Effects of Heat-Stress on Production in Dairy Cattle
Dr. Shiv Kumar Yadav
Assistant Professor cum Junior Scientist
Department of Veterinary Biochemistry, Ranchi Veterinary College, (BAU),
Kanke, Ranchi, Jharkhand
Heat stress can be simply defined as a condition that occurs when an animal cannot dissipate an adequate quantity of heat, whether it is produced or absorbed by the body, to maintain body thermal balance. Under temperature, relative humidity (RH), solar radiation (RAD), and wind speed (WS) that exceed their thermal comfort zone, dairy cows suffer from heat stress. Heat stress reduces milk production, reproductive performance, and profit. This may prompt physiological and behavioral responses, leading to physiological disorders that negatively affect the productive and reproductive performance of farm animals. To study heat stress in livestock, the temperature-humidity index (THI) is a commonly used bioclimatic index which uses dry bulb temperature (Tdb) and wet bulb temperature, was initially developed by Thom (1959) as a heat index. The THI is expressed as a single value representing the combined effects of air temperature and humidity, which is commonly used to evaluate the degree of heat stress in dairy cattle. Heat stress in warm environments, is one of the major factors that can negatively affect milk production, reproduction, and the health of dairy cows. However, the length and severity of the heat stress period and the point at which the THI starts to have a negative effect on production and reproduction have not been studied in depth. Because the primary non evaporative means of cooling for the cow become less effective with rising ambient temperature, the cow becomes increasingly reliant upon evaporative cooling in the form of sweating and panting. High relative humidity compromises evaporative cooling, so that under hot, humid conditions in summer the dairy cow cannot dissipate sufficient body heat to prevent a rise in body temperature. Increasing air temperature, temperature-humidity index and rising rectal temperature above critical thresholds are related to decreased dry matter intake (DMI) and milk yield and to reduced efficiency of milk yield. Modifications including shade, barns which enhance passive ventilation, and the addition of fans and sprinklers increase body heat loss, lowering body temperature and improving DMI.
One of the greatest challenges to production facing dairy farmers is heat stress and the strain that it causes the lactating dairy cow. Climatic conditions are such that the hot season is relatively long, there is intense radiant energy for an extended period of time, and there is generally the presence of high relative humidity. Thus heat stress is chronic in nature, there is often little relief from the heat during the evening hours, and intense bursts of combined heat and humidity further depress performance. Lactating dairy cows create a large quantity of metabolic heat and accumulate additional heat from radiant energy. Heat production and accumulation, coupled with compromised cooling capability because of environmental conditions, causes heat load in the cow to increase to the point that body temperature rises, intake declines and ultimately the cow’s productivity declines.
Climatic Conditions:
Climate is a combination of elements that include temperature, humidity, rainfall, air movement, radiation, barometric pressure, and ionization. Climatic zones differ around the world and are dependent on latitude, prevailing winds, evaporative conditions, availability of water, elevation, proximity to mountains and other factors. The range and duration of ambient temperature is largely dependent on latitude, with latitudes closer to the equator experiencing conditions increasingly conducive to heat stress.
Metabolic Heat Production:
An attractive hypothesis, partly based on the fact that heat-stressed cows do not appear to mobilize adipose tissue, is that heat stress causes metabolic adaptations preventing the enlistment of glucose-sparing mechanisms that normally prevent severe reductions in milk yield during periods of inadequate nutrient intake. Heat production of metabolic functions accounts for approximately 31% of intake energy by a 600 kg cow producing 40 kg of milk containing 4% fat. Physical activity increases the amount of heat produced by skeletal muscles and body tissues. Maintenance expenditures at 35°C increase by 20% over thermoneutral conditions thus increasing the cow’s energy expenditure, often at the expense of milk yield. Body heat production associated with milk yield increases as metabolic processes, feed intake, and digestive requirements increase with yield. The heat load accumulated by the cow subjected to heat stress is the sum of heat accumulated from the environment and the failure to dissipate heat associated with metabolic processes. Obviously with similar body size and surface area, the lactating cow has significantly more heat to dissipate than a non-lactating cow and will have greater difficulty dissipating the heat during hot, humid conditions.
Physiologic Effects of Heat Stress:
Numerous physiologic changes occur in the digestive system, acid-base chemistry, and blood hormones during hot weather; some in response to reduced nutrient intake, but many changes occur as a result of strain in the cow. Neurons that are temperature sensitive are located throughout the animal’s body and send information to the hypothalamus, which invokes numerous physiological, anatomical or behavioral changes in the attempt to maintain heat balance. During heat stress cows exhibit reduced feed intake, decreased activity, seek shade and wind, increase respiratory rate, and increase both peripheral blood flow and sweating. These responses have a deleterious effect on both production and physiologic status of the cow.
Cows that were fed ad libitum in a thermal comfort environment, fed ad libitum in a thermal stress environment, or fed a restricted intake in a thermal comfort environment had similar milk yields for both restricted intake and thermal stress treatments, and mammary blood flow tended to be lower compared with ad libitum fed cows in thermal comfort, suggesting blood flow was responsive to level of DMI. Blood flow shifted to peripheral tissues for cooling purposes may alter nutrient metabolism and contribute to lower milk yield during hot weather.
Hormonal alterations occur with heat strain but it is often difficult to separate effects of lower feed DMI and direct effects of heat strain. Plasma somatotropin to decline with heat stress but no difference due to restricted DMI, while triiodothyronine concentration declined with heat and with restricted intake. Heat stressed cows generally exhibit altered blood acid-base chemistry as a result of the shift in cooling from conductive, convective, and radiation to evaporative cooling. Panting and sweating increase as the reliance on evaporative cooling increases. Panting sharply increases the loss of CO2 via pulmonary ventilation, reducing the blood concentration of carbonic acid and upsetting the critical balance of carbonic acid to bicarbonate necessary to maintain blood pH, resulting in a respiratory alkalosis.
Effects of Altering the Cow’s Environment:
Shading:
One of the first steps that should be taken to moderate the stressful effects of a hot climate is to protect the cow from direct and indirect solar radiation. It was estimated that total heat load could be reduced from 30 to 50% with a well-designed shade and shading is one of the more easily implemented and economical methods to minimize heat from solar radiation. Cows in a shaded versus no shade environment had lower rectal temperatures (38.9 and 39.4°C) and reduced respiratory rate (54 and 82 breaths/min), and yielded 10% more milk when shaded. Cattle with no shade had reduced ruminal contractions, higher rectal temperature and reduced milk yield compared with shaded cows.
Cooling for Dairy Cows:
Although shade reduces heat accumulation from solar radiation there is no effect on air temperature or relative humidity and additional cooling is necessary for lactating dairy cows in a hot, humid climate. A number of cooling options exist for lactating dairy cows based on combinations of the principles of convection, conduction, radiation, and evaporation. Air movement (fans), wetting the cow, evaporation to cool the air, and shade to minimize transfer of solar radiation are used to enhance heat dissipation.
Genetic Selection:
There are many aspects of genetics that influence the response to heat stress, and variation among breeds is large. One of the challenges associated with managing high producing cattle in a hot environment is that selection for increased performance is often in conflict with maintaining homeothermy. The maintenance of body temperature is heritable through characteristics including sweating competence, low tissue resistance, coat structure and color. There was genetic variation of rectal temperature and there was a negative correlation between rectal temperature and fertility, suggesting that selection for lower rectal temperature would improve fertility. There is evidence that hair color influences the susceptibility of the cow to heat stress because coat color is related to the amount of heat absorbed from solar radiation.
Nutritional Management:
There are several key areas of nutritional management which should be considered during hot weather. These include reformulation to account for reduced DMI, greater nutrient requirements during hot weather, dietary heat increment, and avoiding nutrient excesses.
Intake of DM usually declines with hot weather and nutrient density of the diet must increase. The tendency is to increase dietary protein concentration above requirements, but there is an energetic cost associated with feeding excess protein. The most limiting nutrient for lactating dairy cows during summer is usually energy intake and a common approach to increase energy density is to reduce forage and increase concentrate content of the ration. The logic is that less fiber (less bulk) will encourage intake, while more concentrates increase the energy density of the diet. High fiber diets may indeed increase heat production.
Summary:
Extended periods of high ambient temperature coupled with high relative humidity compromise the ability of the lactating dairy cow to dissipate excess body heat. Cows with elevated body temperature exhibit lower DMI and milk yield and produce milk with lower efficiency, reducing profitability for dairy farms in hot, humid climates. Although adequate cooling systems exist their efficiency in humid climates is less than in arid climates and these systems often lack the ability to maintain normal body temperature. Continued genetic selection for improved DMI and milk yield results in cows that are less heat tolerant, and coupled with the unknowns associated with global warming in the future, suggest that heat stress will become worse for dairies in the future. Improved cooling systems that are more efficient and that can cool cows at night when humidity is high are needed to meet challenges in the future. There is genetic variation in cattle for cooling capability, which suggests that more heat tolerant cattle can be selected genetically, and cross-breeding may also offer opportunities. Continued advances in feeding are needed as cattle are selected for greater milk yield, but are subject to lower intake because of environmental stress. Developing nutritional strategies which support yield but which also address metabolic and physiologic disturbances induced by heat strain will help the cow to maintain a more normal metabolism which should enhance performance.
