Nano-Encapsulated Cumin Oil and Bacillus subtilis Enhance Growth Performance, Immunity, Oxidative Stability, and Intestinal Integrity in Growing Rabbits Under High Ambient Temperature

The study evaluated the influence of dietary supplementation with nano-encapsulated cumin oil, <i>B. subtilis</i>, or a combination of both to mitigate the impacts of heat stress on the performance and health of growing rabbits. In the feeding trial, a total of eighty-four growing New Zealand White (35 days, 781.3 ± 1.8 g average body weight) were randomly distributed in a completely randomized design into four groups; each had 21 rabbits arranged in 7 replicates (3 rabbits each). The experiment lasted 42 days (35 days to 77 days). Growing rabbits received a basal diet (first group, CON) without additives, while the other groups were supplemented with nano-encapsulated cumin oil (NECO, 200 mg/kg), <i>B. subtilis</i> (BS, 500 mg/kg), or both (BSNO, 500 mg BS plus 200 mg/kg NECO). Adding BSNO significantly enhanced body weight gain, carcass weight, and feed conversion ratio and reduced mortality rate (<i>p</i> < 0.05). Additionally, the BSNO enhanced digestive system performance by increasing the secretion of trypsin enzymes, as well as nutrient digestibility, especially for protein and fiber (<i>p</i> < 0.05). Supplementing BSNO enhanced oxidative stability and immunity via higher levels of superoxide dismutase (SOD), IgA, IgG, triiodothyronine (T3), thyroxine (T4) and lower malondialdehyde (MDA) levels (<i>p</i> < 0.05), indicating a better ability to adapt to stress. During the examination of gut health, pathogenic bacteria counts decreased, as well as down-regulation of interleukin-6 (IL-6) gene expression and up-regulation of cationic amino acid transporter-1 (CAT-1), interleukin-10 (IL-10), and mucin-2 (MUC-2) gene expression (<i>p</i> < 0.05), supporting gut integrity. This study highlights the potential of mixing nano-encapsulated cumin oil and <i>B. subtilis</i> in growing rabbits' diets as an effective strategy to counteract the negative effects of heat stress caused by high ambient temperatures.