Generally heat stress result in increase in the blood volume and a decline in red blood count. However, longtime heat stress at moderate level leads to haemo-concentration as a result of heat loss.
Heat loss also creates an acid surplus due to formation of lactate, metabolic or respiratory acidiosis which may exhaust the bicarbonate buffer system of the blood resulting in a fall of Pᴴ. The increase in blood volume also leads to a drop in total protein concentration.
The decline in tyroxine secretion is accompanied by increase in ACTH (growth hormone) released and also induces the release of vasopressin from the posterior pituitary gland. Other effects of heat stress on specific productive parameters are discussed as follow:
1. Effect of heat of reproductive Function: In the male, heat stress impairs testicular function resulting in depressed spermatogenesis, lower testosterone function before leydig cell, delayed puberty and decrease libido. There is also an increase in sperm abnormalities and impaired integrity of sertoli cells. Heating the testises to abnormally high temperature cause a complete cessation of spermatogenesis. There is also a decline in sperm mobility, sperm density and fluctuation in seminal Pᴴ.
In female animals there is delayed puberty, reduce ovulation rate increased incident of silent heat. Short oestrous and prolonged oestrous cycle. Other effect are reduced conception rate, increased rate of embryonic mortality, increases of abortion and therefore depresses little size, low birth rate, poor growth rate and poor lactation.
2. Effect of Light on Reproduction: Certain animals are seasonal breeders while other breed throughout the year. Experiments have shown that exposing sheep to constant photoperiod reduces spermatogenesis and if the photoperiod is reduced to 13 hours daily there is increase in ovulation rate. Seasonal fluctuation in day length is an important factor affecting the length of breeding in some other parts of the world except the tropics. In ewes the constant photoperiod is not as efficient as reducing photoperiod in inducing oestrus.
3. Effect of Climate on Egg production: It has been demonstrated that among all factors affecting egg production, temperature, humidity and light play a major role. An increase in temper
ture above 27°C, reduce the number of egg laid per year and egg shell thickness. If temperature is above 27°C the shell thickness is reduced and the egg quality is reduced. Of all the part produced, the yolk is the least susceptible to heat stress while the albumen is most susceptible to high relative humidity also lower egg production.
In birds, the control of sexual maturity depends largely on the pattern of photoperiod. Exposing bird to increasing day length is known to hasten sexual maturity while shorter day length delays sexual maturity. Increasing the day length also causes an increase in the number of egg laid per year.
However, beyond certain period of day length, increasing the photoperiod will make the birth photo-refractory i.e. resistant to photoperiod with cessation of reproductive.
Summary / Conclusion
Animals living in different ecologies of the world have for several decades and for every moment of the day developed means for coping with their environment as a matter of survival.
Farm animal expectedly must go beyond survival to improve their productivity notwithstanding the degree of stress to contend with.
Breeding or introduction of animal into an environment should recognize important adaptive traits and concepts that may help to achieve good performance. Consideration must be given to effect and adaptive mechanisms for different stress factor.
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