Effect of early-age thermal conditioning and food restriction on performance and thermotolerance of male broiler chickens

1. The effects of early-age thermal conditioning and food restriction on pe rformance and thermotolerance were studied in male broiler chickens, in 2 t rials. 2. Chickens were exposed to 36 degrees+/-1 degrees C and 70% to 80% relativ e humidity (RH) for 24 h at the age of 5 d (thermal conditioning; TC), or t o food restriction (FR) at the age of 7 to 14 d, or to both treatments (TCFR), while a control group was reared under standard conditions. At the age of 42 d, chickens were thermally challenged by a heat stress of 35 degrees +/-1 degrees C and 20% to 30% RH for 6 h. 3. In both experiments, weight gain of the TC chickens between the ages of 7 and 42 d was significantly higher than those of other treatments and was associated with higher food intake. 4. Early-age TC significantly increased body temperature (T-b). Thermal cha llenge at the age of 42 d markedly increased T-b in all groups but that of the TC groups was the lowest. 5. Mortality during thermal challenge was significantly lower in the treate d chickens, except for the FR group in trial 2. 6. Plasma triiodothyronine (T-3) concentration was greatly depressed in all treatment groups during the thermal challenge. However the lowest concentr ation was observed in the TC group, suggesting that these chickens exhibit the lowest rate of heat production under such conditions. 7. Thermal conditioning reduced the increase of haematocrit with age, where as food restriction resulted in an increase in haematocrit immediately afte r FR. Thermal challenge resulted in a haematocrit decline in all groups, wi th the lowest values in the TC and TC+FR chickens. 8. It can be concluded that. because the TC treatment improved thermotolera nce (possibly by reducing heat production) and performance, it has advantag es over the FR and TC+FR treatments.