Effects of Pre-Slaughter Dietary Supplementation with a Phytogenic Additive Containing Passiflora incarnata and Stevia rebaudiana Bertoni Extracts on Oxidative Status, Inflammatory Responses, Carcass Quality, Meat Composition, and Bone Morphology in

Background: The final days before slaughter represent one of the most stressful periods in commercial broiler production, during which acute stress stimulates endocrine, oxidative, and inflammatory responses that may compromise animal welfare and slaughter outcomes. This study evaluated the effects of short-term dietary supplementation with a phytogenic additive containing Passiflora incarnata and Stevia rebaudiana extracts (Passiflora–Stevia phytogenic additive; PSPA) during the final five days before slaughter on carcass characteristics, physiological stress, oxidative status, inflammatory responses, tissue composition, and bone morphology in broilers. Methods: A total of 106,240 Ross 308 broilers were assigned to either a control group or a PSPA-supplemented group (1 kg/t) during the final five days before slaughter (days 38–42). Carcass characteristics, breast muscle and liver composition, serum corticosterone, inflammatory cytokines, oxidative stress biomarkers, and bone morphology using three-dimensional geometric morphometrics were evaluated. Results: PSPA supplementation significantly reduced serum IL-1β, IL-6, TNF-α, corticosterone, total oxidant status, and oxidative stress index, while increasing IL-10 concentrations (p < 0.05). Total antioxidant status was unaffected. After adjustment for pre-slaughter live weight, carcass weight was significantly higher in the PSPA group (p = 0.002), whereas carcass yield remained unchanged. PSPA-treated broilers also showed favorable numerical trends in carcass quality, including a higher proportion of Grade A carcasses and fewer carcass defects, although these differences were not statistically significant. Breast muscle and liver composition were unaffected, and only limited treatment-related differences were detected in bone morphology. Conclusions: Short-term PSPA supplementation improved physiological resilience by attenuating corticosterone secretion, oxidative stress, and inflammatory responses before slaughter. Although carcass quality was not significantly altered, the favorable numerical trends and improved physiological biomarkers suggest that targeted preslaughter phytogenic supplementation may represent a practical nutritional strategy to mitigate preslaughter stress in broiler chickens. Longer-term studies are warranted to determine whether these physiological benefits translate into improvements in bone characteristics and slaughter outcomes.