Piestun, Y., Inst. of Animal Science, ARO the Volcani Center, Bet Dagan 50250, Israel, Dept. of Animal Sciences, The Robert H. Smith Fac. of Agriculture, Food and Environment, The Hebrew Univ. of Jerusalem, Rehovot 76100, Israel
Halevy, O., Dept. of Animal Sciences, The Robert H. Smith Fac. of Agriculture, Food and Environment, The Hebrew Univ. of Jerusalem, Rehovot 76100, Israel
Shinder, D., Inst. of Animal Science, ARO the Volcani Center, Bet Dagan 50250, Israel
Ruzal, M., Inst. of Animal Science, ARO the Volcani Center, Bet Dagan 50250, Israel
Druyan, S., Inst. of Animal Science, ARO the Volcani Center, Bet Dagan 50250, Israel
Yahav, S., Inst. of Animal Science, ARO the Volcani Center, Bet Dagan 50250, Israel

Previous studies on broilers conducted in our laboratory showed that intermittent thermal manipulation (TM) of 39.5°C every 12. h during embryonic days E7-E16 improved thermoregulative parameters in the embryo and post-hatch, as well as the broilers' ability to cope with heat challenge at marketing age. The aim of this study was to investigate the effect of intermittent TM of 39.5°C (65% RH) for 12. h/d (12. H), during E7-E16, on post-hatch performance and thermoregulation under both optimal and hot conditions. Chicks from the control and TM treatments were raised under "regular" conditions until 21. d of age. Then, each treatment was divided into 2 sub-treatments (growth conditions) - optimal conditions (25°C) and hot conditions of 32°C for 12. h/d - until 35. d of age. Under optimal conditions, no difference in growth rates was found between TM and control throughout the growth period, but TM had significantly larger relative breast muscle weight, and on days 13, 25, and 35 breast muscle in the TM treatment contained a higher percentage of large-diameter fibers than controls. Consequently, average fiber diameter was significantly larger in TM than in control. Relative weight of the abdominal fat pad was significantly lower in the TM treatment than in control, under both environmental conditions. Hot conditions negatively affected growth rates of broilers in both treatments. The TM chickens showed no body-weight advantage over controls, nevertheless, feed conversion between days 21 and 35 was lower in the TM chickens of both sexes than in controls. This could be attributed to lower body temperature and lower plasma thyroid hormone levels found in the TM chickens, indicating lower heat production rates. Greater muscle growth and lower relative weight of abdominal fat pad were found in the TM chickens than in controls, under hot conditions. It can be concluded that TM during embryonic development improves performance as well as thermoregulation. © 2011 Elsevier Ltd.