Mechanism of Animal Body Defence against Stress

The concept of fitness of farm animal body extends from ability to survive now and withstand environmental demands in future, to ability to produce sufficiently to justify cost of domestication. Homesostatis, Physiological, Biological and Genetic adaptations are concepts in understanding the means by which animal cope with their environment.

The concept of energy balance forms the central pivot which tilts the environmental stress in different directions for animal to respond. Meanwhile, effective responses of animals to environmental stress often result in depressed productivity even in attempt to apply mechanism a toward off the prevading stress condition.

The responsibility of the producer is to understand these concepts in the management of the stock for survival and higher productivity by controlling the overbearing influence of the environment.

Menchanism of Animal Body Defence against Cold

The animal body can defend itself against cold by three means namely: storing or conserving heat, through insulation and by increasing heat production or a combination of all. Increasing the body insulation against cold is more economical considering energy expenditure involved. Differences in species nurtured by adaptation have favored economic ways of supporting higher body insulation to animals living in cold climates. The body insulation is in three classes: 1. Peripheral Tissue:  This act by vasoconstriction of the coetaneous and sub-coetaneous to reduce the temperature gradient from the skin surface to the environment and also by the aid of subcutaneous fat. 2. Hair coat Insulation: This depend entirely on trapped air which occupies over 95 per cent of volume of the air coat. The insulating capacity increases with thickness and air density of the air coat. For example, temperature and arctic species of animals tend to develop thick air coat while most tropical animal have thin air coat. There is the a non-linear fall of temperature along the hair coat, so that as the body size of animal decreases below certain level, the level of the hair coat decreases. However, wind and rain greatly reduce the efficiency of hair coat as insulating mechanism. But the impending effect of wind diminishes with increasing hair coat density. Related: The Environment That Watermelon Grow Best In 3. Insulation of the Air: This insulation is caused by the layer of air or boundary adhering to the surface of the hair coat in the hairy species and to surface of the body in non-hairy species. It varies from one specie to another and is almost independent of the body size. The insulating mechanisms of the boundary layer decreases with increasing air speed.

Mechanism of Animal Body Defence against Heat

This can be effect by: 1. Behavioral means e.g. moving away from heat source, drinking more water, looking for shed or cold surface 2. Reduction in body insulation e.g. (a) vasodilatation to the ears, legs and tongues as more blood flows there to dissipate heat by taking advantages of hairlessness of the body parts. (b) Shedding of hair: If environment temperature is equals to body temperature, vasodialation ceases to be very effective. 3.  Increase in temperature loss: This occurs either from the skin or respiratory tract. The evaporation from skin is by sweating through sensible and insensible heat loss. Loss of heat energy from respiratory track is by panting as often noted in chicken or dog. 4. By lower rate of heat production if exposed to heat stress. The appetite drops and animal consume less feed. It also reduces its motor and thyroid activities. The thyroid gland regulates basal metabolism for homeotherm. 5. Increase in the reflectance of hair coat to solar radiation. Animal with lighter hair coat reflect more heat than those with darker coat color. The relative importance of cutaneous and respiratory evaporation varies from specie to another. A sweating animal controls the amount of water while a panting animal controls the amount for larger proportion of total evaporation than European type of cattle. Also within breed, heat tolerant animal have higher cutaneous and lower respiratory evaporation than heat.
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