Climate change is a long-term shift in the statistics of the environmental variables such as temperature, humidity, radiation, wind and rainfall of a particular region. Climate change may appear as rapid changes in climate in the short period or may appear over decades. Generally climate change is associated with an increasing global temperature. Various climate model projections suggest that by the year 2100, mean global temperature may be 1.1–6.4°C warmer than in 2010. The difficulty facing livestock is weather extremes e.g. intense heat waves, floods and droughts. In addition to production losses, extreme events also result in death of livestock. Animals Livestock systems occupy about 30% of the earth’s terrestrial area directly supporting the livelihoods smallholding farmers in the developing countries. Therefore livestock production is a key factor of world agriculture. Human populations around the globe largely depend on domestic animals for multiple purposes such as meat, fat, milk, and other dairy products, eggs and wool as well as other purposes such as transport, draft, and provision of fertilizers. However livestock production is adversely affected by detrimental effects of extreme climatic conditions. As a result adaptation and mitigation have played a major role to overcome the detrimental effects of climate changes in livestock production. Climate change directly influence on animal performance: growth, milk production, wool production and reproduction. Extreme climatic and seasonal fluctuations in herbage quantity and quality will affect the well-being of livestock, and will lead to declines in production and reproduction efficiency. Impact of climate change can be direct or indirect. The direct effects of climate change include high temperature and change in rainfall pattern, which could result into the increased incidence of existing vector-borne diseases and macro-parasites, accompanied by the appearance and circulation of new diseases. The indirect effects are bestowed by changes in feed resources associated shortage of feed arising from the increasingly demands of food, feed and fuel production, and use of land
Impact of Global Warming on Dairy Cattle:
When ambient temperature increases, animal body attempts to regulate the core body temperature by altering physiological and metabolic function. Many of the behavioural, health and performance disturbances are attributable to these physiological and metabolic alterations .Increased ambient temperature affects animal production system by affecting the health, reproduction, nutrition etc. of the animals resulting in poor performance, inferior product quality, outbreak of novel diseases, etc..
Dairy cattle are particularly more susceptible to increased ambient temperature than other ruminants because of their high metabolic rate and poor water retention mechanism in kidney and gastrointestinal tract .Similarly, neonatal, postpubertal and lactating cattle are especially prone to thermal stress .Effects of thermal stress vary among individuals according to breeds, production level, prior experience etc. Bosindicus (Zebu) cattle are more thermotolerant than Bos taurus cattle due to possession of thermotolerant gene by zebu cattle.
Intensification of thermal stress and more frequent occurrence of this problem is probably the most obvious consequence of global climate change in dairy cattle which is attributable to increased atmospheric temperature. The possibility that the problem of thermal stress to become more prominent with increased atmospheric temperature in future cannot be overlooked. Thermal stress is not only the concern of only tropical region but also a matter of major apprehension in sub-tropical and temperate regions too as frequent spells of high ambient temperature and gradual rise in global atmospheric temperature is being experienced in many of the temperate regions of the world due to global climate change.
Impacts of Increased Ambient Temperature on Dry Matter Intake (DMI):
Decrease in feed intake is one of the thermoregulatory physiological attempts of animals which decreases the metabolic rate, hence reducing metabolic heat production .Reduction in dry matter intake (DMI) indirectly helps to maintain core body temperature by reducing generation of heat during ruminal fermentation and nutrient metabolism.
Feed consumption by dairy cattle starts to decline when average daily temperature reaches 25 to 27 0C and voluntary feed intake can be decreased by 10-35% when ambient temperature reaches 35 0C and above. Mallonee et al. reported that during hot weather feed consumption in cows is reduced by 56% during the day time in no-shed condition as compared to cows kept in shed. In the same study, feed consumption was increased by 19% during the night time and overall feed consumption is 13% less in cattle kept in no-shed condition as compared to cattle kept in shed.
Decrease in dry matter intake is more prominent in animals fed with roughage based diet than in animals fed with concentrate based diet. Similarly, reduction in DMI is more severe and rapid when food is poorly digestible. Decreased rumen motility due to thermal stress together with increased water intake results in gut-fill which in turn reduce feed intake. Thermal stress may have direct effect in appetite centre in hypothalamus to inhibit feed intake. Also, decrease in feed intake is more prominent in Bos taurus cattle than in Bos indicus cattle.
Impacts of Increased Ambient Temperature on Physiological Parameter:
Different physiological and metabolic alteration can be seen in animals under thermal stress many of which are body response to maintain the core body temperature constant. Rectal temperature and respiration rate of the animals are often higher in thermally stressed animals than those in the animals in thermoneutral zone . Increase in respiration rate and rectal temperature are more severe in Bos taurus cattle than those in Bos indicus cattle when exposed to similar thermal stress indicating more thermotolerant nature of Bos indicus . Similarly heart rate of the animal under thermal stress is higher to ensure more blood flow towards peripheral tissue to dissipate heat from body core to the skin and then to the ambient.
Respiration rate of the animal can be used as an indicator of severity of thermal load but several other factors like animal condition, prior exposures to high temperature etc. should be considered to interpret the respiration rate .
Effects of Thermal Stress on Endocrine System:
Process of adaptation and acclimation to thermal stress in animal is generally mediated by alteration in hormonal profile in the body. Levels of secretion of different endocrine glands and activity of hormones have been found to be altered during both active and chronic thermal stress.
Thermal stress in animal has found to alter the activity of thyroid gland resulting reduced concentration of thyroxine (T4) and increased concentration of triiodothyronine (T3) in plasma. It has been speculated that reduced thyroid activity reduces GI tract motility and rate of ingesta passage. Similarly, secretion of adrenal hormone aldosterone is decreased due to thermal stress which causes reduced sodium reabsorption in kidney tubules resulting electrolytes imbalance. Level of catecholamines (adrenaline and noradrenaline) and glucocorticoides (hydrocortisone) were found to be sharply increased when Holstein cattle were exposed to high ambient temperature (40 – 43 0C) with glucocorticoides level returning to normal after long heat exposure but level of catecholamines remained persistent. Likewise, level of prolactin hormone was found to be increased during thermal stress. In contrast, secretion of plasma somatotropin was marginally reduced during thermal stress which is independent of reduced dry matter intake.
Impact of Increased Ambient Temperature on Energy Balance and Metabolism
Thermal stress condition results in 20-30% more maintenance energy requirement ensuing reduced amount of net energy for growth and production . Increased expenditure of energy for maintenance together with reduced intake of energy results in negative energy balance which is responsible for many of the consequences of thermal stress . Thermal stress independent negative energy balance condition causes lower blood insulin and decreased tissue sensitivity to insulin but thermal stress causes increased level of circulating insulin and increased insulin response .
Thermal stress causes reduction in blood glucose and non esterified fatty acid (NEFA) level due to reduction in hepatic glucose synthesis Reduction of non esterified fatty acid (NEFA) level during thermal stress is peculiar and contrasting than expected because increased level of catecholamines and glucocorticoides were supposed to cause lipolysis and mobilize adipose tissue . This phenomenon proved direct effect of thermal stress which is independent of reduced dry matter intake . Reports about level of growth hormone (GH) during thermal stress are inconsistent as both increase and decrease secretion of this hormone in response to thermal stress has been reported .
Impact of thermal stress on electrolyte and acid base balance
Increased potassium loss through skin due to increased sweating together with increased urinary sodium excretion due to lower aldosterone during thermal stress results in electrolyte imbalance in rumen fluid and plasma. Decreased net mineral intake due to reduced appetite and reduced absorption of minerals during hot ambient temperature results further imbalance in electrolytes and chemical reaction in blood and rumen. Similarly, hyperventilation due to increased respiratory rate reduces the level of bicarbonate (HCO3-) in blood resulting respiratory alkalosis.
Impacts of Thermal Stress on Animal Health
Thermal stress may have both direct and indirect effects on animal health. Direct effects of thermal stress range from simple physiological disturbances to organ dysfunction and death [9, 18]. Reduction in feed intake together with diversion of more energy to maintain normal body function creates negative energy balance which compromise health and deteriorate animal body condition score. Immunity of nutritionally challenged animals with poor body condition is compromised making animals prone to infectious diseases. Reduced disease resistance of the animals, enhanced multiplication of microorganisms and altered vector population cause increased incidence of certain diseases like mastitis during summer when ambient temperature is high. Furthermore, high ambient temperature and moisture level creates the environment suitable for fungus growth in feed and feedstuff which may lead to mycotoxicosis in animals due to mycotoxin produced by fungi.
Mortality of the animal is reported to be directly related with temperature humidity index (THI) above certain break point . In a 6-year extensive study in Italy, it was found that mortality rate in dairy cows is the highest in summer and the lowest in spring. From this study, Vitali and his colleagues reported that mortality in a dairy cows increases sharply when maximum and minimum temperature humidity index (THI) increases from 80 and 70 respectively. Similarly, calves born in summer and winter have higher mortality rate .
Excessive water loss through sweating and panting during thermal stress may cause cardiovascular disturbances . Similarly, modification in glucose and fatty acid metabolism together with reduced liver function and oxidative stress during thermal stress causes more incidence of metabolic disorders resulting reduced productivity and reproductive efficiencies.
Impacts of Thermal Stress on Rumen Health and Ph
Increase in ambient temperature causes panting and increased respiration rate as an attempt to maintain body temperature through evaporative cooling. Increased respiration rate leads to hyperventilation and increased exhalation of CO2 resulting low level of bicarbonate (HCO3-) in blood. Increased secretion of bicarbonate (HCO3-) from kidney and decreased secretion of HCO3- in saliva are some of the consequences of hyperventilation. Buffering action of saliva is impaired due to this which results in disturbances in rumen pH. Lower volume of saliva due to less feed intake as an attempt to reduce metabolic heat production further intensify the instability in rumen acid base balance. The imbalance in rumen PH may leads to rumen acidosis, laminitis and reduction in milk fat production. Attebery and Johnson reported decreases in amplitude and frequency of rumen contractions when Holstein cattle were exposed to 38 0C ambient temperature for 5 days. Similarly, rumination also decreases during thermal stress.
Impacts of thermal stress on proportion of VFAs produced in the rumen
Total amount of volatile fatty acids (VFAs) and proportion of different VFAs is altered when ambient temperature is above the thermoneutral zone for cattle. Kelley et al. reported that molar proportion of acetate, propionate and total VFAs altered from 94.7, 33.3 and 147.9 to 47.2, 10.6 and 66.3 mEq/L respectively when ambient temperature was raised from 18.2 to 37.7 0C and feed intake was controlled at constant level by force feeding through rumen canula. From this experiment, it is evident that molar percentage of acetate is increased and that of propionate is decreased when cattle undergoes thermal stress condition.
Impact of thermal stress on nutrient absorption from GI tract
Impact of Thermal Stress on Immunity
Development of resistance against disease in calves is largely influenced by amount of immunoglobulin present in colostrum. Passive transfer of immunity from dam to neonatal calves through colostrum is found to be decreased with increased ambient temperature and concentration of immunoglobulins (IgG and IgA) in colostrum is lower when cow is exposed to high ambient temperature during late pregnancy and early postpartum period . In contrast to above result, Lacetera et al. have found that level of IgM secretion in periparturient cows calved in summer is higher than the cows calved in spring. Altered metabolic status of the animals may also cause reduction in immunity making animals more susceptible to diseases . Decline in immune function is breed dependent characteristics therefore different breeds may have different immunological response to high ambient temperature . Increased incidence of mycotoxicosis during hot ambient temperature also compromise immunity in animals .