The Effect of Air Temperature on Large Broiler Body Temperatures

A study was conducted at the University of Georgia Poultry Research Center to investigate the relationship between air temperature and broiler body temperatures (46 to 58 days of age, 7.3 lbs and 10.4 lbs respectively). The body temperatures of seven broilers were monitored using a micro data logger that the birds swallowed. The birds were placed in a 4' X 5' floor pen, along with others without data loggers, at an initial density of approximately six pounds per square foot. The room’s ventila...tion system operated constantly, providing an air exchange rate of less than once a minute, which resulted in the room's air temperature closely following outside air temperature, which varied between 65 and 85°F. The room’s evaporative cooling system was not used, nor were the room’s circulation fans used to provide supplemental air movement over the birds. Room air temperature, and body temperatures were recorded every minute throughout the two-week study.

Figure 1 illustrates how bird body temperature varied over a fairly typical spring day where room air temperature was in the sixties at night and climbed to the low eighties during the day. When room temperatures were at their lowest, bird body temperatures were around 106oF, which is typically considered “normal.” As the room temperature climbed into the eighties, the birds' body temperatures rose to around 108°F, indicating that they were heat-stressed. This was confirmed by the fact that the birds were observed panting.

Figure 2 illustrates the relationship between room air temperature and bird body temperatures between the hours of 6 AM and 12 PM over the entire 12 day study. Data between the hours of 12 PM and 6 AM were excluded because body temperatures tended to spike during the four-hour dark period and didn’t return to “normal” until approximately 6 AM (Figure 3). This trend has been observed in several field trials conducted in the past. During dark periods, bird movement is minimal, resulting in little to no heat loss from the undersides of the birds due to the insulating properties of the litter on which they sit. The variations in body temperatures between birds seen in Figure 3 are likely due to differences in each birds proximity to other birds in the pen or differences in bird size and/or metabolism.

Although data was limited at the lower room temperatures (< 68oF), it appears that bird body temperatures tended to remain around 106°F between room temperatures of 64°F and 66°F, after which they began to increase as the room temperatures increased slowly. This data suggests that for large broilers such as these, the thermal neutral temperature—the temperature at which a bird can essentially sit still and easily dissipate the heat it is producing—is around 65°F. As temperatures increase above this point, heat slowly begins to build up within the birds, indicating that the birds could benefit from the introduction of some low level of air movement to aid them in heat removal so that they can maintain a body temperature closer to the norm of 106°F. This fact was demonstrated one afternoon when the circulation fans were turned on between 1 PM and 5 PM, resulting in an air speed at bird level of approximately 300 ft/min. Average body temperatures decreased during this time from approximately 108°F to essentially a normal body temperature of 106.5°F (Figure 4).

 

Figure 5 is a graph of the average bird body temperature versus room air temperature over the course of the study (excluding 12 AM to 6 AM). A linear regression performed on the data indicates that bird body temperatures increase approximately 0.1°F for every degree increase in air temperature above an air temperature of approximately 66°F. Though the precise increase in body temperature varied between birds, it was found that it was relatively minimal and nearly constant as room temperatures varied between 65 and 85°F (Standard deviation of approximately +/- 0.4 F, Figure 6).

In conclusion, this study, though very limited in scope, provides interesting insights into the thermal regulation of large broilers in response to varying ambient temperatures. The data suggest that the thermal neutral zone for broilers weighing between 7.3 and 10.4 pounds is around 65°F, beyond which body temperatures begin to rise steadily, indicating an increase in thermal stress. The consistent increase of about 0.1°F in body temperature per degree rise in room temperature above this threshold, along with observed panting behavior, underscores the importance of monitoring birds for signs of elevated body temperatures at even relatively low air temperatures. The most effective method of helping a bird lose excess heat is through the use of tunnel ventilation. Moving air over the birds will remove heat from them, resulting in lower body temperatures and, in turn, better performance and welfare. The precise level of air movement required will vary depending on factors such as bird density, air temperature, and relative humidity.