The relationship between weight gain and body temperature in broiler chickens

A rapidly growing broiler generates an astonishing amount of heat, producing approximately 10 to 15 Btu/hr per pound of body weight. For comparison, one matchstick produces one Btu of heat, indicating that a 4-pound (1.8kg) broiler actually generates 40 to 60 Btu units of heat internally. An 8-pound broiler produces 80 to 120 Btu units of heat, which it must continuously expel to maintain normal body temperature. As the chicken's body temperature rises, its feed intake decreases. This is because the heat generated internally by digesting feed can potentially elevate the body temperature to dangerous levels. Consequently, a chicken with an elevated body temperature tends to reduce its feed intake, which contributes to its temperature rise. After all, who would enjoy a heavy meal on a hot summer afternoon? Reducing feed intake, in turn, slows down the growth rate. It's very easy. If we want to maximize the production performance of broiler chickens, we must create an environment that allows the flock to easily expel the excess heat they generate.
A recent field study conducted at a broiler chicken farm provides a good illustration of the relationship between body temperature and weight gain during the last week of the broiler flock. In one of the chicken houses at the farm, eight 34-day-old broiler chickens were equipped with a small temperature recorder that could record their body temperature every minute. These eight chickens were placed in a pen within the chicken house and remained there until they were slaughtered five days later. We weighed the flock at the beginning and end of the study (Table 1). The air temperature inside the chicken house was measured every minute, ranging from 23.9 to 29.4 degrees Celsius (Figure 2). The air velocity was measured at a location four feet (0.1 meters) above the pen, with an average of approximately 500 feet per minute (2.5 m/s).

During the five-day study, the weight gain of each chicken was determined, and their average body temperatures were plotted into a graph (Figure 2). The normal deep body temperature of broiler chickens is approximately 106°F (41.1°C). During the study period, the average body temperature of the chickens ranged from 106.5°F (41.1°C) to 108.8°F (42.7°C), which is extremely high. Two chickens with average body temperatures exceeding 108°F (42.2°C) did not gain weight during the study period (one of them actually lost weight). The two birds with the lowest average body temperatures (around 106.5°F - 41.1°C) had the highest weight gain, approximately 1.3 pounds (0.59 kg). There was no correlation between initial weight and body temperature. This may be because there was minimal difference in the initial weights of the chickens.

As documented in this article and previous field studies, there are significant variations in chicken body temperature within the same shed (Figure 3). These variations may be attributed to differences in location within the shed, density of nearby chickens, chicken size, genetics, growth rate, health status, and other factors. For instance, the chicken with the highest body temperature was found to have a mild respiratory infection. Another chicken with an average body temperature exceeding 108°F (42.2°C) may have other health issues, but an autopsy did not reveal any apparent health problems. The fact remains that regardless of whether the increase in body temperature is due to disease or insufficient heat removal caused by high environmental temperature and humidity, the response of the flock is the same; they will reduce feed intake, and consequently, their weight gain will also decrease. In hot weather, the goal of environmental management is straightforward: to ensure excess heat is removed from the flock by adjusting wind speed and wet curtain cooling, so that they do not have to limit heat production by reducing feed intake. If chickens have to work harder to lower their body temperature, their performance will be affected. They will pant, which may lead to poor feed conversion efficiency or reduced feed intake, thereby slowing down their growth rate. The more heat we can remove from the flock, the more likely they are to maintain a stable body temperature and continue to consume the necessary feed, maximizing their weight gain and overall performance.

In summer, do we need to worry about broilers nearing slaughter being excessively cooled? In most cases, no. Broilers destined for slaughter have greater heat retention capabilities rather than increased heat loss. For instance, by simply fluffing their feathers, the flock can significantly reduce heat loss. The air trapped between feathers enhances their insulation properties, reducing heat loss even when there is a large airflow above the body. Chickens can sit closer to each other or draw their wings closer to their bodies, further minimizing potential heat loss. Compared to panting, the primary method for chickens to expel excess heat, these heat retention measures require almost no effort from the birds. Therefore, for broilers nearing slaughter, it is safer to provide them with an environment maximizing wind speed for 24 hours, rather than worrying about the flock "getting chilled" at night and turning off some fans. Although this field study is limited in scope and further experiments are needed, it does emphasize a basic principle of flock management in hot weather: the more heat producers can remove from the flock, the better their performance will be. The best way for producers to remove excess heat from the flock in hot weather is by increasing wind speed. The higher the wind speed inside the shed, the lower the body temperature of the broilers, the more feed they consume, and the more weight they gain.