Thermoregulation and the Effects of Heat Stress on Dairy Cattle







Bryan Scott Kennedy

Mississippi State University College of Veterinary Medicine

Production Medicine Graduate Program

March 4, 1999







Introduction

The effects of heat stress (HS) have proven to be a great hindrance to the efficiency and productivity of cattle, especially dairy cattle. Dairy cattle are more susceptible to HS because of their select inherited ability to ingest vast quantities of dry matter, thus producing greater metabolic heat from rumen fermentation as well as from the process of milk production 1. Older, heavier, higher-producing cows are most susceptible to HS 2,3,4. During periods of thermal stress cows are unable to dissipate this additional metabolic heat produced, especially in areas of high humidity. As a result cows voluntarily reduce dry matter intake, thus limiting milk production. Reproduction also suffers as a result of thermal stress on folliculogenesis and oocyte maturation as well as embryonic development and survival 1. The decrease in productivity and negative reproductive effects render periods of HS, and those periods following, less profitable for dairy operations, making heat stress abatement practices cost effective and advantageous. HS brings many challenges, but with proper facilities and management HS can be alleviated to an acceptable level. The veterinarian's understanding of the physiologic consequences of HS is essential for providing appropriate recommendations for treatment and management of heat stressed cattle 5.



Reasons to Reduce Heat Stress:

1. Increased Dry Matter Intake

2. Increased Milk Yield

3. Increased Estrus Activity

4. Increased Conception Rates-Pregnancy Rates

5. Decreased Service Per Conception

6. Decreased Average Days Open

7. Decreased Early Embryonic Death



Thermoregulation Physiology

Like all mammals cattle are homeotherms, that is they must maintain a constant body temperature of an acceptable level 6. Normal body temperature in cattle ranges from 101.1 to 102.2 F (38.4- 36.0 C) 6,7. Cold and heat stress can result in an inability to maintain this constant body temperature causing animal health, productivity and efficiency to suffer as a result. The mammalian body is set up to constantly regulate peripheral and internal body temperature with assistance of cutaneous sensors and internal temperature sensors (located in the hypothalamus) along with integration of the endocrine system 8. Cattle utilize their respiratory systems to dissipate heat by evaporation 9. Cattle pant with their mouths closed so heat exchange occurs from the nasal mucosa of the upper respiratory tract near the turbinate bones 9. Heat is provided by the blood supply to the nasal mucosa and the cool blood drains into the venous sinuses at the skull base, where it joins venous drainage of the ears and poll 9. The rete mirabile, a network of small arteries that make up the blood supply to the brain base, functions as a countercurrent heat exchanger with blood supplying the brain being cooled by blood draining the nasal mucosa, the ears, and the poll 9.



All signals received by the Peripheral Nervous System (PNS) are relayed to the Central Nervous System (CNS) for processing 10. Processing of this information occurs in the thermoregulatory region of the hypothalamus in the brain 10. Through constant monitoring, information is received and processed in the thermoregulatory center and appropriate physiologic and metabolic modifications occur as a result 10. The CNS integrates and involves the cardiovascular, respiratory, digestive, and endocrine systems in the regulatory process 10. Through changes in these systems heat loss or gain can be regulated accordingly.



An animal's body temperature can be summed up with the simple formula:

Body Temp=Metabolic Heat Conduction Convection Radiation + Evaporation 9.

In order to maintain a constant temperature heat production has to equal heat loss 1. This concept is complicated by several factors, including environmental temperature and humidity. This can be represented in the simple equations: Heat Loss=Heat Produced+Environmental Heat, and heat stress results when: HS=(HP+EH)>HL 1. Animals are most comfortable, productive, and efficient existing in a temperature range known as the Thermoneutral Zone (TNZ) 2,11 . The TNZ is considered the range of ambient temperature within which the metabolic rate is at a minimum, and within which temperature regulation is achieved by non-evaporative physical processes alone, by autonomic nervous control and behavioral responses 11. Disregarding humidity, the TNZ of cattle is 41-77 F (16-25 C) 11,12. When temperatures exceed the upper limits of the TNZ milk production, reproduction, and overall health suffer. Several mathematical formulas are available to estimate the effects of HS on cattle. Maybe the most commonly used is THI=T + (0.36) Tdp + 41.5 8. This formula utilizes the temperature (T) measurement along with an account for humidity using the dewpoint temperature (Tdp) to result in a measurement termed the temperature humidity index (THI) 8. When the THI is above 72 HS occurs. Another commonly noted formula utilizes BGT representing interactions of air temperature, multidirectional radiation and mean air velocity effects 13. BGT is a temperature reading from inside a black hollow sphere that has been equated to simply a toilet-bowl float painted black with a mercury thermometer inserted so as not to touch the inside of the float 1. BGT tends to take solar radiation more into account than simply a temperature measurement 13. Body temperature lags behind ambient air temperature by 2 to 5 hrs 14. Many complex mathematical formulas exist to estimate the effects of heat has on cattle, but regardless of environmental variables a cow's body temperature (rectal or core) is the best indicator of the physiologic response to HS.

Mechanisms of Heat Exchange

Heat exchange with the environment occurs basically through four ways 15. The first is conduction, which occurs when heat is transferred through direct contact with material. The second is convection, which occurs by transfer of heat to surrounding air. Convection is greatly limited by high ambient air temperatures. Radiation is the third method of heat exchange and likely the most important for cattle residing in sub-tropical and tropical climates. Research has concluded that black cows may absorb more than twice the solar radiation as white cows 1. Radiation is also an important means of heat loss whenever all or part of the surroundings are cooler than the cows surface 2,15. During periods of HS there is generally a net heat gain from radiation in the daytime and a net heat loss from radiation at night with the animal radiating to its relatively cooler surroundings 15. Facilities should be designed with this in mind. It is thought that cooling cows continuously throughout the day is more beneficial than only cooling during the daylight hours. The fourth and final means of heat exchange is evaporation, occurs during sweating, panting and when cows are wet. Evaporation allows for the conversion of water from a liquid phase to a vapor or gaseous phase, thus transferring heat from the cow's body to the surrounding air with the evaporating water. As the ambient temperature rises , evaporative heat loss becomes the major avenue of heat loss because it is not dependent on the thermal gradient, as are conduction and convection 15.



Other environmental factors have great influence on the ability of a cow to exchange heat with the surroundings. Clouds and dust particles in the air as well as grass on the ground can affect heat exchange 16. Hair coat also has a profound influence on ability to dissipate heat 16. Summer hair coats of cattle tend to be less dense than those of other seasons. Cattle that have not fully shed out their winter hair coats should be full body clipped to help avoid HS 17.



Responses to Heat Stress

Cattle have adapted to HS with physiologic changes and behavioral responses. Behavioral responses exhibited are seeking out shade, reducing DMI, increasing water intake, standing rather than lying down, and bathing in ponds or mud 18. Heat stressed cattle tend to have increased body temperatures (core and rectal). As a result of HS cattle decrease dry matter intake (DMI) and increase water intake.



Respiration and sweating are both increased in cattle under HS 10. The skin and respiratory tract of cattle have great capacity to dissipate heat by evaporation, through sweating and panting, respectively 10. Increased respiratory rates consequently increase daily maintenance energy requirements by 7-25% 1. Respiratory changes of heat stressed cattle are characterized by increased rates and tidal volumes (dead space ventilation), thus increasing evaporative effectiveness, but also results in rapid loss of carbon dioxide leading to respiratory alkalosis 1. Cattle compensate for respiratory alkalosis by increasing urinary excretion of bicarbonate 1. Salivary bicarbonate is also lost, by drooling secondary to panting, thereby decreasing bicarbonate for ruminal buffering 1. Reduced rumen pH, leading to ruminal acidosis, along with increased standing increase the risk of developing laminits1. Cows likely stand more during HS because it is easier to pant standing rather than lying. Blood flow is also re-directed from internal systems to the peripheral integumentary system 10. Vasodilation occurs in peripheral cutaneous vessels increasing flood flow and heat transfer 10. Due to the redistribution of the finite circulating volume, less blood flow is available for gastrointestinal perfusion. This effect has been represented by decreased portal blood flow in experimentally heat stressed cattle 18,19.



Sweat glands on the body of cattle are of the apocrine type, meaning the apocrine cells shed the apical part of the cell during the sweating process 20. Catecholamines, which are elevated during periods of HS, are believed to mediate sweating by -adrenergic stimulation of apocrine glands 20. Increased catecholamine levels also tend to be correlated with decreased rumen contraction rates 10. Cattle also have a decrease in circulating ADH (anti-diuretic hormone), which normally increases in response to decreased blood volume (dehydration) in most animals 20. ADH normally increases the permeability of water in the collecting ducts and collecting tubules of the distal nephron of the kidney 20. Prolactin (PRL), which is similar to growth hormone (GH), is luteotropic (supports the CL) in some species, other than cattle, and is important for the initiation and subsequent maintenance of lactation 20. PRL also acts on the CNS to induce maternal behavior. PRL levels increase during periods of HS, and may be related to a sodium-potassium imbalance 20. During periods of HS considerable amounts of potassium (K+) are lost 1. Cattle increase urinary excretion of sodium (Na+) in order to preserve K+, thus increasing the dietary need for both Na+ and K+ 1. Low plasma K+ is likely the main factor regulating aldosterone activity, with Na+ regulation being controlled by hormones 21. Aldosterone acts to increase active uptake of Na+, in exchange for excretion of K+, in the cortical collecting tubules of the nephron 20. In most animals, aldosterone increases to conserve water, but appears to function differently in cattle .



Most mammalian cells possess the capability to produce what are known as heat shock proteins (HSP) and antioxidants in response to thermal stress, thereby limiting the damaging effects of HS 1,6. T3 and T4 (thyroxine and tri-iodothyronine) levels decrease to lower heat production, assist in acclimation, and lower DMI 2,17,21. These decreases have negative effects on mammary development and lactogenesis, as well as, likely negative reproductive effects 2,17. Stress in general, including HS, causes a release of ACTH from the anterior pituitary, which results in release of glucocorticoids from the adrenal cortex 22. Glucocorticoids also inhibit the release of LH from the anterior pituitary gland 22. The increase in corticosteriod levels inhibits the function of neutrophils, which are important for immune response against infections, especially those of the mammary gland 12. Corticosteriods suppress neutrophil adhesiveness, chemotaxis, receptor activity, lysosomal enzyme release, bactericidal activity, and phagocytic activity 12.



Effects of Heat Stress

Nutritional and Dietary Effects

Water is considered the most important nutrient during periods of HS 23. Water's high heat capacity provides a thermal buffer by conserving body heat in cold climates and conserving body water in warmer climates 23. Milk is approximately 87% water, and inadequate water supply will decrease milk production more rapidly and dramatically than any other nutritional factor 23. Water intake can increase by nearly 33% during HS 18,23. Research has shown that total body water loss increased by 58% in dry cows maintained at 82 compared to 28 F 23. Although not economically advantageous, providing cooled water sources has been shown to be beneficial, and cows generally prefer warmer water over cooler water anyway 22, 23.



Supplemental buffers, such as bicarbonate (HCO3-) and additional potassium (K+) may potentiate appetite and elevate rumen pH 17. Fiber quantity not only increases the metabolic heat of rumen fermentation, but fiber quality is also important. Fiber sources with high levels of structural components add to the heat of rumen fermentation 15. Grazing cattle during periods of heat stress also tends to increase maintenance energy requirements of 40 to 50% higher than those of confined cattle 15.



Dairy cattle may reduce DMI as much as 25% during periods of HS 1,12. HS increases energy maintenance requirements which leads to decreased feed efficiency 18. Most cattle normally experience a negative energy balance during the periparturient period. Decreased DMI coupled with increased energy needs has negative effects on milk production and reproduction. HS tends to cause an increase in rumen lactic acid levels, thus reducing rumen pH, which predisposes cattle to laminitis 24. An increased loss of nitrogen compounds in the skin secretions occur in cattle experiencing HS 15. Protein requirements may increase during periods of HS 15. Dairy cows will respond to increased dietary protein with increased milk production 19.



Milk Yield and Components

Each 0.45 kg of milk a 454-kg cow produces generates 10 kcal of metabolic heat per hour 15. Milk production not only decreases as a result of HS but milk fat is also lowered with a relative increase in long-chain fatty acids as a result of decreased circulating acetate (a volatile fatty acid) levels, resulting from decreased fiber intake 25. Milk protein appears to be minimally affected by HS 15,25. Somatic cell counts tend to increase in cows under HS 1,26,27. Mammary blood flow appears to decrease during HS 1,5,12. Cortisol and growth hormone (GH) may influence mammary metabolism, cortisol by altering glucose supply to the mammary epithelial cells while GH may alter the circulation of long-chain fatty acids to the udder 25.



Reproduction

Reproductive Pathophysiology

The major consequences of HS on reproduction is a delay in conception of cows after calving 1. These consequences can result in a seasonal calving pattern, and subsequent seasonal variation in milk prices 1. Two of the major causes of delayed re-breeding are a decrease in the number of cycling cows detected in estrus and a decrease in the number of inseminated cows that establish and maintain a pregnancy 1. HS has such a negative impact on conception rates in summer of hot, humid areas some dairy operations do not breed cattle during this period 17,28. Both luteal progesterone levels and proestrous estradiol/progesterone ratios may be related to the quality of the developing preovulatory follicle, intensity of estrous behavior, and uterine and oviduct micro-environment, as well as early embryonic development, that soon follows 5.



Ovarian Dysfunction

Cattle normally have about two to three follicular waves during an estrous cycle 29. HS can retard growth and impair function of dominant ovarian follicles 1. Follicles produce estradiol during development. HS causes a decrease in circulating estradiol concentrations during first wave dominant follicle development and during proestrus 1,6,30. The dominant follicular cells that are destined to become corpus luteum (CL) tissue may influence function of the CL, to produce and secrete progesterone, if they experience thermal stress during development. Cows experiencing HS show lower luteal progesterone levels to a degree that may interfere with the LH surge of estrus and thus estradiol concentrations 31. Decreased estradiol concentrations in cattle under HS may be related to a reduction in follicular size associated with decreased steriodogenesis within theca cells, granulosa cells, or both 30. Reduced viability of granulosa cells or more specific changes in steriodogenic enzymes may be possible causes of abnormal ovarian dynamics involved in HS 30. Second wave follicles are more likely to ovulate in cows in the TNZ 30. Cattle under HS exhibit more follicular waves per estrous cycle and longer luteal phases 30. Reductions in DMI may further compromise follicular growth by altering LH secretion patterns or reducing blood IGF-I concentrations 30. Prolactin, which is luteotropic in some species other than cattle, and many other hormones may be important in development of normal ovarian follicles 20. Increasing blood concentrations of estradiol from the preovulatory follicle also up-regulate the synthesis of endometrial oxytocin receptors and activate enzymes associated with PGF2 that lead to luteolysis 30. Administration of GnRH at observed estrus is thought to cause a predictable release of LH thereby enhancing secretion of luteal progesterone and improving embryo survival 31. Observations in heifers under HS are similar to those of heat stressed lactating cows 32. Although the exact stages of folliculogenesis and oocyte maturation that are disrupted by HS are not known, follicular memory is likely disrupted by occurrences in the 40-60 day period of bovine oocyte development and maturation 1,30.



Uterine Micro-environment

The re-distribution of the circulating blood flow internally to externally in heat stressed cattle leads to elevated reproductive tract temperature, reduced nutrient exchange, and alterations in the volume and content of oviductal and uterine secretions 6,33. It is not clear whether progesterone support for endometrial function is compromised by HS 1. A 0.5 C rise above normal in uterine temperature has been correlated with a decline in pregnancy rate of approximately 12.8% 1. Increased uterine temperature can also damage spermatozoa during capacitation in the female tract 22.

Embryo Development and Survival

Early embryonic death (EED) is perhaps the most unrecognized consequence of HS 18,33. HS can result in an increase in abnormal embryos as well as a decrease in embryo survivability 18,33. The first seven days is a critical period of development for embryos 18. EED by Day 35 has been estimated at 17.2% to 25.5% 3. Bovine embryos have begun to demonstrate thermotolerance by day three of development1. Bovine pregnancy recognition normally occurs on days 1 to 17 of the reproductive cycle3. Bovine trophoblast protein-1, now known as interferon- (tau), is produced by the early developing embryo and acts as an anitluteolytic agent, inhibiting maternal prostaglandin production, during early pregnancy 12,34. Interferon- production tends to be decreased in thermally stressed embryonic cells 34.



Unlike most cells, the peri-ovulation oocyte and the early embryo cannot produce HSPs in response to thermal stress 1. Antioxidants and pharmacologic agents, such as alanine, taurine and glutathione have all shown ability to protect developing embryos from thermal stress 6. Embryo transfer results in higher pregnancy rates when compared to cattle bred by artificial insemination 6. Although decreased pregnancy rate is a multifactorial disorder, EED plays a major role in low pregnancy rates 1.



Effect on Reproduction Parameters

Heat Detection

Inadequate heat detection is the most common cause of reproductive inefficiency in the southeast United States 33. HS is responsible for a delay in functional luteolysis and a decrease in circulating estradiol 1,30. Cows under HS show a reduction in the intensity and duration of estrus 15,33. Estrus periods may be reduced from 18 to about 10 hours 5,15,33. Cows in estrus will show most activity during the cooler parts of day 22. Cows bred during the summer season generally exhibit more average days open, apparently because of irregular or missed estrus periods. Estrous synchronization programs should improve heat detection rates by predicting when cows will exhibit estrus and because the number of cows in estrus at one time increases, increasing heat detection efficiency 1.





Fetal Growth, Gestation Length, and Calving

Uterine growth precedes placental growth, which in turn precedes fetal growth1,33. Approximately 0 per cent of fetal growth occurs during the last trimester of pregnancy , and is coupled with simultaneous development of the mammary gland 5. Decreased weights of the embryos exposed to HS have been detected as early as 17 days after fertilization 35. Calves born in mid to late summer generally weigh less than those born during winter months 33. Several studies have indicated a correlation between calf birth weight and subsequent milk yield 12,33. Fetal size has been shown to be consistently lower in cattle under HS compared to cattle in their TNZ. Cows calving under HS also have reduced total dry placental weight 5. Protecting cows, especially dry cows, from HS will result in healthier calves born, increases in productivity, and improvements in the overall health of cows 6.



Late gestation cows may calve early, resulting in retained fetal membranes (RFM) and metabolic disorders, like periparturient hypocalcemia and ketosis, which are exacerbated by decreased DMI 17,33,36. RFMs may cause increased days to first service, increased days open, and increased services per conception 33. It has also been speculated that increased incidence of dystocia and stillborn calves occurs in cattle located in hot, humid areas 12. Cows and especially first calf heifers, that have recently been transferred to warmer areas as replacements, could experience hyperthermia, heat stroke and even dystocia in calving lots and should have shade provided to protect them from solar heat 33. Serum IgG levels in neonates born in sub-tropical climates have been shown to be lower during summer months compared to winter months 33. The decreased IgG levels could be the result of: 1) Increased serum corticosteroids in neonates leading to reduced small intestinal permeability to IgGs; 2) Decreased suckling response of neonates during HS 37.



Management Schemes for Heat Abatement

Currently Used Methods:

1. Shades, fans, sprinklers, and evaporative cooling methods

2. Nutritional management programs such as total mixed rations (TMR), low quantity- high quality fiber rations, and rations supplemented with necessary amino acids





Potentially Useful Methods:

1. Strategic cooling 1

2. Embryo Transfer/In-vitro fertilization 1

3. Administration of pharmacological thermoprotective agents for developing embryos 1

4. Development of less heat sensitive breeds by use of genetics, records, and modern reproductive techniques 1



Facilities

Facilities should be designed, using economically justified inputs, to reduce the effects of HS 13. Areas that prove beneficial for cooling systems are feed lines, free stalls, holding pens, milking parlors, and exit lanes. Feed line shade has proven beneficial in increasing DMI and milk production 18. Feed line cooling costs approximately $4 per cow 1. Cattle feeding under cooling systems will go directly to the feed bunk and eat for longer durations than cows that have no cooling system available 18. The longer duration of standing and eating also allows sufficient time for the teat sphincter to close and thus may reduce the opportunity for pathogenic microbes to more easily enter the teat. Strategic cooling, a system for improving pregnancy rates during the summer based on estrous synchronization and extensive environmental modification during early pregnancy, may prove effective 1,6. The preovulatory LH surge and luteal progesterone concentration is higher in cooled cows, compared to cows under HS 28,38. Strategic cooling barns, such as for fresh cows, could be thought of as analogous to maintaining a sick barn for unhealthy cattle6. Pasturing cattle seems to provide inadequate relief from HS 12.





Shade

Response of cattle to shades in areas of high humidity are generally less predictable than those of cattle in arid regions 15. Trees serve as the best shades, but can be killed when cow density is high 1. If trees are to be used, they must be protected from cattle by fences 2. Improved efficiency of metal roofs has been noted by painting tops white, underside black, by placing about 2.5 cm of insulation directly beneath the underside, and by sprinkling the roof with water 1,2,15,18,39. Polypropolene fabric for shade cloth has become popular, especially the fabric providing 80% shade 2,18. It is recommended that cattle in arid climates have 3.5 to 4.5 m2 per lactating cow (38-45 sq ft)18. Shades should be at least 4 m (12 ft), preferably 14 to 16 ft, high to minimize radiation reflection from the shade roof back to the cow and maximize air velocity flowing over the roof, thereby reducing roof temperature13,18. Ridge vents of barns for warm climates should be at least 1 ft wide with an additional 2 inches for each 10 ft of structure width over 20 ft 2. A good rule of thumb for ridge vent width is 12 per 20, then 2 per 10 (in inches and feet, respectively) 7. Roof slopes should be at least 4:12 (33%) 7. Temporary shade structures should be oriented with the long axis north and south in order for the sun to assist in drying the ground under the shade during morning and afternoon 1,18. Permanent shade structures and those in hot, arid areas should be oriented in an east-west manner as to keep the area under the shade cooler 1,18. Temporary shades cost approximately $1.75 to $2.25 per square foot, about $25 to $50 per cow, and last approximately 5 years 7,18. Permanent shades cost about $150 to $300 per cow, and last approximately 3 years, making the benefit to cost ratio about 2.2:1 7. Therefore shaded cattle need to produce about 700 pounds of additional milk for shades to be cost effective 18.



Fans and Sprinklers

Fans are considered a practical method for increasing cooling, especially at night, by increasing heat loss at the animal surface through evaporative and convective means 15. Fans, that increase heat loss by convection, do not cool the air but work best if the ambient temperature is less than the cows body temperature 13,18. Fans reduce dependence upon panting, allowing cows to more easily feed or ruminate, consequently increasing DMI 13. Sprinklers should wet the hair coat of the cow, not just form a fine layer of mist over it. Sprinkling, as opposed to misting, allows direct evaporative cooling 18. Whereas misting relies upon convection cooling with evaporatively cooled air 18. Misting is also thought to be less effective in humid climates than arid climates because it forms an insulating barrier between the body surface and the ambient air, thereby decreasing effective heat exchange 1.

Evaporative Cooling

Although evaporative cooling works best in areas of low humidity it is the most economical way of cooling cattle in the hot, humid southeast U.S.14. Evaporative cooling, also known as forced-air ventilation, works by using heat energy from the air to evaporate water, lowering the temperature of the air and raising relative humidity 14,22. Fog and mist systems spray water into the air cooling the air as the droplets evaporate 14. Evaporative cooling pad systems and fans are effective in hot, arid climates and lower air temperature by 8-12 F, but raise relative humidity 14. An effective evaporative cooling system is to run the misters 1-3 minutes every 1 minutes and the overhead fans the remaining time of each cycle 1. Another option is to run fans and misters during the day and just fans at night.





Conclusion

Optimal milk production require facilities that will prevent excessive environmental heat load, while assisting cattle to dissipate surplus body heat, in addition to feeding and management practices that help minimize internal heat production 33. Combined use of shade, sprinklers, and fans can alleviate much HS when the systems are properly designed 1. Management implements such as strategic cooling and reproductive technologies such as ET hold potential for reproductive improvement 1. All the reasons to reduce HS mentioned earlier lead to increased cow comfort and productivity, ultimately increasing profits for dairy operations. Dairy veterinarians should be familiar with the effects of HS on dairy cattle and current methods to reduce its negative impact, making cows more comfortable, productive, and profitable for their clients.

























References



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Bryan Scott Kennedy

Production Medicine Graduate Program

March 4, 1999



Thermoregulation and the Effects of Heat Stress on Dairy Cattle



Introduction

Dairy cattle are predisposed to Heat Stress (HS)

Genetics

Metabolism

Physical attributes

Size

Skin thickness

Heat stress has negative influences on:

Nutrition and Diet

Decrease dry matter intake (DMI)

Increased energy demands

Negative energy balance

Increase water intake

Milk production

Decreased milk volume

Decreased milk fat

Decreased milk protein (minimally affected)

Increased SCC

Reproduction

Changes in:

Decreased heat detection rate (HDR)

Intensity and duration of estrous decreases

Decrease in cows establishing and maintaining pregnancy

Ovarian dysfunction

Decreased Progesterone concentrations

Decreased Estradiol concentrations

Decreased LH surge

Follicular memory

Uterine Micro-environment changes

Decreased repro tract blood flow

Elevated repro tract temperature

Reduced nutrient exchange

Uterine and oviductal secretions

Decreased volume

Altered secretion

Early embryonic death (EED)

Main consequence of HS

Embryo thermally stressed

Inability to produce HSPs

Increase in abnormal embryos

Decrease in embryo survivability

Decreased IFN- production?

Fetal Growth

20% growth occurs during last trimester

Calves born under HS weigh less

Gestation Length

Early calving

Retained placenta

Predisposed to periparturient disease

Hypocalcemia

Ketosis

Increased SPC, ADO, & days to 1st service



Veterinarians need to understand the effects of HS and how to reduce them:

Reasons to Reduce Heat Stress:

1. Increased Dry Matter Intake

2. Increased Milk Yield

3. Increased Estrus Activity

4. Increased Conception Rates-Pregnancy Rates

5. Decreased Service Per Conception

6. Decreased Average Days Open

7. Decreased Early Embryonic Death

Thermoregulation Physiology

Cattle are homeotherms

Normal bovine body temperature is 101.1-102.2 F (38.4-36.0 C)

HS causes increased body temperature

CNS receives temperature information from cutaneous sensors

CNS regulates body temperature through integration of:

Cardiovascular system

Increased peripheral blood flow/Decreased internal blood flow

Decreased portal and GI blood flow

Decreased mammary blood flow

Increased cutaneous blood flow

Respiratory system

Increased dead space ventilation

Increased respiratory rate

Increased tidal volume

Digestive system

Decreased Portal and GI blood flow

Decreased GI motility

Altered rumen fermentation

Endocrine system

Hormonal changes

Decreased TSH, T3, and T4

Increased Catecholamines

Increased ACTH and Cortisol

Decreased Estradiol, Progesterone, and LH

Increased Prolactin

Decreased ADH

Decreased Aldosterone

Mechanisms of Heat Exchange

Conduction

Occurs on contact

Convection

Occurs with air movement

Radiation

Occurs from absorption of solar radiation

Most significant means of gaining heat in hot, humid climate

Net gain day/Net loss night

Evaporation

Occurs by

Sweating through apocrine glands in skin

Panting-increasing evaporation of upper respiratory tract

Formulas:

Body Temp=Metabolic Heat Conduction Convection Radiation +Evaporation

Heat Loss=Heat Production + Environmental Heat

HS=(HP+EH)>HL

THI=T + (0.36) Tdp + 41.5

HS occurs THI >72

Responses to HS

Physiologic Responses

Increased respiration rate

Sweating

Re-directed blood flow

Behavioral Responses

Seeking shade

Reducing DMI

Standing rather than lying

Bathing in pond or mud

Management Schemes for Heat Abatement

Currently Used Methods:

1. Shades, fans, sprinklers, and evaporative cooling methods

2. Nutritional management programs such as total mixed rations (TMR), low quantity- high quality fiber rations, and rations supplemented with necessary amino acids

Potentially Useful Methods:

1. Strategic cooling

2. Embryo Transfer/In-vitro fertilization

3. Administration of pharmacological thermoprotective agents for developing embryos

4. Development of less heat sensitive breeds by use of genetics, records, and modern reproductive techniques Facilities

Facilities

Shade

Fans

Sprinklers

Evaporative cooling systems

Conclusion

Optimal production and efficiency requires

Feeding and management practices to reduce HS

Use of shade, fans, sprinklers, and evaporative cooling

Reproductive improvements

Modern technologies

ET

In-vitro fertilization

Strategic cooling

Profitable dairy's will require

Comfortable, productive cows

Veterinarians and animal health consultants with appropriate recommendations