Heat Stress Amelioration Strategies
The combination of high environmental temperature, relative humidity, solar radiation and air movement exceeds thermoneutral zone which causes heat stress. The high- producing dairy animals are more susceptible to heat stress due to their high metabolic heat production and more feed consumption.
It is highly essential to maintain the optimum temperature to maximize productive performance. Hence, the productivity of animals during heat stress may sustained through the physical modifications of environment, nutritional management and genetic development of heat- tolerant breeds.28.22.1 Physical Modification of Environment
The optimum maintenance of livestock environment is highly essential due to the changing climatic point of view. This management aspect is a challenge to reduce the detrimental effects of environmental conditions on animal production. Further, it is also important to reduce the cost of environmental protection methods for domestic animals. The physical modification of environment including provision of shade, shelter with cooling mechanisms is important in tropical and subtropical areas to maintain productivity and reproduction during heat stress. The simple and basic attempt to reduce heat load from direct solar radiation in cattle is the use of shades which can be natural or artificial. Trees are most effective in providing shade since they protect from the sun and capture radiation by evaporative humidity in the leaves. The painting of upper part of the shade unit or roof with white colour and installation of 2.5 cm thick isolating material reduce solar radiation. Further, the height of shades at the corral must be 3.6-4.2 m2 in order to ensure reduction in solar radiation. The shading units must be high enough from the ground to facilitate air circulation and tractor access for corral cleaning. The shades have to be placed in the centre of the corral that could prevent the accumulation of humid beneath the structure.
The cooling systems are more effective in reducing heat load from dairy cows through evaporation. However, the ameliorative responses vary with techniques where cooling has consistently improved feed intake and milk production during heat stress in dairy cows. The sprinkling and ventilation of dairy cows also enhanced the feed intake with less quantity of water and sustained the milk, fat and protein production during heat stress.28.22.2 Nutritional and Feeding Managements
Nutritional intervention is one of the important heat stress ameliorative procedures to combat heat stress and to maintain the production performance during hot environmental conditions. The modification of ration balancing is essential in reducing the adverse effects of heat stress in dairy cows. Animals should be fed at cool hours of the day with proper time intervals, and it is important to modify the composition of macro and micro-nutrients of feed in addition to supplementation of vitamins, minerals and feed additives. The feeding of animals during the cooler periods of the day improves the feed intake that helps in minimizing of metabolic and climatic heat load. The frequent feeding evades the diurnal variations in ruminal metabolites and improves the effective feed utilization in the rumen. The encouragement of grazing at cooler periods of the day, like early mornings and late evening, would reduce negative effects of heat stress. Further, nutrient requirements change during heat stress which needs to be modified to ensure normal feed intake and performance.
The digestive and metabolic processes are the additional sources of heat production from the animals during heat stress where high fibre content of feed increases the heat production. The rectification in the dietary fibre content is important to cut down heat augmentation in rumen fermentation. Therefore, low fibre content of feed is advocated to prevent high heat generation in rumen during heat stress. The feed intake is improved when the neutral detergent fibre (NDF) level of roughage ranging between 27% and 35% in the ratio which was also reflected as reduced the respiration rate and rectal temperature in animals during heat stress.
Further, reduction of dietary roughage NDF from 18% to 12% on dry matter basis significantly reduced the rectal temperature. However, supply of sufficient dietary high-quality fibre forage is essential to establish normal rumen activity. Further, highly fermentable carbohydrates may help in sustaining the feed intake in heat stress, and care must be taken to counterbalance the high-grain diets which results in rumen acidosis. However, it is necessary to maintain the ideal rumen function with an adequate level of 18% acid detergent fibre and 28% NDF on dry matter basis of the feed.Energy is the foremost important nutrient, and it has to be enhanced in the diet in addition to concentrate to decrease forage level. The energy level of the feed should be higher to meet the maintenance requirement as well as to support the extra demand for thermoregulation during heat stress. The reduction of forage to concentrate ratio enhances the efficiency of nutrients utilization in heat stressed animals. The addition of extra fat in the feed enhances the net energy intake during heat stress due to higher energy density with lower metabolic heat generation than fibre and starch. The inclusion of fat in the diet must be limited up to the level of 5% without any adverse effects on ruminal microbes. Heat stressed animals are in negative nitrogen balance due to decreased feed intake. Hence, good quality of protein source with crude protein (CP) content of 16% with low degradability is ideal because highly degradable CP increases the endogenous heat generation in ruminants. The high content of undegradable protein, calcium soaps of fatty acids and monopropylene glycol in feed enhance the performance of animals with reduced plasma urea. The supplementation of dietary essential amino acids is also necessary for the restoration of protein producing machineries or processes such as transcription and translation to maintain the production performance in heat stressed animals. The inclusion of lipoic acid enhances the thermotolerance by promoting insulin and antioxidant status during heat stress.
The feeding of rumen protected glutamine, arginine, tryptophan and citrulline intensifies the immune status particularly cell- mediated immune response in heat stressed ruminants.28.22.2.1 Water
Water is an essential element in the body of animals and is essential for the maintenance of physiological functions including tonicity of tissue, lubrication, thermoregulation, nutrient transport and excretion. Water metabolism is highly associated with the thermoregulatory mechanisms of the ruminants in the regulation of homoeothermic status. Water is the primary heat carrier medium in the removal of excessive heat load from the core body through evaporative heat loss. Water requirements of animals are regulated by dry matter intake, environmental temperature and loss of water through evaporation, urine, faces and milk. Heat stress enhances the water requirement to facilitate higher heat dissipation. Ruminants are experiencing moderate to severe water restriction during different environmental conditions and the demand for water increase due to high ambient temperature and solar radiation. Sheep and goats are better adapted for the drought conditions particularly goats are having high potential to conserve water. However, goats drink doubled their water requirement to enhance heat loss by sweating and panting during heat stress conditions. Therefore, the best way to reduce heat stress is to provide clean fresh cool drinking water ad libitum to ensure optimum performance.
28.22.2.2 Vitamins and Minerals
The decrease in feed intake during hot environmental conditions influences the requirement of vitamins and minerals which are associated with health status and immunity. Hence, it is advocated to include vitamins and minerals in the diet to minimize the effects heat stress in animals. The incorporation of selenium, copper and zinc, in addition to Vitamin A and E more than NRC recommendation, may improve the immunity and health status of animals during heat stress.
The cations requirements are increased by the kidney especially Na+ and K+ during heat stress in animals due to their higher rate of excretion up to 80% and 18%, respectively. NaHCO3, K2CO3 and KHCO3 are the source of Na+ and K+ and its inclusion in the diet improved the feed intake in heat stressed animals. The feed additives, fungal cultures and plant extracts enhance the feed intake and favour the rumen metabolism and thermoregulation during heat stress. The inclusion of yeast increases the nutrient digestibility and feed efficiency by maintaining rumen pH. The administration of plant extract daidzein alleviates the heat stress in ruminants and enhances the antioxidant potential with higher level of glutathione peroxidase.28.22.3 Genetic Selection of Heat-Tolerant Breeds
The current research findings and approaches in together positively improved the physical modification of environment and nutritional management procedure in ameliorating the impacts of heat stress on animal performance. However, long-term approaches are essential to establish heat resilient breeds or animals in the view of variances in thermal tolerance among livestock species. The selective breeding of dairy animals for high milk production has increased the susceptibility to heat stress with compromised summer production and reproduction. The selection for high milk production resulted in decreased ability of thermotolerance and depression in fertility. Therefore, the recognition of heat-tolerant animals among high-producing breeds will be more effective to sustain the productivity and survivability during heat stress. The cattle having shorter hair with higher diameter and lighter coat colour are more adapted to hot environmental conditions in comparison to those with longer hair coats and darker colours. This phenotypic character has been identified in tropical B. taurus, and this dominant gene facilitates higher sweating rate, lower rectal temperature and respiratory rate in homozygous cattle in the tropical regions.
The genes of heat shock proteins which are highly in association with thermotolerance could be distinguished biomarkers in the marker-assisted selection programmes. The incidence of polymorphisms in relationship with heat-tolerant genes is expressed in different breeds such as HSP90AB1 in Thai native cattle, Sahiwal and Frieswal; HSF1 gene, HSP70A1A gene, HSP70A1A gene and HSBP1 in Chinese Holstein cattle. Apart from HSPs, few more thermo-tolerant genes have been identified in livestock species that are changing in their expression pattern during heat stress like insulinlike growth factor-1 (IGF), toll-like receptors (TLR), etc. In addition, the genes that are economically important were perceived in heat stressed animals such as ATP1B2, thyroid hormone receptor, interleukins, fibroblast growth factor, protein kinase C, NADH dehydrogenase, phosphofructokinase and glycosyl transferase. Further comprehensive research findings are essential to elucidate the expression pattern of these genes in diversified animal species in prior to be designated as biological markers that may be used in marker assisted selection programmes to develop thermo-tolerant breeds.Learning Outcomes
• The chapter highlighted the various adaptive responses of farm animals and birds to cope with environmental challenges.
• Special emphasis was given to understand all important definitions and terms pertaining to environmental physiology of farm animals which could be very useful for the readers to prepare themselves for any competitive exams.
• Further the chapter also highlights the various indices to quantify heat stress response and identified different associated biomarkers.
• The chapter also signified the importance of channelizing the energy resources towards adaptation pathway to activate life sustaining process in heat stressed animals and thereby leading to compromised production performance.
• Lastly the chapter describes in brief the various strategies which could be implemented to ensure animal welfare during heat stress exposure.
Exercises
Objective Questions
Q1. What is the study of animals in their natural habitat?
Q2. Climate comprised of the atmospheric variables over a period of how many years?
Q3. What are the components of environment?
Q4. What is higher threshold temperature for animal health and welfare?
Q5. What are cardinal signs of heat stress?
Q6. What are the methods of sensible heat loss?
Q7. How does the animal dissipate heat as sensible heat loss?
Q8. At what temperature an animal can perform its maximum?
Q9. In which type of animals do you find brown fat?
Q10. What are homeotherms?
Q11. Define Poikilotherms?
Q12. How animals are classified based on the source of body heat production?
Q13. What is the method to estimate the level of heat stress in animals?
Q14. Enlist the types of sweat glands?
Q15. What is the evaporate rate in the skin of heat stressed cattle at 40 °C?
Q16. Enlist the major ways of heat production during cold stress?
Q17. What is the primary endocrine indicator of heat stress?
Subjective Questions
Q1. Impact of heat stress on animal production performance?
Q2. What are the environmental factors which affects animal performance?
Q3. Impact of high altitude on animal performance?
Q4. What are the thermal exchange methods?
Q5. Define thermoneutral zone?
Q6. What is the role of skin in animal thermoregulation?
Q7. How is heat stress measured?
Q8. Describe in brief the response of animals to stress?
Q9. What are the neuron-endocrine response to stress in animals?
Q10. What is the impact of heat stress in female animals? Q11. Explain the cellular response to heat stress?
Q12. What are the heat stress ameliorative measures?
Answer to Objective Questions
A1. Whole-animal physiology/physiological ecology/envi- ronmental physiology.
A2. Over a period of 30 years.
A3. Ambient temperature, relative humidity, radiant heat, precipitation, atmospheric pressure and wind velocity.
A4. 30 °C with relative humidity below 80% and 27 °C with RH above 80%.
A5. Increased rectal temperature, respiration and heart rate.
A6. Convection, conduction and radiation.
A7. Respiratory tract and skin.
A8. Thermoneutral zone.
A9. Hibernating animals.
A10. Animals having the ability to control their body temperature.
A11. Animals do not have a control over their body temperature and also called as temperature conformers.
A12. Endotherms, ectotherms and heterotherms.
A13. The temperature humidity index (THI) is a simple, reliable and easy method to assess the potential level of heat stress in animals.
A14. There are two types of sweat glands, eccrine and apocrine glands.
A15. The evaporative heat loss from the skin surface of cows is around 150 g/m2/h.
A16. Shivering and non-shivering thermogenesis.
A17. Cortisol.
Answer to Subjective Questions
A1. Heat stress affects feed intake, diverts more energy towards maintenance of homeostasis, decrease in body weight, decrease in milk production.
A2. Environmental temperature, relative humidity, solar radiation, wind velocity, precipitation.
A3. High-altitude environment, extreme cold, mountainous terrain, reduced oxygen, high solar radiation, short vegetations.
A4. Radiation, conduction, convection, evaporation.
A5. Comfort zone, upper critical temperature, lower critical temperature, lethal body temperature.
A6. Integumentary system, hair coat types, blood vessels, sweat glands.
A7. THI, BGHI, TCI, GCI.
A8. Respiration, sweating, cardiovascular system.
A9. HPA axis, HPT axis, autonomic and CNS.
A10. Heat stress, GnRH-LH and FSH-estradiol, progesterone, low fertility.
A11. Heat stress, activation of HSFs, synthesis of HSPs.
A12. Physical modification of environment, nutritional management, genetic selection.
Further Reading
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