Heat stress on poultry birds is the major complaint by poultry farmers and this impacts heavily on food scarcity. Heat stress caused by high ambient temperature occurs when the level of heat produced by animals surpasses their capacity to dissipate the additional heat to the surrounding environment. Indeed, heat stress was found to affect poultry production, particularly, within the tropical regions (Renaudeau et al, 2012, Vandana, et al; 2020). The death rate in broiler birds, caused by excess heat inside their housing, is often substantial and this may impact the capacity to meet the food needs of the teeming population. Heat stress results in annual economic losses of $128 to $168 million in the poultry industry alone (Nwab et al, 2018). This situation was further compounded by the continuous increase in global temperature which has risen beyond the 1.5oC that was projected by the International Panel on Climate Change (Masson-Delmotte, 2018). It has been projected that the temperatures in Africa are likely to rise faster compared to the other parts of the world (Ademakinwa & Rodrigues, 2017).
Hazardous climatic environments limit efficient food production in tropical and subtropical areas, and the broiler feed intake is reduced when the ambient temperature rises to a high level. A poultry house is expected to have an indoor environment that has a moderate temperature and adequate air circulation to enable the birds to suffer less from heat stress, thereby reducing mortality. Thus, it is important to know the thermal conditions of these birds that are reared inside poultry houses and the various ways the environment can help to cushion the effect of the rising global temperature on these birds.
Effects Of Heat Stress On Poultry
Heat stress in chickens occurs when the temperature experienced by the birds is higher than their core body temperature. When this occurs, the feeding appetite of the animal reduces (Elijah et al, 2008), resulting in weight loss. The discomfort zone causes heat stress and the birds spend less time feeding, moving, walking, drinking, elevating wings, and resting; thus affecting the general performance of the birds. Furthermore, exposing poultry to the environmental temperature outside its thermo-neutral zone may adversely affect the immune functions of the birds. Heat stress caused by a rise in temperature might cause viruses and bacteria to grow in chickens. Heat stress results in various harmful influences on the physiological and performance traits of poultry (El-Khorty et al, 2017). Kim et al (2017) further posited that heat stress causes minerals, and vitamins to be excreted from the body of chickens resulting in mineral and vitamin deficiency.
Air temperature, air movement, humidity, and metabolism are factors that influence heat absorption in animals. Some research works were definite in mentioning the range of temperatures the birds are more impacted. At a temperature of 30.0oC, the appetite of the animals (livestock) is suppressed, and they can eventually reduce their feed intake by three to five percent for every added degree of temperature (Igbal, 2022). Ahaotu et al (2019) added that as the ambient temperature increases to 34.0oC, the mortality due to heat will be higher in broiler by 8.4%. Also, seasonal temperature variation interferes with the broiler’s comfort and suppresses production efficiency, growth rate, feed conversion, and live weight gain (Okonkwo & Ahaotu, 2019). High temperature affects the physiological functions of poultry production and chronic heat stress in broilers negatively, affects fat metabolism, and muscle growth, and reduces meat quality (Damir et al, 2018). Even during the transportation of the birds, high ambient temperature has been associated with higher mortality and welfare issues (Vecerek et al., 2016).
Thermal Comfort
Poultry birds strive to be thermally comfortable inside their housing but that depends on how they interact with their immediate environment. The environmental parameters, such as air temperature, relative humidity, and air velocity, and the physical parameter such as their housing come into play in determining their comfort. Thermal comfort is defined by the American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) as ‘that condition of the mind which expresses satisfaction with the thermal environment’ (ASHRAE 2017). For humans, it is a subjective assessment in form of the psychological condition of the mind. In either of the two states (too hot or too cold), the individual feels thermally neutral which is an indication of the acceptability of the indoor thermal conditions. In the case of animals, this thermal neutrality or thermal comfort is difficult to assess subjectively. Humans can also be objectively assessed by allowing them to respond to questions posed to them about how they feel the impact of the environmental parameters. But assessing animals objectively is also impossible. The option available to assess how poultry birds feel the impact of heat stress in buildings is by observing their behavioral traits. For example, when poultry chicks feel hot, they often display some behavioral tendencies such as wing-raising, panting, prostrating, drinking more water, and staying in a cooler environment (if available). In the same way, people resort to behavioral adaptations, such as clothing change, posture adjustment, drinking more water, and looking for a cooler environment (when they feel hot). These adaptive behaviors help the chickens and human beings combat the effect of heat stress.
Adaptation has always been adopted by humans and animals to be comfortable. ‘People like animals, have been adapted too, and where fuel was scarce and warmth needed, they largely controlled their comfort behaviorally – adaptively’ (Humphreys et al., 2015). The proponents of the adaptive comfort model believe that people and animals tend to adapt naturally to the changing surrounding environment. There is a linear relationship between neutral temperature (comfort temperature) and indoor temperature, on one hand, and the relationship between adaptation and indoor comfort temperature is expounded by the adaptive comfort model. Climates in each locality may influence different perceptions of thermal comfort. This may have been the reason why local birds grown in tropics and subtropics are well adapted to high temperatures, whereas new animals that have been introduced to the tropics are possible to be affected by heat stress (King et al; 2006), because of differences in climate. The adaptive comfort model also recognizes that the difference between outdoor and indoor air temperature and airflow can also affect the perception of comfort (Du, Bokel, & van den Dobbelsteen, 2019). Air temperature above the core body temperature of birds can trigger heat stress which may be detrimental to the welfare and productivity of broiler chickens. When the daily temperatures reach their extremes, especially during the summer months, it becomes critical for the birds to dissipate body heat to the surrounding environment. Poultry birds do not sweat and therefore must dissipate heat in other ways to maintain their body temperature at approximately 105°F (40oC). However, their body temperature is regulated mainly by the loss of heat to the surrounding environment through conduction, convection, radiation, and evaporation. Birds expend energy when panting and during the process, their body heat is dissipated by the evaporative process. Panting removes heat by the evaporation of water from the moist lining of the tract. The evaporative cooling can be hindered by high humidity and low airflow. For the birds to maintain thermal neutrality, about 60% of heat is dissipated through evaporation (Daghir, 2008). Also, high feed intake by the birds can cause heat gain in chickens from high metabolism.