This study aims to utilise thyme oil (TO) as a natural growth promoter in comparison to mannanoligosaccharides (MOS) for broilers raised under hot climates from 1 to 28 days of age. Thus, a total of 180, day-old broilers chicks were divided into five groups (36 broilers/group in 6 replicates of 6 broilers/replicate). The chickens were fed the same corn-soybean meal basal diet and were submitted to one of the following five dietary treatments: (a) positive control group, fed the basal diet supplemented with MOS at 1g/kg feed; (b) negative control group, fed the basal diet without supplementation; (c) thyme oil 1.0 (TO_1.0) group fed TO at 1.0 g/kg feed, (d) thyme oil 1.5 (TO_1.5) group fed TO at 1.5 g/kg feed and (e) thyme oil 2.0 (TO_2.0) group fed TO at 2.0 g/kg feed. The TO_1.0 displayed a better feed conversion ratio (FCR) than did the other treatments (p < .01). The MOS, TO_1.5 and TO_2.0 groups had higher (p < .01) plasma total protein than the control, and TO_1.5 also increased plasma globulin (p < .01) compared to the control, but decreased plasma albumin/globulin ratio. Moreover, the TO groups significantly decreased the plasma AST. Groups on the MOS and TO_1.0 diets showed higher (p < .01) white blood cells (WBCs) than the other groups. In addition, the MOS and TO_2.0 groups exhibited a greater (p < .01) antibody titre to infectious bursa disease (IBD) than the control group. In conclusion, TO at 1.0 g/kg diet may be used as a potential growth enhancer for broilers in hot region during 1-28 days.ARTICLE HISTORY

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