Heat stress is one of the major challenges which the poultry industry faces during summer in tropical and subtropical regions. This study was conducted to evaluate the impact of Vitamin (Vit) E, C and/or probiotics on improving tolerance of broiler chickens to chronic heat stress (CHS). A total of 294, 1-day-old Cobb-500 broiler chicks were allocated into seven treatment groups; Thermoneutral group was raised under a thermoneutral condition during 25–42 d of age. The other six groups were raised for three successive days per week at 36±2ºC and 75–85% relative humidity for 7 h daily: heat stressed group, Vit E (100 mg/kg diet), Vit C (200 mg/kg diet), Vit C+Vit E, probiotics (Saccharomyces cerevisiae and Lactobacillus acidophilus at 2 g/kg diet) and Vit C+Vit E+probiotics. Exposure to CHS decreased body weight gain (BWG), feed intake (FI), and abdominal fat. It had adverse impact on feed conversion ratio (FCR), packed cell volume (PCV), monocyte, basophil, total protein, and phagocytic activity while increased plasma cholesterol and aspartate aminotransferase (AST) compared to the thermoneutral group. Vit E, Vit C or probiotics alone decreased the adverse effects of CHS on growth rate throughout the experimental period. Vit C and E were equally potent during the experimental period, but were less effective than the combination of both vitamins. Vit E increased the dressing percentage and abdominal fat as compared to the thermoneutral group, but decreased AST while increasing basophil, monocyte and globulin compared to the heat stressed group. In addition, serum albumin and AST of Vit E were lower compared to Vit C, but cholesterol was higher. Vit E increased red blood cells and white blood cells, but plasma cholesterol was increased compared with the heat stressed group. Vit C increased PCV, lymphocytes, monocyte, basophil and albumin and decreased neutrophil. Both vitamins without/with probiotic induced a further increase in basophil, serum total protein, and albumin. It could be concluded that supplementation of Vit E, Vit C, probiotics, and different combinations reduced some of the adverse effects of CHS, and Vit E+Vit C+probiotics was the most effective for economic traits followed by Vit E+Vit C or probiotics.
