Effects of Heat Stress on Production
28.4.1 Impact of Heat Stress on Livestock Growth
Growth, the increase in the live body mass or cell multiplication, is controlled genetically and environmentally. The higher environmental temperature influences average daily body weight gain.
The elevated ambient temperature reduces growth rate by decreasing the anabolic activity with enhanced tissue catabolism as an effect of increased catecholamines and glucocorticoids during heat stress in livestock.28.4.2 Impact of Heat Stress on Livestock Milk Production
The increased environmental temperatures and humidity enhance the animal’s body temperatures that resulted in reduced feed intake, decreased milk yield and disrupted reproductive functions. Heat stress causes a reduction in feed intake and growth rate which ultimately affects milk production and reproduction performance in dairy cows. Further, the cellular and molecular reactions to heat stress influence mammary gland metabolism, energy distribution and health status of animal. The elevated temperature with high heat load negatively impacts ovarian activity exclusively in buffalo and crossbred cows that are inefficient in eliminating heat from the skin. The water scarcity further impacts production and reproduction in farm animals. Heat stress negatively affects the secretory functions of the udder that results in decreased milk production during high environmental conditions. The lactating cows are incompetent to cope with heat stress particularly during early lactation which adversely affects the total milk production. The high- producing dairy animals are more susceptible to heat stress due to greater metabolic heat production and eventually decreased milk yield with lower level of fat, solids, lactose and protein. Hence, high-producing animals are more affected by heat stress than low producers. Heat stress affects milk production during the first 60 days of lactation when the high-producing cows are in negative energy balance.
The catabolic processes which are initiated to mobilize energy to meet the lactation demands further add on to the heat load by increasing the metabolic heat production.The impact of climatic stress on milk production of dairy animals is predicted to be 1.8 million tonnes at present. The models-based prediction of loss of milk production on different climatic scenarios suggested to be 1.6 million tonnes by 2020 and more than 15 million tonnes by 2050. The northern part of India is predicted to experience greater climate-related reduction in milk production of cows and buffalos.
28.4.3 Impact of Heat Stress on Meat Production
There are significant research reports on impact of heat stress on meat quality and composition in cattle, sheep, goat, pig and broilers. The increased temperature with relative humidity resulted in higher meat pH, loss of juiciness, high cooking and drip loss. Heat stress reduces the energy utilization and enhances the energy expenditure for thermoregulation which deteriorates the meat quality by decreasing the muscle glycogen that increases the muscle pH. The primary functional properties of meat such as colour, water holding capacity and myofibrillar fragmentation index were also negatively influenced during heat stress in ruminants. In addition, the animal management practices during heat stress indirectly affect the meat quality. The rearing of heat-tolerant Bos indicus cattle is an effective adaptation strategy against the prevailing harsh climatic conditions which results in tougher and less juicy beef. Besides the qualitative alterations driven by the heat load on the animals, carcass weight losses in heat stressed animals also have economic significance. Antemortem temperature stress is a major determinant for live carcass weight losses, hot carcass weight and retail meat yield. The diversion of energy towards thermoregulation in combination with reduced feed intake during heat stress resulted in live weight losses. Therefore, the above information confirms that heat stress deteriorates the qualitative and quantitative characteristics of meat depending upon thermotolerance and animal origin.
28.4.4 Impact of Heat Stress on Livestock Reproduction
28.4.4.1 Female Reproduction
The environmental temperature influences the reproductive performance of female animals at different stages of pubertal development, conception and embryo development. Stress suppresses reproductive efficiency of animals through activating the hypothalamic-pituitary-adrenal (HPA) axis which enhances the secretion of adrenocorticotropic hormone (ATCH) from the pituitary gland. ACTH acts on the adrenal and secretes glucocorticoids and catecholamines which are primarily stress relievers. Heat stress reduces the extent and magnitude of estrous cycle and affects follicular development with higher incidence of apoptosis in the antral and pre-antral follicles. Heat stress suppresses growth and development of follicles, thereby altering the oocyte function. The prolonged secretion of ACTH prevents ovulation and follicular development by modifying potency of follicular selection, dominance and follicular steroidogenesis. The higher level of glucocorticoids prevents the meiotic maturation of oocytes and corticotropic releasing hormone which consequently suppress the ovarian steroidogenesis. The hot environmental condition delays the commencement of puberty in female animals. In addition, high temperature decreases production of gonadotropin-releasing hormone (GnRH) which in turn results in delayed estrus. The reduction in GnRH also decreases luteinizing hormone (LH) and estradiol that lowers length and intensity of estrus in heat stressed animals. The comprehensive negative impact of heat stress modifies the reproductive behaviour of higher prevalence of anestrus and silent heat in farm animals. The higher level of ACTH and cortisol during heat stress decreases the activity of granulosa cells aromatase that results in lower estradiol secretion resulting in reduction of estradiol-induced sexual behaviour. The reduction in the concentration of estradiol depresses the estrus signs, gonadotropin surge, ovulation, transport of gametes with eventual reduction in fertilization rate.
Heat stress impairs the follicular dynamics by altering the development of follicles with reduced follicular dominance and encouraging numerous large follicles with prolonged dominance which disrupts the normal estrus cycle. In addition, lower level LH and negative energy balance of animals in hot environmental conditions inhibit the maturation and ovulation of dominant follicles. The extended follicular dominance alters the normal oocyte maturation and reduces their developmental competence. The development of such small dominant follicle results in ovulation of defective oocyte. Heat stress also decrease the oocyte developmental competence by inhibiting growth and maturation with higher oxidative damage and apoptotic cell death that results in irreversible alterations on cytoskeleton and meiotic spindle. The heat stress reduces fertilization capacity of oocyte and further having a negative impact on its embryo development stages, thereby reducing the fertility in animals. The embryonic death during heat stress could be due to impaired protein synthesis, significant oxidative cell damage, decreased pregnancy recognition and lower progesterone levels.28.4.4.2 Male Reproduction
Bull is known as half of the herd where bull’s fertility is equally or more important for fertilization of oocyte to produce a good, viable and genetically potential conceptus. Males possess an exceptional physiological mechanism of testicular thermoregulation to sustain its reproductive competence during hot environmental conditions. Further, presence of numerous sweat glands in scrotum of male animals is efficiently involved in the local thermoregulation. The testicular temperature is 4-5 °C lower than the rectal temperature which is prerequisite for the optimum semen production. The male animals are vulnerable to heat stress that directly affects the sperm quantity and quality and results in low fertility. Therefore, preferable environmental temperature for efficient semen production varies between 15 and 20 °C.
The high environmental temperatures enhance the oxidative metabolism of glucose in spermatic cells due to mitochondrial dysfunctions and increased the reactive oxygen species. Therefore, the testicular temperature must be 2-6 °C cooler than core body temperature for the desired production of fertile semen with good quality and fertile sperm.Heat stress reduces the level of testosterone, which has an impact on libido and results in low sperm concentration and motility with increased dead and abnormal sperm. The higher testicular temperature also enhances spermatogonial germ cell apoptosis, degradation of Sertoli and Leydig cells, DNA damage specially in pachytene spermatocytes and spermatids. The sexual behavioural alteration of bulls is attributed to the reduced levels of testosterone. Heat stress impairs the semen production and quality not only on day of collection but also during epididymal maturation or spermatogenesis which can go up to 70 days before collection. Further, biochemical elements of semen including fructose, sodium and potassium cation, citric acid, total phosphorus and calcium levels are decreased in heat stressed bulls. Further, higher level of lipid peroxidation during heat stress due to oxidative stress deteriorates the semen characters in bulls.
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