Heat stress in poultry is a serious problem in many countries and causes enormous economic losses. Heat stress has been associated with oxidative stress. Hence, nutritional interventions with antioxidants might be beneficial. Therefore, the effects of dietary Curcuma xanthorrhiza (CX) and Oreganum compactum (OC) essential oils; both rich in simple phenolic compounds, on performance, mRNA levels of heat shock protein 70 (HSP70) and antioxidant enzymes, oxidative status and meat quality of heat challenged broilers were studied. Starting on d 25 of age of Ross 308 broilers, dietary treatments were applied, i.e., a control diet and 4 diets containing 200 or 400 mg/kg feed of CX or OC (CX200, CX400, OC200, OC400 diets). Each diet was given to 3 pen replicates of 20 chickens each in a completely randomized design. Commencing on d 28 of age, the temperature was increased from 22°C to 34°C with 50% relative humidity for 5 h daily during 2 wk. Dietary CX or OC did not affect zootechnical performance of the chickens (P > 0.05). Compared to control, mRNA levels of HSP70 were reduced at d 31 by feeding CX400 and OC400 in kidney and liver, respectively (P < 0.05). The mRNA levels of SOD were increased at d 31 on the OC400 diet in kidney and on the CX400 diet in heart (P < 0.05). Feeding both 400 mg/kg of CX and OC increased the mRNA levels of CAT at d 31 (P < 0.05). In heart, at d 31, both dietary levels of CX and OC200 resulted in higher GSH-Px activity (P < 0.05). Feeding CX400 increased SOD activity in liver, kidney, and heart at d 31 (P < 0.05). A higher activity of CAT was observed in the CX200 and OC400groups at d 42 (P < 0.05). Feeding CX at both levels decreased the ferric reducing ability in plasma at d 42 (P < 0.05). Feeding CX at both levels and OC200 decreased plasma malondialdehyde concentrations at d 42 (P < 0.05). There were no differences in breast WHC, pH, and the a* value in fresh breast meat (P > 0.05), whereas feeding CX400 and both levels of OC increased the a* value in stored breast meat (P < 0.05). Feeding OC diets tended to decrease the TBARS values in fresh breast muscle (P = 0.061). In conclusion, these results indicate that dietary essential oils rich in simple phenolic compounds offer potential for improving the antioxidant defense against heat stress induced changes.

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