Heat stress (HS), one of the most serious climate problems of tropical and subtropical countries, negatively affects the production performance of broilers. Keeping this in view, the current study was aimed at elucidating the effects of supplementing betaine (Bet) and dried turmeric rhizome powder (TRP), either singly or in combination, on growth performance, leukocyte profile, humoral immunity, and antioxidant status in broilers kept under chronic HS. A total of 625 oneday- old Ross male chicks were randomly assigned to five treatment groups (5 replicates of 25 birds per replicate pen). From day 1, the birds were either kept at the thermoneutral zone (TN) or exposed to HS (33±1 °C) to the conclusion of study, day 42. The treatment groups were as follows: thermoneutral control (TN-CON), HS-CON, HS-Bet, HSTRP, and HS-BT (fed Bet and TRP). The results showed that decreases in body weight gain, feed intake, and increases in feed-to-gain ratio and mortality induced by HS were partially restored by dietary supplementation of Bet and TRP. The heterophil/lymphocyte ratio, total, and IgG antibody titers against sheep red blood cell for secondary responses in the HS-TRP and HS-BT groups were also similar to those of the broilers in the TN-CON group but better (P<0.05) than for HS-CONgroup.An increase (P<0.05) in serumconcentration of malondialdehyde induced by HS was significantly decreased by dietary supplementations. The serum glutathione peroxidase and superoxide dismutase activities were also higher (P<0.05) in the supplemented groups compared to both TN and HS-CON groups. In conclusion, dietary supplementation of either Bet or TRP alone or in combination can partially ameliorate some of the detrimental effects of HS in broilers. Results also suggest that TRP might be better than Bet for improving stress tolerance and immune response in heat-stressed broilers.
