic expression of stress-responsive genes was markedly modulated. CEO at 100 mg/kg downregulated HSP70 by 58.1% (P < 0.001), HSP60 by 62.5% (P < 0.001), and HSPA9 by 62% (P < 0.001). Antioxidant enzymes SOD2, GPX1, and GPX4 were reduced by 75.2% (P < 0.001), 90.1% (P < 0.001), and 83.2% (P < 0.001), respectively. Pro-oxidant NOS2 and glucocorticoid receptor (GR) expression decreased by 50% (P < 0.05) and 55% (P < 0.05), respectively, while NRF2 and HMOX1 were downregulated by 51.5% (P < 0.05) and 62.7% (P < 0.05). Jejunal GLUT2 expression was upregulated by 227.3% (P < 0.05), whereas sGLT1 remained unchanged. No significant changes in liver enzyme activity or glucose levels were observed, suggesting that the treatments did not markedly impair hepatic function under heat-stress conditions (P > 0.05). These findings indicate that encapsulated thyme, cinnamon, and clove oils at 100 mg/kg can substantially enhance growth performance, antioxidant capacity, and cellular stress resilience in broiler chickens under heat stress.