by Yadav Sharma Bajagai
Warming of climate system of the earth is a unanimously accepted reality  and probably one of the most prominent challenges for scientists, development workers, policy makers and other relevant stakeholders regarding development and sustainability in international and national arena during past several years. Intergovernmental panel on climate change (IPPC) has described climate change as any anthropogenic or naturally occurring alteration in the climate over time . The World Bank has published ‘world development report 2010’ with the title “development and climate change” as an example to depict the importance of this issue.
Global warming is attributable to increase in atmospheric concentration of green house gases (mainly CO2, CH4 and NO2) as a result of human activities since the industrial revolution [1, 2]. Concentration of total green house gases in the atmosphere has been increased by more than 75% from 1970 to 2004  (figure 1). These trace gases have significant contribution to increase radiative forcing at the atmosphere  resulting in net positive forcing of +1.6 W m-2 since 1750 . Emissions of green house gases (GHG) at current rate would result in more warming of global climate in 21st century than during 20th century . Atmospheric temperature of the earth has been increased by 0.74±0.18 0C in 20th century and predicted to be increased by 1.8 to 4 0C by the end of 21st century . Scientists have envisioned that global temperature rise above 20C may be beyond the bearable limit of present-day societies causing extended and widespread societal and environmental disruptions .
Warming of global climate has multifaceted effects in many natural, economic and social systems including ecosystem, agriculture, health, soil, water resources etc. across the world  and these effects will most probably be continued for centuries in future . Among several global fingerprints of climate change; increase in global mean surface temperature (figure 1) and global ocean temperature, sea-level rise and arctic sea ice extent, ocean acidification, and more frequent extreme climatic events are some of the key variations in natural environmental system of the earth .
Figure 1: Instrumental record of global average temperatures as compiled by National Aeronautics and Space Administration (NASA). Anomalies in temperature are depicted with the zero of the coordinate as the mean temperature from 1961 to 1990. Source: http://www.globalwarmingart.com
2 Global climate change and animals
Climate change has complicated impacts on animals affecting distribution, growth, incidence of diseases, availability of prey, productivity and even extinction of species in extreme cases due to habitat loss [8-11]. Although both domestic and wild animals ranging from insects, amphibians, birds to mammals have been reported to be affected by global climate change, information on direct impact of climate change on animals is scarce [9, 12-15]. Most of the impacts of climate change are attributable to increased ambient temperature.
Climate change has complex impacts on domestic animal production system affecting feed supply, challenging thermoregulatory mechanism resulting thermal stress, emerging new diseases due to change in epidemiology of diseases and causing many other indirect impacts . Thermal stress is one of the greatest climatic challenges faced by the domestic animals  and global climate warming may further aggravate the condition and even provoke new episode of thermal stress condition. Global warming has two way effects on animal production system. In one hand it directly affects the health, reproduction, nutrition etc. of the animals resulting in poor performance, inferior product quality, outbreak of novel diseases, etc. while on the other hand there are indirect effects on animal production due to change in soil fertility, decrease in preferred vegetation, rangeland degradation, desertification and decrease in production of feed stuffs .
3 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 [18, 19]. 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. [9, 16].
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. . Bos indicus (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 .
3.1 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 [24, 27]. 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 .
3.2 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 [19, 26, 29, 30, 31]. 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 .
3.3 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 [30, 31, 34]. 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 .
3.4 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 [37, 38]. 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 [18, 39, 40]. 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 [19, 41].
Thermal stress causes reduction in blood glucose and non esterified fatty acid (NEFA) level due to reduction in hepatic glucose synthesis [19, 39, 40]. 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 [18, 19]. 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 [31, 40].
3.5 Impact of thermal stress on electrolyte and acid base balance
Increased potassium loss through skin due to increased sweating [26, 42] 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 .
3.6 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 [18, 39, 44, 45]. 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 [47, 48, 49]. Furthermore, high ambient temperature and moisture level creates the environment suitable for fungus growth in feed and feedstuffs which may lead to mycotoxicosis in animals due to mycotoxin produced by fungi [50, 51].
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. The same study specified 87 and 77 as upper maximum and minimum critical THI above which the mortality reached maximum. 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 .
3.7 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 [19, 55]. 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 [9, 55]. 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 .
3.8 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.
3.9 Impact of thermal stress on nutrient absorption from GI tract
Although digestibility of feed was reported to be increased at higher ambient temperature , absorption of nutrient from gastrointestinal tract is impaired during thermal stress. When ambient temperature is more than the normal body temperature, the blood circulation to the skin and peripheral tissue increases with vasodilation of peripheral blood vessels to transfer more heat from core to the skin surface and to hasten evaporative and convective heat loss from skin thereby reducing blood supply to visceral organs including GI tract [59, 60]. Reduction in intestinal blood flow may reduce the absorption of nutrients from the intestine.
3.10 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 .
3.11 Impacts of thermal stress on reproduction
Thermal stress causes imbalance in secretion of reproductive hormones . High ambient temperature has been reported to increase incidence of ovarian cysts . Plasma progesterone level in animals under high ambient temperature is low as compared to animals under thermal comfort . Badinga et al.  have reported that high ambient temperature causes poor quality of ovarian follicles resulting poor reproductive performance in cattle. Fertility of cattle is also reduced due to low intensity and duration of estrus caused by reduced luteinizing hormone (LH) and estradiol secretion during thermal stress . Reduced libido, decreased length and intensity of heat and increased embryonic mortality in cattle suffered with thermal stress reduce reproductive efficiency [20, 38, 67].
Conception rates were reported to be less in cattle under thermal stress as compared to those in cattle in thermoneutral zone [29, 68]. Thermal stress prior to and immediately after artificial insemination (AI) causes reduction in conception rate in high producing lactating cows . In addition, thermal stress also causes decrease reproductive efficiency by increasing calving interval due to reduction in 90-day non-return rate after calving . Similarly, Claves borne from dams under thermal stress were found to be of lower body weight than those from normal cows followed by reduced lactational performance of the dams due to carryover effects of thermal stress during prepartum period [20, 30].
Similarly, climatic warming also affects reproductive performance in bulls. Concentration of semen, motility and spermatozoa per ejaculation is lower in summer than in winter . Impaired spermatogenesis during thermal stress results in poor quality semen . In addition, defects in spermatozoa is higher during summer than during winter .
3.12 Impacts of thermal stress on milk production
Reduction in milk production is one of the major economic impacts of climatic stress in dairy cattle. Decrease in milk yield due to thermal stress is more prominent in Holstein than in Jersey cattle . Decreased synthesis of hepatic glucose and lower non esterified fatty acid (NEFA) level in blood during thermal stress [19, 39, 40] causes reduced glucose supply to the mammary glands resulting low lactose synthesis which in turn ensues low milk yield . Reduction in milk yield is further intensified by decrease in feed consumption by the animals to compensate high environmental temperature [9, 23].
Reduced milk production due to thermal stress is attributable only partly to decrease in feed intake . Actually 35% of reduced milk production is due to decreased feed intake while remaining 65% is attributable to direct effect of thermal stress . Other factors resulting reduced milk production during thermal stress are decreased nutrient absorption, effect in rumen function and hormonal status and increased maintenance requirement resulting reduced net energy supply for production [18, 19].
Milk production in cow has been found to be reduced when ambient temperature and temperature humidity index increases above critical threshold [17, 75]. Thermal stress during 60 days prepartum period negatively affects postpartum milk production  and cows parturated during summer produce less milk as compared to other season . Similarly, quantity of milk protein and solid not fat (SNF) have been found to be reduced during thermal stress in dairy cattle [40, 78, 79]. Mallonee  reported 20% less milk yield in cattle kept in sun than milk yield in cattle kept in shed. Similarly, Roman-Ponce  found 10.7% higher milk production in cows kept in shed than that in cows kept in sun during hot weather.
Atmospheric temperature of the earth has been increased due to cumulative effects of greenhouse gases in the atmosphere emitted from different industrial and agricultural activities of human. Warming of the climate system of the earth has multifaceted affects on animals. Intensification and increase frequency of thermal stress is the most prominent impact of global warming in dairy cattle resulting in different physiological, metabolic and production disturbances. Importance of thermal stress has been increased to the dairy farmers in tropical, subtropical and even in temperate region of the world due to atmospheric warming.
This article has been published in Nepalese Veterinary Journal vol. 30. Therefore, suggested to be cited as:
This article has been published in Nepalese Veterinary Journal vol. 30. Therefore, suggested to be cited as:
Bajagai, YS 2011, 'Global climate change and its impacts on dairy cattle.' Nepalese Veterinary Journal, vol. 30, pp. 2-16
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