Impacts of Climate Change on Dairy Cattle
Introduction – Climate Change
The most direct consequence of climate change is on agriculture—and this includes the health of livestock. Livestock systems occupy 30% of the earth’s terrestrial area and directly support the livelihood of small-holding farmers in developing countries. Human populations around the globe depend on domestic animals for a variety of purposes, such as meat, eggs and dairy products as well as wool, transport and fertilizer production. Climate change both directly and indirectly affects an animal’s health and development, and by consequence, its capacity for productivity. Extreme climatic conditions result in seasonal fluctuations in the quantity and quality of fodder, which will affect the well-being and production efficiency of livestock. High temperatures and changes in rainfall patterns (climate change) could also result in an increase in vector-borne diseases and macro-parasites that cause new diseases. An increase in the global demand for food, fuel and land impacts the feed resources available for livestock, and thereby indirectly impacts the health and development of cattle.
Impact of Global Warming on Dairy Cattle
When there is an increase in ambient temperature, the body of an animal attempts to regulate the core body temperature by altering physiological and metabolic functions. These alterations can negatively impact of the climate change like health, behaviour and performance of the animal, including its capacity for reproduction, and make it susceptible to disease.
Dairy cattle are particularly susceptible to an increase in ambient temperature because of their high metabolic rate and their poor capacity for water retention in the kidneys and gastrointestinal tract. Neo-natal, post-pubertal and lactating cattle are especially prone to thermal stress. The effect of thermal stress varies depending on the breed of the animal, as well as its capacity for production or history of exposure to similar stress. It is important to know that Bos indicus (Zebu) cattle are genetically more thermo-tolerant than Bos taurus cattle.
The problems that are associated with thermal stress will only intensify as the atmospheric temperature rises in the future. It is important to remember that this temperature increase will not be felt only in the tropics—the temperate zones of the globe are also going to experience an equivalent rise in temperature and atmospheric pressure.
Impacts of Ambient Temperature on DMI
One of the thermoregulatory physiological alterations that an animal’s body will make in response to thermal stress is to decrease the appetite. A lowered food intake lowers the metabolic rate, and therefore reduces the heat produced by processes like ruminal fermentation or nutrient metabolism. A reduction in DMI or dry matter intake therefore indirectly helps to maintain the animal’s core body temperature.
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.
Feed consumption by dairy cattle starts to decline when the average daily temperature reaches 25C to 27C and voluntary feed intake could decrease by 10-35% when ambient temperature reaches 35C. Mallonee et al. reported that during hot weather, daytime feed consumption in outdoor cattle decreases upto 56% more than it does in cows who are kept in a shed. In the same study, feed consumption was increased by 19% during the night time, and the overall feed consumption was found to be 13% less in outdoor cattle as compared to cattle kept in a shed.
Decrease in dry matter intake is more prominent in animals that are fed a roughage-based diet than in animals fed a concentrate-based diet. Similarly, reduction in DMI is more severe and rapid when the food is not easily digestible. Decreased rumen motility due to thermal stress along with an increased water intake results in gut-fill which in turn leads to a reduction in feed intake. Thermal stress may have a direct effect on the appetite centre in the hypothalamus to inhibit feed intake. A decrease in feed intake is more prominent in Bos taurus cattle than in Bos indicus cattle.
Impacts of Increased Ambient Temperature on Physiological Parameter
To regulate the core body temperature during times of thermal stress, the body of an animal undergoes various physiological and metabolic alterations. Their rectal temperature and respiration rate is often higher—and this disturbance is especially true of Bos taurus cattle, which are less thermotolerant than Bos indicus. Similarly, the heart rate is also higher to ensure an increased blood flow to the peripheral tissue, in order to dissipate heat out of the body through the skin.
The respiration rate of an animal can offer an indication of the severity of the thermal stress, but several other factors like the overall health of the animal and its prior exposure to high temperature should also be considered when attempting to interpret the respiration rate.
Effects of Thermal Stress on Endocrine System
The process of adaptation and acclimation to thermal stress is generally mediated by an alteration in the hormonal profile. The levels of secretion of different endocrine glands and the activity of these hormones have been found to be altered during both active and chronic thermal stress.
Thermal stress has been found to alter the activity of the thyroid gland resulting in a reduced concentration of thyroxine (T4) and an increased concentration of triiodothyronine (T3) in the plasma. It has been speculated that reduced thyroid activity reduces motility in the gastrointestinal tract. Similarly, the secretion of the adrenal hormone aldosterone is decreased due to thermal stress which causes reduced sodium reabsorption in the kidney tubules, resulting in an imbalance of electrolytes. The level of catecholamines (adrenaline and noradrenaline) and glucocorticoids (hydrocortisone) were found to be sharply increased in Holstein cattle that were exposed to high ambient temperature (40 – 43 0C). The level of glucocorticoids returned to normal after long heat exposure but the level of catecholamines remained persistent. Likewise, the level of prolactin was found to be increased during thermal stress. In contrast, the secretion of plasma somatotropin was marginally reduced during thermal stress, independent of reduced dry matter intake.
Impact of Increased Ambient Temperature on Energy Balance and Metabolism
During times of thermal stress, metabolic maintenance requires 20-30% more energy than usual, thereby reducing the amount of energy available for growth and production. This, combined with the animal’s tendency to reduce its feed intake, results in a negative energy balance, which is responsible for many of the consequences of thermal stress. Negative energy balance that is independent of thermal stress causes lowered blood insulin and decreased insulin sensitivity, but thermal stress causes an increase in levels of circulating insulin and an increased insulin response.
Thermal stress also causes a reduction in blood glucose and non-esterified fatty acid (NEFA) levels due to a reduction in hepatic glucose synthesis. The reduction of non-esterified fatty acid (NEFA) levels during thermal stress is peculiar and contradictory to what might be expected, because increased levels of catecholamines and glucocorticoids were supposed to cause lipolysis and mobilize adipose tissue. Reports about the level of growth hormone (GH) during thermal stress are inconsistent as both an increased and decreased secretion of this hormone in response to thermal stress has been reported.
Impact of Thermal Stress on Electrolyte and Acid base Balance
An increased loss of potassium through sweat, along with increased urinary sodium excretion, results in an imbalance of electrolytes in rumen fluid and plasma. A decrease in the net mineral intake due to reduced appetite and reduced absorption of minerals during hot ambient temperature results in a further imbalance in electrolytes in the blood and rumen. Similarly, hyperventilation due to an increased respiratory rate reduces the level of bicarbonate (HCO3 ) in blood, resulting respiratory alkalosis.
Impacts of Thermal Stress on Animal Health
Thermal stress (climate change) 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. Reduction in feed intake along with an increased demand for energy for normal body functions creates a negative energy balance, which compromises the overall health of the animal. This could result in a lowered immunity as well, which makes the animal more susceptible to illness. Reduced disease resistance, along with a multiplication of microorganisms and an altered vector population due to climate change, could cause increased incidences of certain diseases like mastitis. Furthermore, high ambient temperature and moisture levels create an environment suitable for fungus growth in feed and feedstuff, which may cause mycotoxicosis.
Animal mortality is reported to be directly related with temperature humidity index (THI) above a certain break point. In a 6-year extensive study in Italy, it was found that the mortality rate in dairy cows is highest in the summer and lowest in the spring. From this study, Vitali and his colleagues reported that mortality in a dairy cow increases sharply when maximum and minimum temperature humidity index (THI) increases from 80 and 70 respectively. Similarly, calves born in summer and winter have a higher mortality rate.
Excessive water loss through sweating and panting during thermal stress may cause cardiovascular disturbances. Similarly, a modification in glucose and fatty acid metabolism along with reduced liver function and oxidative stress causes more incidences of metabolic disorders resulting in reduced productivity.
Impacts of Thermal Stress on Rumen Health and Ph
An increase in ambient temperature causes panting and a higher respiration rate in an attempt to maintain body temperature through evaporative cooling. An increased respiration rate leads to hyperventilation and increased exhalation of CO2 resulting in a low level of bicarbonate (HCO3-) in the blood. The increased secretion of bicarbonate (HCO3-) from the kidney and decreased secretion of HCO3- in the saliva are some of the consequences of hyperventilation. The buffering action of saliva is impaired, which results in disturbances in rumen pH. The lower volume of saliva due to the lowered feed intake further intensifies the instability in rumen acid-base balance. The imbalance in rumen pH leads to rumen acidosis, laminitis and reduction in milk fat production. Attebery and Johnson reported a decrease in the amplitude and frequency of rumen contractions in Holstein cattle which were exposed to 38 C ambient temperature for 5 days. Similarly, rumination also decreases during thermal stress.
Impacts of Thermal Stress on Proportion of VFAs Produced in the Rumen
The total amount of volatile fatty acids (VFAs) and the proportion of different VFAs is altered when the ambient temperature is above the thermoneutral zone. Kelley et al. reported that the 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 mg/L respectively when ambient temperature was raised from 18.2 to 37.7 C and feed intake was controlled at a constant level by force feeding through rumen canula. From this experiment, it is evident that the molar percentage of acetate is increased while that of propionate is decreased when cattle undergoes thermal stress.
Impact of Thermal Stress on Nutrient Absorption from GI tract
Although the digestibility of feed was reported to be increased at a higher ambient temperature, the absorption of nutrients from the gastrointestinal tract is impaired during thermal stress. When the ambient temperature is more than the normal body temperature, the blood circulation to the skin and peripheral tissue increases so as to transfer heat from core to the skin. This reduces the blood supply to visceral organs including the gastrointestinal tract. The reduction in intestinal blood flow may reduce the absorption of nutrients from the intestine.
Impact of Thermal Stress on Immunity
The development of resistance against disease in calves is largely influenced by the amount of immunoglobulin present in the colostrum. The passive transfer of immunity from dam to neonatal calves through the colostrum is found to decrease with an increase in ambient temperature. The concentration of immunoglobulins (IgG and IgA) in the colostrum is lower when the 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 the level of IgM secretion in periparturient cows calved in the summer is higher than that of cows calved in the spring. An altered metabolic status of the animals may also cause a reduction in their immunity, making them more susceptible to diseases. A decline in immune function is breed- dependent and therefore different breeds may have different immunological response to high ambient temperature. An increased incidence of mycotoxicosis during high ambient temperature could also compromise immunity in animals.
Impacts of Thermal Stress on Reproduction
Thermal stress causes an imbalance in the secretion of reproductive hormones. High ambient temperature has been reported to increase incidences of ovarian cysts. The level of plasma progesterone in animals under high ambient temperature is lower than that of animals who are in a zone of thermal comfort. Badinga et al. have reported that high ambient temperature decreases the quality of ovarian follicles resulting in poor reproductive performance in cattle. The fertility of cattle is also reduced due to low intensity and duration of estrus, caused by a reduction in luteinizing hormone (LH) and estradiol secretion during thermal stress. Reduced libido, decreased length and intensity of heat and increased embryonic mortality in cattle suffering from thermal stress reduces reproductive efficiency. Conception rates were reported to be lower in cattle who were under thermal stress as compared to those in a thermoneutral zone Thermal stress prior to and immediately after artificial insemination (AI) causes a reduction in the conception rate in high producing lactating cows. Additionally, thermal stress also causes a decrease in reproductive efficiency by increasing the calving interval. Similarly, claves borne from dams who are under thermal stress were found to be of a lower body weight than those borne from normal cows. This was followed by a lower lactational performance in the dams that experienced thermal stress during the prepartum period. Climatic warming also affects the reproductive performance of bulls. Concentration of semen, motility and spermatozoa per ejaculation is lower in the summer than in the winter. Impaired spermatogenesis during thermal stress results in poor quality semen. Additionally, defects in spermatozoa are higher during the summer than during the winter.
Impacts of Thermal Stress on Milk Reproduction
One of the major economic impacts of climate change on dairy cattle is that it results in a reduction in milk production. Decrease in milk yield due to thermal stress is more prominent in Holstein cattle than Jersey cattle. The decreased synthesis of hepatic glucose and a lowered blood level of non esterified fatty acid (NEFA) during thermal stress leads to a reduction in the supply of glucose to the mammary glands. This in turn results in low lactose synthesis and a lower milk yield. Reduction in milk yield is further intensified by the decrease in feed consumption. Reduced milk production due to thermal stress is attributable only partly to decrease in feed intake: 35% of reduced milk production is due to decreased feed intake while the remaining 65% is a direct effect of thermal stress. Other factors resulting in reduced milk production during thermal stress are decreased nutrient absorption, a disturbance in rumen function and the overall hormonal profile, as well as a reduction in the energy available for milk production. It has been found that exposure to thermal stress as early as 60 days pre-partum negatively affects postpartum milk production. Cows who give birth during the summer produce less milk than cows who give birth during any other season. Similarly, the quantity of milk protein and solid not fat (SNF) have been found to be reduced during thermal stress in dairy cattle. Mallonee reported 20% less milk yield in cattle kept as compared to that of cattle kept in a shed. Similarly, Roman-Ponce found 10.7% higher milk production in cows kept in a shed than in cows kept in the sun during hot weather.
Dr. Rajesh Kumar Singh
Jamshedpur, Jharkhand, India
Mob No: 9431309542
Email ID: rajeshsinghvet@gmail.com