Optimising rabbit performance under cyclic heat stress: effects of nano-chromium chloride on growth, immunity, antioxidant defense, and gene expression in New Zealand White and Rex breeds

Heat stress significantly impairs rabbit productivity, especially in hot climates. This study evaluated the impact of nano-chromium chloride (nano-CrCl3) supplementation on growth performance, antioxidant status, immunity, and gene expression in New Zealand White (NZW) and Rex rabbits under heat stress. A total of 216 rabbits (108 per breed) were used in a 2 × 2 × 3 factorial design: two temperature conditions—comfortable (CT, 24.5 ± 1.3 °C) and cyclic heat stress (CHS, 32.8 ± 1.5 °C)—and three nano-CrCl3 levels (0, 1, and 2 mg/L). Supplementation with 2 mg/L nano-CrCl3 improved body weight gain, feed intake, and feed conversion ratio in both breeds, with NZW rabbits performing better under CHS. Nano-CrCl3 enhanced immunity by increasing phagocytic activity and reducing heterophil-to-lymphocyte ratios, especially under CT. Under CHS, it boosted IgG levels and reduced TNF-α, IL-4, and IFN-γ, indicating an anti-inflammatory effect. Antioxidant activity (SOD and CAT) was higher under CT, while nano-CrCl3 reduced oxidative stress markers such as MDA. Gene expression analysis revealed no direct effect of nano-CrCl3 on HSP70, GHR, or IGF1 expression. However, breed–temperature interactions were significant: NZW rabbits showed HSP70 upregulation under CT and downregulation under CHS; IGF1 was downregulated under CHS in NZW rabbits, while GHR was upregulated in Rex rabbits under the same conditions. In conclusion, nano-CrCl3 at 2 mg/L effectively improved rabbit growth, immunity, and oxidative stress resistance, mitigating the negative effects of heat stress.