Fennel essential oil as a natural feed additive to mitigate heat stress in broilers: Effects on growth, physiological and immunological responses, and intestinal morphology.

This study examined how dietary fennel essential oil (FEO) supplementation affected heat-stressed (HS) broiler chickens' growth, economic efficiency, hematological and biochemical blood profiles, immunological response, antioxidant status, and intestinal histomorphology. A total of 600 one-day (d)-old Ross 308 chicks were assigned to 6 treatments: thermoneutral control (TNC), HS control (HSC), three HS groups with 1, 2, or 3 g FEO/kg diet, and a HS group given paracetamol (500 mg/L drinking water). Birds were raised under TN settings until d 21, exposed to 36 ± 0.5 °C for 6 h/d from d 22-25, and then returned to TN conditions until d 35. Dietary FEO supplementation had no effect on growth during the starter phase (P > 0.05). The grower phase showed significant treatment effects on growth performance indices (P < 0.05), with FEO- and paracetamol-supplemented birds exhibiting comparable body weight (BW) and body weight gain (BWG) to HSC. FI was reduced in FEO-fed groups relative to both HSC and paracetamol group (P < 0.001), while FCR remained comparable among these groups. Finisher phase and overall performance showed enhanced BW, BWG, and FCR in broilers fed 1-3 g FEO/kg versus HSC, despite comparable FI among these groups (P < 0.001). FEO supplementation increased red blood cell count, hemoglobin concentration, packed cell volume, lymphocyte count, and enhanced key biochemical, immunological, and antioxidant markers (P < 0.05). Improved small intestinal histomorphology, increased splenic CD3+ T-cell and CD20+ B-cell immunostaining, enhanced breast muscle fatty acid profile, and greater economic efficiency were also observed in FEO-supplemented birds (P < 0.05). Overall, dietary FEO supplementation improved the resilience of broilers exposed to HS, suggesting it may serve as a viable nutritional strategy for mitigating performance losses associated with HS.