Thermoregulation During Cold and Heat
The maintenance of Tc at normal range during cold when the environmental temperature goes below the LCT depends upon the ability of animal to increase the metabolic rate. The increment in metabolic rate of small mammals is higher which is proportionate to the three-fourth power of body weight (six times) as that of basal metabolic rate.
New born animals have maximum metabolic rate by a factor five and the metabolic rate reduces as they grow. However large animals maintain homeothermy during cooler temperatures, primarily by taking advantage of their insulation capacity than increasing metabolic rate. Sheep are more tolerant to cold than the high-producing cows. The hypothermia causes cold injuries in the extremities such as the ears but the arteriovenous anastomoses give some protection against frostbite. However, maintenance of homeothermy is more critical during hot environmental conditions than a cold. The heat tolerance capacity of the animals depends on the evaporative cooling mechanisms where the sweating species tolerate higher environmental temperatures than panting species. Animals establish a new equilibrium of body temperature and continue as the heat load is moderate. When the heat load turn into severe, animal losses its capacity to control Tc, which gradually increases and results in hyperthermia. Hyperthermia occurs at relatively low Ta when RH and solar radiation are high with increased metabolic heat production. The intensification of hyperthermia eventually causes failure in sweating and respiratory mechanisms and finally results in a breakdown of thermoregulation.28.17.1 Impact of Heat/Cold on Metabolism
The metabolic heat is the principal source of heat aggregation in animals which is generated within the body for each and every biochemical reaction associated with body function such as growth, lactation and pregnancy.
The metabolic heat increment is necessary during cold to protect body temperature while metabolic heat has to be eliminated away from the body during warm periods. Whenever the animals are not able to transfer sufficient heat to their surroundings and accumulate it within the body, it results in hyperthermia. Animals that are well adapted to hot conditions consistently decrease heat production or enhance heat loss mechanisms to sustain the homeothermy. The environmental temperatures influence the metabolic and endocrine profile of animals. Exposure of animals to cold or heat stress reduces the productivity and feed efficiency. The susceptibility of animals to cold stress varies with the stage of life, production phase and breed. The energy requirement increases with decreasing temperatures in winter to elevate resting heat production to maintain the Tc by shivering or non-shivering thermogenesis.Shivering is an involuntary function of the body and consists of muscle contractions usually preceded by an increased muscular tone. The circuit consisting of gamma motoneurons, muscle spindles and muscle afferent fibres is apparently of great importance in the control of shivering. The shivering thermogenesis is initiated and regulated by peripheral and central temperatures where peripheral cooling activates shivering without any change in brain temperature. The local cooling of the anterior hypothalamus or the spinal cord also induces shivering thermogenesis with a constant environmental temperature. The non-shivering thermogenesis occurs through calorigenic effect of epinephrine and nor-epinephrine that is secreted during cold stress. Further, the higher level of thyroxine during cold potentiates the calorigenic action of epinephrine. Therefore, thermal stress caused by the variations in Ta above and below TNZ results in decreased performance of animals. Heat stress induces a decrease in feed intake which could be in an effort of animal to reduce the metabolic heat production.
Reduced feed intake accounts for around 30-50% of the total decline in milk production. Therefore, the impacts of heat stress on livestock productivity are probably arbitrated by alterations in the metabolism, which could not be assumed on the level of nutrition nevertheless heat stress modifies metabolism independent of nutrient intake. Heat stress influences the post- absorptive metabolisms that are independent of decreased feed intake and energy balance in livestock. The changes may be an adaptive approach when the animal attempts to sustain Tc during extreme environmental conditions. In general, milk yield of dairy cow is reduced during cold stress and the initial phase of lactation is highly susceptible. Further, the milk production decreases when the Ta falls below —5 °C depending upon the breed, level of feed intake and acclimatization.28.17.2 Adjustment of Energy Requirements During Heat/Cold Stress
The homeotherms are able to adjustments their energy requirement for maintenance during different environmental conditions. The maintenance energy requirement of animal depends on level of heat stress or the severity of heat stress that could vary among animals based on the acclimatization, availability of feed, level of productivity and diurnal fluctuations in radiant heat load. The severe heat stress condition increases the maintenance requirements by the enhanced panting and alterations in tissue metabolism due to elevated tissue temperature with reduced metabolic rate. However, severe heat stress reduces appetite of animal that results in reduced productivity and metabolic heat production. The adjustment of energy requirement of animal’s maintenance during cold stress depends on the level of acclimatization ability to cold conditions. The animals acclimatized to cold stress have an increased metabolic rate with an enhanced capacity to increase their rate of metabolic heat production to prevent hypothermia during severe cold stress. The enhanced heat generation during cold stress requires an effective usage of energy substrates either from diet or tissue reserves.
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