Broilers’ Response to Sudden Changes in Air Temperature

A study was conducted to investigate the impact of sudden increases in air temperature on the body temperatures of broiler chickens. Seven-week-old broilers were monitored using micro temperature data loggers that they ingested. The birds were placed in a 4' x 5' floor pen, and additional birds were placed in the pens without data loggers to create a starting density of approximately 6 pounds per square foot of floor space. The room temperature was generally maintained at approximately 74°F, ex...cept during specific 2- to 6-hour intervals when it was raised to 82°F, 87°F, or 92°F. Throughout the seven-day study, room temperature, relative humidity, and bird body temperatures were recorded every minute.

Figure 1 shows the body temperatures of the seven broilers on the first day of the study (6:00 a.m. to 9:00 p.m.), where an air temperature of 74°F - 76°F was maintained. During this period, body temperatures remained relatively stable, ranging from 106°F to 106.5°F—within the normal range for broilers, as reported in previous research.

On the second day, the room temperature was increased to 82°F from 12:00 p.m. to 3:00 p.m. (Figure 2). During this period, the average body temperature of the birds gradually rose to 107°F over the first two hours and then plateaued for the remainder of the three-hour test period. Although elevated, a body temperature of this level is typically not considered problematic. Within an hour of the room temperature returning to 74°F, the birds’ body temperatures returned to normal levels.

On the third day of the study, the room temperature was raised to 87°F between 10 a.m. and 3:30 p.m. (Figure 3). This time, the average bird body temperature plateaued at approximately 108°F within the first two hours, after the room temperature reached 87°F. A body temperature of 108°F has been shown to reduce broiler feed consumption and weight gain dramatically. Bird body temperatures returned to normal within two hours after the room temperature was restored to 74°F.

On the fourth day, the room temperature was increased to 92°F from 10:30 a.m. to 12:30 p.m. (Figure 4). As expected, the higher room temperature resulted in a greater increase in body temperature (109°F). A body temperature of 109°F would likely have a more negative impact on broiler performance. How much broiler performance would be impacted would depend to a large extent on how long a bird’s body temperature remained elevated. As with previous days, body temperatures returned to normal within two hours of the room temperature returning to 74°F.

On the final day of the study, the room temperature was again raised to 92°F, this time for a duration of four hours. As in the previous day’s trial, the average body temperature plateaued at approximately 109°F within the two hours of the room temperature reaching 92°F. Notably, unlike the earlier trial, in this trial it took over five hours for the birds’ body temperatures to return to approximately 106°F.

There were two differences on this day. First, the room temperature could not be quickly lowered back to 74°F due to the temperature of the incoming air, which was significantly higher (80-82°F). Secondly, the humidity was approximately 20% higher during the second trial. The combination of these factors made it more difficult for the birds to quickly rid themselves of excess heat, thereby increasing the time required for their body temperature to return to normal. Although this should not be surprising, it does reinforce the importance of cooling the birds at the end of the day when they are exhibiting signs of heat stress. Bottom line: the quicker a bird's body temperature can be brought back to normal, the less likely its performance will suffer.

Here are a few things to consider when you are looking to bring near-market age birds' body temperatures quickly back to normal after a stressful hot summer afternoon:

1) Make sure that all the tunnel fans operate throughout the night.

2) Consider keeping the lights on at night. During dark periods, bird movement is minimal, resulting in reduced heat loss from the undersides of the birds due to the insulating properties of the litter on which they sit.

3) Double check to make sure your tunnel fans are properly maintained (belts are tight and not overly worn, shutters and screens are clean).

4) Do not operate evaporative cooling pads late at night. Outside humidity tends to climb above 80% as the outside temperature drops below 80°F. Operating evaporative cooling under these conditions tends to produce little temperature reduction and brings the relative humidity of the incoming air to near saturation, thereby actually reducing heat loss from the birds.