Effects of heat conditioning and dietary ascorbic acid supplementation on heat shock protein 70 expression, blood parameters and fear-related behavior in broilers subjected to heat stress.

Background: Heat stress is one of the most important environmental factors. Exposure of broilers to high temperature causes significant changes in physiological response. Early heat conditioning induces the heat tolerance of broiler chickens at later growth stage prior to marketing. Ascorbic acid supplementation has been reported to may alleviate the negative effects of heat stress on the performance of broilers. This study was carried out to investigate the effects of heat conditioning and dietary ascorbic acid supplementation on heat shock protein 70 expression, blood parameters and fear-related behavior in broilers subjected to heat stress. Material, Methods & Results: A total of 320 male broilers were used as the material of this study. Broilers were randomly assigned to four treatments. Each group contained four replicates with 20 chicks in each pen. Until the 21 days of age, all animals were raised at comfort temperature. The brooding temperature was maintained at approximately 32o C for the first 3 days and then decreased 3o C weekly until 21 days. Broilers in control group were kept under thermo-neutral conditions (24o C) and fed with the basal diet throughout experimental period. Other three groups were exposed to heat stress from 22 to 42 days of age. Heat stress was applied by exposing the broilers to a temperature of 35o C for 6 h/day between 10:00 h and 16:00 h. Ascorbic acid supplemented group was fed a diet supplemented with 500 mg of L-ascorbic acid/kg ration and exposed to heat stress from 22 to 42 days of age. Heat conditioned group was exposed to a temperature of 36o C for 24 h at the age of 5 days; fed with basal diet and exposed to heat stress from 22 to 42 days of age. Heat stress group was fed with the basal diet, no subjected to heat conditioning and exposed to heat stress from 22 to 42 days of age. The broilers in heat stress group had higher body temperature, H/L, TI duration, serum corticosterone, glucose, cholesterol, triglyceride concentrations and Hsp 70 expression in brain, liver and kidney tissues, but lower antibody titer against NDV than those in control group. The lower H/L, TI duration, serum corticosterone, glucose, cholesterol, triglyceride concentrations and Hsp 70 expression in brain, liver and kidney, whereas the higher antibody titer against NDV were obtained in heat conditioned and ascorbic acid supplemented diet fed groups, compared with heat stress group. Discussion: The results obtained in this study showed that heat stress caused to increase in H/L ratio, TI duration, body temperature, serum corticosterone, glucose, cholesterol, triglyceride concentrations and Hsp 70 expression in brain, liver and kidney, whereas it decreased antibody titer against to NDV. Heat conditioning and dietary ascorbic acid supplementation decreased H/L ratio, TI duration, serum corticosterone concentration and Hsp 70 expression and also increased antibody titer against to NDV, indicating the lower stress level in these groups. From these results, it can be said that heat conditioning and ascorbic acid supplementation alleviated the negative effects of heat stress in broilers. Heat conditioning was more effective than ascorbic acid supplementation in alleviating the adverse effects of heat stress on brain, liver and kidney tissues. In conclusion, heat conditioning of broilers by exposure to 36o C for 24 h at the age of 5 days and dietary ascorbic acid supplementation (500 mg/kg of diet) may offer a potential protective management practice in preventing heat stress in broilers.