Hemodynamic changes were studied in broiler chickens during exposure to constant temperatures (10 to 35 C) or diurnal temperature cycles (10:30 C and 15:35 C), and during acute heat or cold. Packed cell volume (PCV) was significantly high at low constant temperatures (10 and 15 C), whereas at high temperatures (30 and 35 C) plasma volume (PV) was significantly high. A linear relationship between hematocrit and heart weight was observed and indicates an adaptation of heart mass to changes in cardiac output and hematocrit to meet the demands of increased basal metabolic rate. Only during the diurnal temperature cycle of 15:35 C did a significant increase in PV occur when ambient temperature (Ta) was raised form 15 to 35 C. Acute exposure of chickens to high temperature did not affect PV or PCV, but resulted in hyperthermia (44.7 +/- 0.4 C). Changes in PCV are probably related to modulation of the supply of oxygen to accommodate changes in heat production. The significant hypervolemia observed at high temperatures could occur to provide the fluid needed for heat dissipation by panting. The lack of response of the blood system to acute temperature changes may be at least partially responsible for the chickens' failure to control body temperature.

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.

1. Male broiler chickens were exposed in separate experiments to: (a) constant ambient temperatures (Ta) ranging between 10 and 35 degrees C; (b) 12 h:l2 h diurnal high-low temperature of 10:30 degrees C, 15:35 degrees C and 15:30 degrees C in different trials; and (c) 12 h of 30 or 35 degrees C and 12 h of variable lower temperature. 2. A progressive decline in weight gain and food intake was obtained as Ta increased from 18 to 35 degrees C. Under diurnally cycling temperature, weight gain and food intake were lower than in the average corresponding constant temperature, with the exception of chickens exposed to 15 : 30 degrees C, where weight gain and food intake were not significantly different from those of the constant average temperature. 3. As the diurnal cold period was made colder, chickens exposed during half of the diurnal cycle to high temperature (30, 35 degrees C) demonstrated a significant (P <= 0.05) increase in weight gain and food intake. 4. There was a good correlation between plasma triiodothyronine (T3) and Ta. The correlation between T3 and food intake was highly significant, but that between T3 and weight gain was poor. 5. The results indicate the difficulties in predicting the performance of broilers exposed to diurnally cycling temperatures from knowledge of the average temperature.

The effect of air velocity (AV) on weight gain, feed intake, and feed efficiency was studied in male broiler chickens exposed to a constant 35 C, 60% RH, and continuous illumination from 4 through 7 wk of age. In two separate trials, AV of 0.5, 1.5, 2.0, and 3.0 m/s (Trial 1) and 1.0, 1.5, 2.0, and 2.5 m/s (Trial 2) were used. The fifth week was used for acclimation to the targeted environmental conditions. Weight gain was significantly higher in broilers exposed to AV of 1.5 and 2.0 m/s in both trials, which coincided with significantly higher feed intake and feed efficiency. Exposure to the low AV (0.5 and 1.0 m/s) resulted in weight gain, feed intake, and feed efficiency that were less than those of broilers exposed to high AV (2.5 and 3.0 m/s). In all treatments, broilers developed hyperthermia, but there were no significant differences in body temperature (Tb) among the birds. It can be concluded that AV has a significant effect on broiler performance; the optimal AV was found to be 1.5 to 2.0 m/s.