Optimal dietary dose of Carnosic acid–chitosan nanoparticles for improving growth performance and health status of newly weaned rabbits under severe heat stress

This study was conducted to evaluate the effects of dietary Carnosic acid–chitosan nanoparticles (CA-CNPs) on growth performance, feed utilisation, and overall health status of heat-stressed growing rabbits. A total of 150 weaned New Zealand White rabbits (5 weeks old; 693.92 ± 15.42 g) were assigned to five groups: a control group fed a basal diet and four groups supplemented with 25, 50, 100, or 200 mg CA-CNPs/kg diet for eight weeks. The selected doses were based on a pilot study evaluating tolerance and preliminary growth responses. The temperature–humidity index (29.99 ± 0.82) indicated severe heat stress. Dietary CA-CNPs supplementation significantly improved body weight gain, feed conversion ratio, and performance index in a quadratic manner, with optimal responses observed at approximately 140 mg/kg diet. Rectal temperature and respiratory rate decreased quadratically, suggesting enhanced thermoregulation. Carcase yield and edible organ weights were significantly increased without affecting meat composition. CA-CNPs supplementation linearly increased serum total protein, albumin, globulin, thyroid hormones, immunoglobulins, and nitric oxide, while linearly reducing cholesterol, triglycerides, and glucose concentrations. Hepatic and renal biomarkers, MDA, 8–OHdG, and pro-inflammatory cytokines (TNF-α, IFN-γ, IL-6) decreased quadratically, whereas antioxidant enzymes (SOD, CAT, GPX) and total antioxidant capacity increased quadratically. Histological examination revealed marked hepatic and renal protection at 100 mg/kg. Quadratic regression analysis indicated optimal CA-CNPs inclusion levels between 125 and 140 mg/kg diet. In conclusion, dietary supplementation with CA-CNPs significantly improved growth performance, physiological adaptability, antioxidant defense, and immune function, while mitigating heat-induced oxidative and inflammatory stress in growing rabbits.