B.V. Sunil Kumar and Simrinder Singh SodhiCollege of Animal Biotechnology, Guru Angad Dev Veterinary and Animal Sciences University, Ludhiana
Simply defined, thermoregulation is the means by which animal maintains its body temperature. It involves a balance between heat gain and heat loss. Metabolic heat includes that necessary for maintenance plus increments for exercise, growth, lactation, gestation and feeding. High rates of these activities will result in more heat gain from metabolism. In addition to the heat gained from metabolism, heat is also gained from environment. Stress is a condition which arises when an animal suddenly faces a change in its environment. It may occur due to a variety of factors, temperature being one of the most important factors. Increased ambient temperature may lead to enhanced heat gain as compared to heat loss from the body and may cause heat stress in animals.
Heat stress in animals
Under heat stress, a number of physiological and behavioral responses vary in intensity and duration in relation to the animal genetic make-up and environmental factors. Climatic, environmental, nutritional, physical, social or physiological stressors are likely to reduce welfare and performance of animals. Heat stress is one of the most important stressors especially in hot regions of the world. Adaptation to heat stress requires the physiological integration of many organs and systems viz. endocrine, cardio-respiratory and immune system. The heterophil/lymphocyte (H/L) ratio has been accepted as a reliable index for determining stress in poultry. It is also found that exposure of birds to heat stress resulted in an increase in the H/L ratio.
Physiological state of dairy animals is a predisposing factor in environmental influences on animal health. Heat stress reduces libido, fertility and embryonic survival in animals. Primary effect of environmental stress in neonates is increased disease incidence associated with reduced immunoglobulin content in plasma. Heat stress in late gestation reduces fetal growth and alters endocrine status of the dam. Carryover effects of heat stress during late gestation on postpartum lactation and reproduction are also detectable.
Effects of heat stress on livestock
Heat stress causes havoc on the overall performance of livestock. In dairy cattle it causes depression in milk production and reduction in reproductive performance. During heat stress there is a change in nutritional requirement of cattle leading to decrease in milk yielding efficiency and dry matter intake. Rectal temperature also rises above threshold due to an increase in temperature and humidity. Heat stress also reduces estrous behavior expression, alters follicular development, affects functioning and growth of dominant follicles, compromises oocyte competence and inhibits embryonic in livestock thereby affecting their reproductive performance. It is not easier to quantify the effect of heat stress as it has both delayed as well as concurrent effect on reproductive system in cattle. Fertility of both male and female cattle is reduced because of heat stress and overall reproductive performance of livestock is reduced. Heat stress also causes reduction in conception rates in cattle.
Heat stress has a direct effect on economy of livestock industry worldwide, the livestock industries have recorded huge economic loses because of heat stress. Sheep exported from Australia showed welfare risks and high mortality rate due to heat stress. Physiologically, sheep initially shows increased rate of respiratory response due to increase in ambient temperature with rapid panting followed by deeper and slower panting.
Heat stress also affects the animals metabolically, affecting the physiology and structure of cells causing disruptive RNA processing, transcription, translation and membrane function and structure, reported high loss of electrolytes through saliva, urine, polypnea and sweat due to excessive heat dissipation because of heat stress leading to decrease in plasma concentration of Na+, Cl– and K+ ions. Steady concentration of free radicals is disturbed due to heat stress resulting in oxidative damage at both mitochondrial and cellular levels. A huge impact of heat stress has been observed in tropical countries where heat stress is a major cause of concern for livestock owners as it changes the level of antioxidants and concentration of electrolytes and increases lipid peroxidation.
Temperature Humidity Index (THI)and its roles
Temperature humidity index is a measure to assess heat stress in an animal. It is calculated by measurement of dry bulb,(usually known as the air temperature) and wet bulb, measured with the help of a container where a cloth soaked in water with free passage of air, (which usually tells about the humidity of the environment).
A proportional relationship between increase in THI and time spent by cows lying in housing system has been reported. A group of scientists conducted experiment on yak making use of THI to determine whether yak were under heat stress or not and they observed that yak showed some physiological changes and concluded that THI of 52 or above subjected yak to heat stress.THI takes into account the restrictions of the heat release of the animals, which vary from species to species. So, separate formulae have been devised for calculation of THI to assess heat stress in different livestock and poultry birds.
Strategies for ameliorating heat stress
The effects of heat stress are costly to dairy farmers, but there are opportunities to recover some of the losses to hot weather. Physical modifications of environment, genetic development of breeds that are less sensitive to heat and nutritional management are the three major key components to sustain production in hot environment.
1. Shelter management
With the help of managemental tools, it is possible to modify the microenvironment to enhance heat dissipation mechanism to relieve heat stress. Sheds if constructed scientifically, provide comfortable environment to animals. There is no doubt that shading is one of the cheapest ways to modify an animal’s environment during hot weather. Although shade reduces heat accumulation, there is no effect on air temperature or relative humidity and additional cooling is necessary for farm animals in a hot humid climate. Cooling ponds and sprinklers can also be used to cool the environment but none has been proved efficient.
2. Genetic modification
There is genetic variations among animals for cooling capability, which suggest that more heat tolerant animals can be selected genetically. Cross breeding offers another opportunity. However, extensive crossbreeding studies have shown little if any heterosis for heat tolerance. Additional studies are needed to examine variability in heat tolerance of high producing animals and what possibilities may exist for intensive selection programmes with these animals. Possibly improved herds could be developed when selected for milk yield and heat tolerance under local conditions.
3. Nutritional management
It has been documented that both low and high ambient temperature cause oxidative stress. Oxidative damage, as a result of heat stress may be minimized by antioxidant defense mechanisms that protect the cells against cellular oxidants and repair system that prevent the accumulation of oxidatively damaged molecules. Antioxidants, both enzymatic and non-enzymatic (certain vitamins and minerals), provide necessary defense against oxidative stress as a result of thermal stress.
Both vitamin C and vitamin E have antioxidant properties. Antioxidant vitamins have proved to protect the biological membranes against the damage of reactive oxygen species (ROS) and the role of vitamin E as an inhibitor –“chain blocker”- of lipid peroxidation has been well established. The effect of vitamin E was studied on monosodium glutamate induced hepatotoxicity and oxidative stress in rats by a group of scientists.They successfully demonstrated the ameliorative effect of vitamin E on stressed rats. Like vitamin E, ascorbate is also a chain breaking antioxidant. It prevents lipid peroxidation due to peroxyl radicals. It also recycles vitamin E. It protects against DNA damage induced by H2O2 radical. Vitamin C has a paradoxical effect as it can also produce ROS by its action on transition metal ions. Both ascorbate and zinc are known to scavenge reactive oxygen species (ROS) during oxidative stress. Vitamin C has an ability to spare other antioxidants in relieving oxidative stress in human subjects. Vitamin C has been found to assist in absorption of folic acid by reducing it to tetrahydrofolate, the latter again acts as an antioxidant. Use of folic acid is impaired when vitamin C is deficient.
- Minerals and trace elements
Zinc and other trace elements like copper and chromium act as typical antioxidants as they work indirectly. Zinc is a catalytic cofactor for Cu/Zn SOD and catalyzes dismutation of superoxide anion, producing molecular oxygen and H2O2, the latter product is usually metabolized by GPx and CAT. Several reports have shown the impact of copper and zinc deficiency on the antioxidant defence system and oxidative damage to cellular components. The activity of Cu/Zn SOD, CAT and GPx is decreased in copper deficient animals. It is also reported that normal copper levels are necessary to maintain the structural integrity of DNA during oxidative stress.
Supplementation of electrolytes is one among the nutritional strategies to combat heat stress in animals. Addition of Na+, K+ and Cl– is benefited in heat stressed dairy cows in terms of milk yield, acid base balance and lower temperature. Supplementation of sodium and potassium in the form of bicarbonate/carbonate also help in better regulation of acid-base balance in the blood.
Fig. Impact of heat stress on livestock and poultry