**1. Introduction**

Climate has been identified as a factor that has direct and indirect effects on animals via the animal's environment [1]. Farmers will bear direct costs of climate change characterised by reduction in poultry yields and indirect costs of adaptation [2]. This implies heat stress is contributing to low production while simultaneously increasing the production costs. This leads to reduction in poultry enterprises profitability and is a threat to the survival of the sector. Notably, all domestic livestock are homoeothermic which means that they are continuously attempting to maintain their body temperatures within the most suitable range for optimal

biological activities. However, poultry are more susceptible to heat stress because they can only tolerate a narrow temperature ranges [3]. Achieving a state of thermal stability without expending too much nutrients would result in high production in systems in where nutritional, health and general animal welfare environments are maintained at optimal states. Heat stress occurs when there is a negative balance between the amount of heat energy produced by the animal and the net amount of heat energy flowing from the animal to the environment [4].

In order to maintain a constant body temperature poultry must preserve a thermal balance between their heat production and gain from the environment. This is achieved physiologically through metabolic and physical means [5]. The metabolic heat production depends on the basal heat production for maintenance of essential body processes [6]. The imbalance in heat production in birds is attributed to various environmental factors such as thermal irradiation, sunlight, air humidity and animal characteristics such as metabolic rate, thermoregulatory mechanisms and species [7].

Whenever ambient temperatures increase beyond the thermo-neutral zone, mechanisms to dissipate heat are triggered (increased respiration and heart rate), and as a result, maintenance requirements increase [8]. Similarly, maintenance requirements also increase during cold weather as the animal needs to generate heat in order to maintain body temperature [6]. The heat production varies and corresponds to the nature of activities the animal is exposed to. Process such as growth, reproduction and health status results to an increase in metabolism and subsequently heat production. Animals are able to respond to decreases and increases in metabolic, digestive and nutrient metabolism, muscular, increased metabolism heats. However, an animal has to maintain basal heat production at constancy since essential or vital body processes must be maintained [5]. There are a number of body heat loss methods [9]. The most important one is evaporation. This method depends on the ambient air temperature, the amount of available moisture, the evaporative surface area, the humidity of the air surrounding the animal and the degree of air movement [7]. The amount of available moisture depends on the quantity of sweat produced by the animal while the area of evaporating surface depends on the surface area of the animal and the size of the lungs, with considerable evaporation being achieved by panting [5].

Loss of heat by livestock through the movement of heat from one object to the other at higher temperature (conduction) is limited. Heat is transferred from the body through conduction at relatively low temperature [9]. Loss of heat from the surface (convection heat loss) increases when a cool breeze blows on the animal and increased air movement. Research has shown that the loss of temperature by convection occurs when heat by from the comb, wattles, face, legs, toes, neck, body and wings is lost to the surrounding air as air circulates inside the poultry house [10]. The cool breeze and increases in air movement significantly increases evaporative heat loss. Livestock houses in the tropics should always be constructed in ways that encourage maximal air movement on and around the animal. This is achievable by ensuring that the animal houses are well ventilated.

The documented normal body temperatures of chicken ranges between 41 and 42°C, while the chicken thermal comfort zone falls within 18–21°C. Study by Wasti et al. [11] showed that temperatures above 25°C results to heat stress. When the balance between body heat production and loss is not maintained then the birds are subjected to heat stress. The balancing occurs in an environment charactersied by interactions of many factors key among them being high environmental temperatures, radiant heat, airspeed and humidity among others. When the ambient temperatures are high, chicken use various physiological and physical mechanisms to maintain a thermal comfort zone [9]. The birds use body energy reserves to

**191**

temperatures.

*The Effects of Heat Stress on Production, Reproduction, Health in Chicken and Its Dietary…*

maintain a thermal comfort zone. This occurs at the expense of production and growth. This implies that heat stress negatively impacts the productivity and profitability of poultry enterprises. Broiler meat and eggs lose their quality under high

Birds do not have sweat glands and rely on evaporative cooling which is aided by panting to regulate their body temperatures. It has been documented that due to lack of sweat glands in birds, most of the heat loss occurs through respiration [5]. Birds have an extensive air-sac system connected with their lungs which is important in heat regulatory function [5]. In the tropical environments, it has been observed that poultry frequently hold their wings slightly separately when standing to allow air flow through while at the same time extending the heat loss surface area [12]. The birds adopt an extended position when lying down to further increase the surface area. Collectively, these behavioural responses maximise on the surface area of the body in contact with the external environment for heat dissipation [12]. Studies have shown that when chicken are exposed to heat stressful conditions, they spend less time moving and more time resting in the shades or cool places [13]. Birds also manage heat by increasing or decreasing feed and water intake [14]. When the temperatures are high, birds tend to reduce their feed intake while increasing their water intake. The reverse holds when the temperatures are low. Generally, chicks from many poultry species are not able to efficiently regulate body temperature during early and post hatching period [15]. However, at an early age, the chicks should be provided with right temperatures in the post-hatching stage without which they get chilled and die almost immediately. This underlines the importance of good brooding heat management by provide adequate heat (artificially) for the young chicks. The temperatures should be set at around 35°C in the first week after hatching, with a 3°C weekly reduction until normal ambient temperature of about 23°C - 26°C is attained. Lin et al. [16] proposed that the ideal temperature range for neonatal chicks should be set at between 32 and 35° C. It is important to maintain the right chicks' temperature range when transporting them from hatcheries to avoid overheating or chilling which reduces their post hatch-survival rate [17]. Abdelazeem [18] reported that, high ambient temperatures reduce the chicks' growth rate and feed intake. There is general sluggishness in chicks when temperatures are high [17]. This tends to reduce the feeding time and results in reduced feed intake and consequent poor performance in growth. This problem can be managed by formulating high nutrients density chick feeds which would ensure a

*DOI: http://dx.doi.org/10.5772/intechopen.97284*

**2. Heat regulation in poultry**

temperatures resulting to further postharvest losses.

good supply of nutrients in the face of low feed intake [19].

It has been observed that free range birds maintain thermal neutrality by hiding

Various environmental factors including temperature, light, rainfall, humidity and altitude collectively impact on an animal's nutrition [14]. The respective factors

under shades during the hot hours of the day while resuming the scavenging in the cool hours of the day [13]. The pattern has also been noted with housed birds which stop feeding during the hot hours of the day and resume feeding in the cool afternoon. This is important for poultry producers to note so as to synchronise their feeding strategies to the behaviour in areas characterised by high ambient

**3. Effect of environment on poultry nutrition and production**

maintain a thermal comfort zone. This occurs at the expense of production and growth. This implies that heat stress negatively impacts the productivity and profitability of poultry enterprises. Broiler meat and eggs lose their quality under high temperatures resulting to further postharvest losses.
