Rapid growth rate has presented broiler chickens with serious difficulties when called on to thermoregulate efficiently in hot environmental conditions. Altering the incubation temperature may induce an improvement in the acquisition of thermotolerance (AT). This study aimed to elucidate the effect of thermal manipulations (TM) during the development of the thyroid and adrenal axis of broiler embryos on the potential of broilers to withstand acute thermal stress at marketing age. Cobb broiler embryos were subjected to TM at 39.5 degrees C and 65% RH from embryonic day 7 to 16 (inclusive), either continuously (24 h) or intermittently (12 h). After hatching chicks were raised under standard conditions to 35 d of age and then subjected to thermal challenge (35 degrees C for 5 h). Continuous TM caused a significant decline in hatchability, coupled with significantly lower BW and body temperature at hatching. The intermittent (12-h) chicks showed results similar to the controls but had significantly lower body temperature. Thermal challenge at marketing age demonstrated a significant improvement in AT in both the 12- and 24-h TM-treated broilers, which was characterized by a significantly lower level of stress (as evidenced by the level of plasma corticosterone) and rate of mortality. It was concluded that TM during the portion of embryogenesis when the thyroid and adrenal axis develop and mature had a long-lasting effect and improved the AT of broiler chickens. Whereas intermittent TM had no significant effect on hatchability and performance parameters, continuous TM negatively affected these parameters.

This study aimed at elucidating the effects of thermal manipulation (TM) during different periods of embryogenesis on hatchability, body weight, and thermoregulation of Ross chicks at hatch, and on the chick's ability to cope with thermal challenge (TC) at 3 d of age. Control embryos were incubated at 37.8 degrees C and 56% RH, whereas the TM embryos were treated for 3 h at 39.5 (1) or 41.0 degrees C (2) and 65% RH, during early embryogenesis (EE), E8 to E10, and late embryogenesis (LE) at E16 to E18. At hatch, after feather drying, BW and body temperature (Tb) were measured, blood was drawn from the jugular vein to measure plasma concentrations of thyroxine (T4), triiodothyronine (T3), and corticosterone. These parameters were also measured in 3-d-old chicks during exposure to TC at 41 degrees C for 6 h. Hatchability was significantly higher in the LE1-treated group than in the other treatments. EE1- and LE1-treated embryos exhibited significantly lower Tb at hatch than the LE2 chicks. Significant hyperthermia was noted in the control chicks during 6 h of TC; it coincided with a higher plasma T3 concentration (P = 0.074). During TC, the LE1-treated chicks exhibited the lowest T3 to T4 ratio and a significantly reduced plasma corticosterone concentration. It can be concluded that TM at 39.5 degrees C for 3 h during E16 to E18 of incubation improved chick's thermotolerance acquisition and reduced the corticosterone level of chicks exposed to TC at the age of 3 d.

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Air velocity (AV) is one of the main environmental factors involved in thermoregulation, especially at high ambient temperatures. To elucidate the effect of AV on performance and thermoregulation of 4- to 7-wk-old broiler chickens, an experiment was conducted using 4 different AV (0.8, 1.5, 2.0, and 3.0 m/s) at constant ambient temperatature (35 +/- 1.0 degrees C) and RH (60 +/- 2.5%). BW, feed intake, and fecal and urinary excretions were monitored in individuals and were used to calculate the amount of energy expended for maintenance. Infrared thermal imaging radiometry was used to measure surface temperatures for the calculation of heat loss by radiation and convection. Brachial vein blood was collected for plasma osmolality and arginine vasotocin analysis. Broilers performed optimally at an AV of 2.0 m/s. Energy expenditure for maintenance was significantly higher under these conditions, suggesting the ability to direct a sufficient amount of energy to control body temperature, while maintaining relatively high growth rates. Convective heat loss increased significantly with increasing AV, whereas radiative heat loss was not affected. Sensible heat loss, expressed as a percentage of energy expenditure for maintenance, was significantly higher at 2.0 m/s compared with 0.8 m/s but significantly lower than that of 3.0 m/s. The high level of heat loss observed at 3.0 m/s probably affected body water balance, as supported by significantly higher plasma osmolality, arginine vasotocin concentration, and the hyperthermic status of these birds. It can be concluded that AV of 2.0 m/s enables broilers to maintain proper performance together with efficient thermoregulation and water balance under harsh environmental conditions.

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