Impact of Heat Stress and Cold Stress on Production Performance in Birds
The exposure of birds to hot or cold environmental conditions in tropical and subtropical regions or temperate regions adversely affects their productive performance and results huge economic losses in the poultry industries.
The thermoneutral zone of birds varies between 18 and 22 °C for their optimum growth performance and egg production (Fig. 29.1). The optimum temperature of 30 °C is the critical point for birds wherein they maintain their production by compensating reduction in dry matter intake through their efficient feed conversion ability. However, beyond 30 °C dry matter intake decreases where birds are incompetent in maintaining body temperature and energy homeostasis, and thus leads in production loss with enhanced mortality. The heat or cold stress activates the hypothalamo-hypophyseal- adrenocortical (HPA) axis in birds and results in enhanced release of corticosteroids. The higher concentration of corticosteroids in birds enhances catabolic effects with increased oxidative stress and results in muscle wasting and reduced growth. Further, heat stress with high relative humidity disrupts the homeostasis of birds. Birds try to decrease its
Fig. 29.1 Thermoregulation of in birds during heat stress. When the environmental temperature exceeds lower critical temperature, the thermogenic mechanisms are initiated to increase heat production. Similarly, ambient temperature beyond upper critical temperature initiates
thermolytic mechanisms to decrease heat production and enhances the heat dissipation away from the body. Inadequate heat stress amelioration measures adopted at this juncture would lead to drastic reduction in production which may also culminate with mortality in birds
metabolic heat generation by decreasing feed intake with consequences of poor performance during hot condition.
Birds dry matter intake increases during cold stress condition in order to maintain the body temperature. The impact of heat or cold stress is more severe in birds with larger body size and growth rate. The elevated or low environmental temperature affects the growth rate and meat production in broiler birds as an outcome of impaired dry matter intake and intestinal development. Heat or cold stress affects the functions of digestive enzymes and absorption of nutrients with impaired immune functions in the intestine. The stocking density is major factor to be considered in view productivity and welfare in broiler production during high ambient temperature. Further, heat or cold stress modifies the nutritional composition of broiler meat by lowering breast muscle yield with higher abdominal and intermuscular fat deposition.The decrease in the dry matter intake during hot condition depends on the severity of heat stress. The reduction in dry matter intake increases with age in meat type birds and reaches 50%, whereas reduction is around 30-50% in layers. Birds reared in cage systems are more prone to heat stress compared to deep litter systems. Further, the elevated environmental temperatures reduce the secretory pattern and function of digestive enzymes trypsin, chymotrypsin and amylase which results in impairment in the digestive processes and absorption of nutrients. The insufficient nutrients and energy demand directly affect egg and meat production. The decrease in quality and quantity of egg production in layers is an effect of decreased dry matter intake and feed conversion ratio. It is also well established that heat stress enhances yolk and serum cholesterol levels in layers. The majority of energy is diverted towards restoration of thermoregulatory mechanisms in layers exposed to heat stress which depresses body weight, number of eggs, egg weight and shell quality (Fig. 29.2). Further, panting in birds during heat stress causes increased production loss of carbon dioxide which is necessary for the development of calcium carbonate that is responsible in formation of eggshell.
The low levels of carbon dioxide causes reduction in eggshell thickness with higher incidents of broken eggs. The hen-day egg production also decreases significantly in layers when they are experiencing a continuous heat stress during their peak production.The reduced dry matter intake and increased mortality in layers during hot conditions may also result in decreased egg production. Further, increased mortality in layers and broilers during hot environment particularly in summer have also been reported. The high environmental temperature reduces protein synthesis with enhanced protein breakdown in broilers. The alterations in protein/fat ratio, increased lipid peroxidation, decreased levels of minerals and vitamins in tissues result in compromised meat quality of broilers and reduced nutritive value of eggs in layers. The meat quality is affected by high drip loss and high occurrence of pale, soft and exudative meat which does not appeal the consumer preference and causes huge economic loss. Therefore, poor production performances of birds during heat stress are related with reduced appetite and lower dry matter intake in birds so as to prevent further heat increment. Further, heat stress damages the intestinal membrane, thereby hampering the absorption of vital nutrients leading to impaired digestibility and metabolism. Additionally, heat stress depresses the reproductive performance male birds by reducing seminiferous epithelial cell differentiation and intracellular iron exchange that result in decreased semen quality and quantity. In addition, semen volume, spermatozoa concentration and consistency are depressed during hot environmental conditions. The occurrence of infertility and low hatchability are more common in heat stressed female birds. Further, heating of fertile eggs beyond the optimum temperature during incubation resulted in impaired tissue growth at various phases of incubation with asymmetrical skeletal development. The heat stressed embryos were developed with shorter face length, reduced lung weight and unsteady gait which resulted in weaker chicks with higher mortality.
Fig. 29.2 Impact heat stress on productive and reproductive performance of birds during heat stress. Heat stress decreases the feed intake, modifies secretory pattern and activity of digestive enzymes resulting in impaired digestive processes and absorption of nutrients. The insufficient nutrients and energy demand directly affect egg production in layers and the growth rate in broilers. Further, heat stress activates the HPA axis that leads to increase in CRH and ACTH release,
consequently increasing glucocorticoids particularly corticosterone in birds. In addition, the decrease in GnRH and simultaneously reduction in LH and FSH impairs the reproductive performance in birds. The majority of energy is diverted towards restoration of thermoregulatory mechanisms in layers during heat stress which depresses body weight, number of eggs, egg weight and shell quality
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