effect of early cold exposure on the endocrine responses of broiler chickens and the incidence of ascites syndrome.

two hundred and fifty day-old broiler chickens were housed in a high altitude farm and divided in two experimental groups (cold-exposed and control). at the age of 3 and 4 d, the cold-exposed chickens were exposed to 15?c for 3 h, while the control birds were reared under normal condition. during the period between 14 and 28 days, both groups of chickens were exposed to a low environmental temperature. for hormonal analyses, blood samples were taken at days three and four (before and after cold exposure), and followed by samples taken at seven, 14, 28 and 42 days. the mean plasma corticosterone level was significantly higher in cold chickens, who were first exposed to cold temperature at days three and four of age. however, this result was reversed at day 28 during the second cold challenge. cold-exposed birds showed significantly higher t3 levels compared to control chickens at day three after exposure to cold temperatures. during the second cold exposure (day 28), the t3 level was significantly lower in cold chickens compared to control chickens. at days three (after cold exposure) and four (before cold exposure), the concentrations of t4 were significantly lower in cold-exposed chickens compared to the control chickens. such hormonal responses coincided with a marked reduction of ascites mortality in the early cold-exposed chickens compared with control chickens. in the present study, repeated cold exposure resulted in a moderate response; lower corticosterone and t3 levels resulted from the second cold exposure than from the first one alone. it can be concluded that early cold exposure improves the thermotolerance of broiler chickens at a later age, which consequently leads to an adaptation that reduces the incidence of the ascites syndrome.