en Español: Estrategias de Enfriamiento Durante el Estrés por Calor
Introduction
Heat stress can be considered as the sum of all the forces acting on the cow to increase body temperature above normal limits. It is difficult for a dairy cow in a hot environment to meet her full potential for either milk yield or fertility. The physiological adjustments the cow makes to prevent body temperature from rising during heat stress help prevent death from heat stroke but also reduce productivity. For example, heat-stressed cows experience a decrease in appetite. The advantage to the cow is that her internal heat production is lowered, and regulation of body temperature becomes easier. The disadvantage to the producer is that the reduction in appetite caused by heat stress decreases milk yield. The increase in body temperature that occurs during heat stress can itself compromise productivity. This is certainly true for fertility because the early embryo in the first one to three days of life is not able to develop when body temperature rises to 104°F (40°C).
The most common way to reduce the effects of heat stress on the dairy cow is to change the cow’s environment to reduce the magnitude of heat stress forces. The goal should be to minimize the rise in body temperature caused by heat stress so that the cow does not need to engage physiological adjustments that compromise production or experience body temperatures that adversely affect the body’s functions.
In this paper, we will discuss four topics:
- Understanding the mechanisms cows use to regulate body temperature as the basis for changes in housing to minimize hyperthermia during heat stress.
- How to measure heat stress to assess the effectiveness of cooling strategies.
- The importance of cooling non-lactating animals.
- Emerging ideas in cow cooling.
Please check this link first if you are interested in organic or specialty dairy production.
Regulation of Body Temperature and Implications for Cooling Cows
One way to think of a cow is as a furnace (Figure 1). It burns fuel (in the case of cows, carbohydrates, fats, and proteins that are mobilized from body stores and that ultimately are derived from feedstuffs) and the heat produced as a result is lost to the environment. Cows, like other warm-blooded animals, attempt to match the amount of heat produced with that lost to the environment so that body temperature remains constant. For cattle, the body’s thermostat is set to about 100.9° to 101.5°F (38.3° to 38.6°C). When it is cold, cows can increase heat production and decrease heat lost to the environment so as to prevent a decline in body temperature. When it is hot, however, cows can increase heat lost to the environment, but the ability to reduce heat production is limited to reducing feed intake. As a result, cows exposed to heat stress often experience increased body temperature (i.e., hyperthermia).
Figure 1. The cow is like a furnace. It burns fuel in the form of metabolites produced from feedstuffs and uses some of the energy in the fuel to do work such as milk synthesis, muscle growth, etc. The heat released from burning metabolites is lost to the environment. When heat stress is a problem, the cow cannot lose all of the heat of metabolism to the environment, and body temperature rises.
It is very difficult for an animal like the lactating dairy cow to successfully regulate body temperature during heat stress because lactation causes a large increase in heat production that cannot easily be dissipated to the environment. This idea is illustrated by data from an experiment in which rectal temperature was measured as a function of dry bulb temperature (i.e., air temperature) in Wisconsin (Sartori et al., 2002). At a variety of dry bulb temperatures characteristic of heat stress, lactating cows experienced hyperthermia while non-lactating heifers did not. For example, a lactating cow exposed to a dry bulb temperature of 77°F (25°C) had an average rectal temperature of 102.4°F (39.1°C), while a non-lactating heifer exposed to the same dry bulb temperature had a body temperature of 101.1°F (38.4°C).
Heat is exchanged between the cow and the environment by two types of mechanisms (see Table 1). Sensible heat loss mechanisms, which include conduction, convection, and radiation, occur at a rate that depends on the magnitude of the gradient between animal surface temperature and the temperature of the object or medium in the environment with which the animal is exchanging heat. Assume, for instance, that the skin temperature of a cow was 86°F (30°C). As the dry bulb temperature increased from 80°F to 84°F, the cow would lose less heat to the air by conduction and convection. At a dry bulb temperature of 90°F, the cow would experience a net gain in heat from the air.
As that example shows, sensible heat loss mechanisms become ineffective as the gradient between environmental temperature and the cow’s skin temperature decreases. In this situation, another heat loss mechanism, latent heat loss, predominates. Latent heat loss refers to the heat associated with the evaporation of water. For a cow, water is evaporated when produced on the skin as a result of sweating, rain, immersion in water, etc. or when a breath of air enters the respiratory tract and is humidified. Converting water from the liquid to the gaseous phase requires large amounts of heat. It takes over 500 times the amount of heat to evaporate an ounce of water as it does to raise the temperature of that water by 1°F. Heat-stressed cows, therefore, increase sweating rate and respiration rate to lose heat by evaporation.
Latent heat loss does not depend directly on dry bulb temperature, but it does depend on the humidity of the surrounding air. At very high humidity, little heat is lost by sweating and panting. When combined with exposure to high air temperatures, a cow in a humid environment is very compromised in its ability to lose its metabolic heat.
Various approaches for using the physics of heat exchange to increase the ability of the cow to lose heat to the environment have been developed and are summarized in Table 1.
|
Mode of heat loss |
Nature of heat exchange |
Environmental factors that determine the rate of heat exchange |
Modifications to facilities to increase heat loss from the cow |
|
Conduction |
Heat transferred from a warm substance to a cool substance without the substances moving; examples are from cow to air or from cow to floor |
Dry bulb temperature, surface temperature of material used for loafing; conductivity of materials exchanging heat |
Air conditioning, including by evaporating water into the air; cooling of bedding |
|
Convection |
Heat transferred from a warm substance to a cool substance where the substances are moving past each other; an example is from cow to wind |
Dry bulb temperature, wind speed; conductivity of materials exchanging heat |
Air conditioning and introduction of fans |
|
Radiation |
Electromagnetic radiation absorbed at the surface of an object; examples include exchange of radiant energy between cow and the sun or between cow and objects such as flooring, roof, trees, etc. |
Surface temperature, shade, cloudiness of night sky |
Shade, access to outdoors |
|
Evaporation |
Heat required to convert liquid water to gaseous water; examples include sweating, behavioral wetting of the skin, and respiration |
Humidity of air, wind speed, precipitation |
Sprayers, soakers, misters, and cooling ponds |
Heat loss by conduction and convection depend on air temperature, and heat loss through these modes can be increased by lowering air temperature. One practical way to do that is to force air to move past a water source. The loss of heat from the air as the water evaporates causes air temperature to drop. This is the principle behind fans mounted with misters. Convective heat loss involves transfer of heat into a medium moving past the animal. Installation of fans to increase wind speed can be an effective way to increase convective heat loss. When air temperature is very high, however, simply increasing the air speed will be ineffective because the gradient between cow temperature and air temperature is small.
Radiation refers to the exchange of heat as electromagnetic radiation (light) is emitted from one object and absorbed by another object. The energy of the sun reaches Earth through radiation. Any object with a temperature greater than absolute zero produces radiation. Thus, cows lose heat via radiation as well as gain radiant heat. The predominant factor determining radiation in cattle housing systems is the shade used to minimize solar radiation. Proper provision of shade is critical to reduce the magnitude of heat stress, and a simple solution like shade cloth is not adequate to completely reduce the heat gained through solar radiation.
At night, cows experience a net loss of heat via radiation to either the roof of the barn or to the sky (either clouds or objects in space). Keep in mind that a cow that is in a barn at night will cool down less effectively than a cow that is outside because the amount of radiant heat exchange will be reduced. Similarly, a cow outside at night will lose less heat if the sky is cloudy than if it is clear because clouds have a warmer temperature than objects in outer space.
Wetting cows through use of sprayers, soakers, or cooling ponds can be a very effective method for cooling cows, particularly when the gradient between cow skin temperature and air temperature is small. Not only does wetting increase evaporative heat loss, but conductive heat loss increases dramatically also. Heat is conducted through water 22 times more effectively than through air so, when a cow is wetted, conductive heat exchange from the skin increases.
How to Measure Heat Stress
Most dairies in warm regions of the world have installed some facilities to minimize heat stress. What is often not known, however, is the degree of heat stress cows experience or the effectiveness of housing for minimizing heat stress. Fortunately, it is not too difficult to evaluate existing cooling systems.
Temperature-Humidity Index (THI)
The THI is a mathematical formula based on dry bulb temperature and humidity that is used as an estimate of the magnitude of heat stress affecting a cow. The most commonly used THI is calculated as follows:
Tdb-(0.55-(0.55*RH/100)*(Tdb-58)
where Tdb=dry bulb temperature in °F and RH=relative humidity.
Through analysis of eight studies of lactating cows in environmental chambers, Zimbelman et al. (2009) estimated that milk yield starts to decline when cows are exposed to a minimum THI of 65 or greater. These authors recommended that cooling be initiated at average THI of 68 to minimize loss of milk due to heat stress. In another paper in which farm records were analyzed, decreased milk yield was observed when average THI was 70 or above (Ravagnolo et al., 2000). Note that in both of those studies, milk yield was affected by heat stress at a THI lower than the value of 72, commonly considered as a threshold for heat stress. It is likely that the increase in milk yield in the last 30 to 40 years has increased cow sensitivity to heat stress.
Analysis of non-return rate at 45 days after insemination indicated that heat stress becomes detrimental at a THI above 70 for cows in Georgia and Florida and at THI above 76 for cows in Tennessee (Ravagnolo and Misztal, 2002).
Body Temperature
The accuracy of THI for estimating the magnitude of heat stress is limited by the fact that THI is not greatly affected by the amount of shade, forced ventilation, and evaporative cooling provided to cows. In a Florida study in which cows were managed in freestall barns with fans and sprinklers, average body temperatures of cows did not become above normal until cows experienced dry bulb temperatures and THIs well above those usually considered as being characteristic of heat stress (Table 2). Probably, cows were cooled more than the THI indicated and were also somewhat adapted to heat stress because they were exposed to it chronically. Another problem with THI is that it is not very useful for identifying which activities and routines during the day make cows susceptible to heat stress (loafing, the holding area or parlor, etc.).
|
Rectal temperature |
Dry bulb temperature, °F (°C) |
Temperature-humidity index |
|
101.3°F (38.5°C) |
83.1°F (28.4°C) |
78.2 |
|
102.2°F (39.0°C) |
85.5°F (29.7°C) |
79.6 |
|
103.1°F (39.5°C) |
88.5°F (31.4°C) |
81.5 |
|
104.0°F (40.0°C) |
89.0°F (32.2°C) |
82.3 |
|
104.9°F (40.5°C) |
93.4°F (34.1°C) |
83.6 |
|
105.8°F (41.0°C) |
95.4°F (35.2°C) |
84.6 |
aData are the average dry bulb temperatures and temperature-humidity indices for cows experiencing the rectal temperatures shown in the left column. Rectal temperature was measured from 3:00 to 5:00 p.m. for cows housed in a free-stall barn with fans and sprinklers. Temperature-humidity index was calculated from measurements collected in the barn at cow side.
b Data are from Dikmen and Hansen (2009).
Rectal temperature is a better predictor of milk yield during heat stress than THI (Zimbelman et al., 2009). A good rule of thumb is that cows experiencing rectal temperatures of 102.2°F in the afternoon are at risk of reduced milk yield and fertility. This recommendation is based on results from two experiments with lactating cows. In one study (Zimbelman et al., 2009), milk yield declined when rectal temperatures reached 102.2°F (39°C). In another study (Gwazdauskas et al., 1973), conception rate declined 6.9% to 12.8% for each 0.9°F (0.5°C) increase in uterine temperature above the mean temperature of 100.9° to 101.5°F (38.3° to 38.6°C). Because rectal temperature is about 0.2°C lower than uterine temperature (Gwazdauskas et al., 1973), it can be estimated that fertility declined when rectal temperature reached about 102.2°F (39°C).
It is very easy to measure body temperature. The simplest way is to use any one of several commercially available rectal thermometers. Measurements should be taken in the afternoon (3:00 to 5:00 p.m.) when cows are most likely to be experiencing elevated body temperature. The thermometer should be placed in the rectum for a full minute to give the thermometer time to stabilize. While waiting for temperature measurements to stabilize, it is easy to also measure respiration rate by counting the number of flank movements for 30 seconds and multiplying by 2. A respiration rate of 60 breaths per minute or more indicates that a cow is experiencing heat stress.
Advances in computer technology have made it very easy to measure body temperature continuously so that times of day and cow routines associated with reduced ability to regulate body temperature can be identified. An easy location in the cow for monitoring body temperature is the vagina since small dataloggers can be easily secured in place in the vagina by attaching the device to a blank CIDR. Keep in mind that vaginal temperature has been reported to be about 0.2° to 0.4°F (0.1° to 0.2°C) higher than rectal temperature (Burfeind et al., 2011).
We monitor vaginal temperature with a reusable device called the Thermocron® iButton from Maxim in Sunnyvale, California, that sells for $25 each. These stainless-steel-encased computer chips have a built-in real-time clock and identification number and measure and record temperature at intervals set by the user of 1 to 255 minutes. Data can be downloaded by touching the iButton to an interface plugged into a personal computer. The iButton fits securely in a hollowed-out space in the center of a blank CIDR device (see Figure 2). A representative example of data collected using the iButton is shown in Figure 3. Data are from two non-lactating and non-pregnant cows, one housed in a freestall barn equipped with fans and foggers, and another housed in a lot with shade cloth.
Figure 2. The iButton and its attachment to a blank CIDR. Note that a groove has been cut into the center of the CIDR. The iButton is placed sideways into the groove and fixed into place with silicone sealant. After removal from the cow, data can be downloaded and the iButton reused.
Figure 3. Examples of data collected on vaginal temperature using the iButton attached to a CIDR. Lines represent data collected every 10 minutes from 8:00 a.m. to 6:00 p.m. from two non-lactating and non-pregnant cows during heat stress. One cow (solid line) was housed in a freestall barn with fans and misters, while the other cow (dashed line) was moved at 8:00 a.m. from the freestall barn to an open lot containing shade cloth.
Placement of temperature-recording devices like the iButton in sentinel cows can increase the effectiveness of heat stress management by providing information as to how well cows regulate body temperature during the day. Shown in Figure 4 are data on average vaginal temperatures recorded from lactating Holsteins on a grazing dairy in Florida during the summer.
Several conclusions can be derived that could not be made from measurements of THI or afternoon rectal temperatures:
- Cows are hyperthermic almost continuously, and there is only one short period after the morning milking when cows experience normal vaginal temperatures.
- The cooling provided in the holding area and parlor was effective in reducing vaginal temperature in the morning and, to a lesser extent, the afternoon.
- While vaginal temperatures increased during the day, they did not get higher than vaginal temperatures at night, suggesting the irrigation pivot cows were given access to provided some cooling. Based on these observations, it is likely that incorporation of additional heat abatement strategies, including providing cooling at night, would increase the cows’ performance.
Figure 4. Daily variation in average vaginal temperature during heat stress for lactating Holsteins maintained on pasture with access to irrigation pivot during the day. While in the parlor, cows were cooled with sprinklers. Data are from Dikmen et al. (2009).
Summer:Winter Ratios
In Israel, producers evaluate the effectiveness of cow cooling systems by assessing the summer:winter ratio of milk yield or conception rate. These ratios are calculated by the Ministry of Agriculture in cooperation with the Israeli Cattle Breeders Association.
The effectiveness of the summer:winter ratio for evaluating effectiveness of cooling systems for improving fertility is illustrated in Table 3. A total of 22 Israeli herds were categorized based on milk yield in the previous year’s winter and as to the intensity of cooling. Herds with intensive cooling managed cows so that they were exposed to 10 cooling sessions per day. In each session, cows were extensively wetted and then exposed to forced ventilation. Cows in the moderate cooling herds were cooled by wetting and forced ventilation only while in the holding pen before each milking (3x). Regardless of the level of cooling, cows were less fertile when inseminated in summer than when inseminated in winter. However, the decrease in fertility in summer was less for herds in which intensive cooling was practiced as compared to herds where only moderate cooling was provided.
While not shown in the table, the summer:winter ratio of economical-corrected milk yield (an Israeli measurement of production) ranged from 0.96 to 1.03 for intensively cooled cows versus 0.84 to 0.90 for moderately cooled cows. Those data point out the fact that reproduction is more sensitive to heat stress than milk production.
|
|
High production |
Low production |
||
|
Intensive cooling |
Moderate cooling |
Intensive cooling |
Moderate cooling |
|
|
Conception rate, winter, % |
39 |
39 |
40 |
39 |
|
Conception rate, summer, % |
19 |
12 |
25 |
3 |
|
Summer: winter ratio |
0.49 |
0.31 |
0.62 |
0.08 |
aFlamenbaum and Galon (2010).
Currently, summer:winter ratio can be calculated by individual producers and should prove useful in assessing effectiveness of heat stress abatement. Very accurate estimates of the ratio will require sophisticated statistical adjustments to the data to correct for seasonal variation in parity, stage of lactation, etc. As part of the USDA grant supporting this article, efforts are being made to develop procedures for calculating summer:winter ratios for dairies in the United States.
Don’t Forget the Heifers and Dry Cows
As mentioned before, lactating cows are more prone to hyperthermia when exposed to heat stress than dry cows or heifers. That does not mean that minimal efforts should be given to heat stress abatement for non-lactating animals. Heifers can become hyperthermic during heat stress and, when that happens, providing cooling improves fertility (Moghaddam et al., 2009). Monitoring of rectal or vaginal temperatures can provide clues as to whether housing is adequate to prevent heat stress. Even young calves can be heat stressed; hutch-reared calves experienced slower growth when born during periods of heat stress (Broucek et al., 2009). Providing something as simple as shade is likely to improve health and performance of growing heifers, and more extensive cooling may be in order. Again, measurements of body temperature can be an important diagnostic.
Cooling dry cows can provide benefits that extend into lactation. One of the consequences of heat stress during late pregnancy is that placental function becomes compromised so that hormones like placental lactogen that prepare the mammary gland for the next lactation are reduced in secretion. Not surprisingly, then, cooling cows during the dry period can increase subsequent milk yield. In a recent experiment in Florida (do Amaral et al., 2009), housing dry cows in a barn with fans and sprinklers increased subsequent milk yield by 16.5 lb/day (74.3 lb/day vs. 57.8 lb/day) as compared to cows housed in a similar barn without fans and sprinklers (see Figure 5).
Figure 5. Effect of cooling cows during the dry period (from 46 days before expected calving) on subsequent milk yield. Shown on the y-axis is 3.5% fat-corrected milk (FCM) yield. The line with solid squares represents dry cows that were cooled by being housed in a freestall barn with fans and sprinklers during the dry period. The line with open circles represents dry cows that were housed in a freestall barn without fans and sprinklers. The asterisks indicate significant differences between groups. Data are from do Amaral et al. (2009).
Strategic Cooling
One of the difficulties with getting cows pregnant in the summer is that many events necessary for establishment of pregnancy can be compromised by heat stress.
- The follicle that ovulates today started growing two to four months earlier; heat stress can affect that follicle for at least the last 26 days of its growth.
- Heat stress also damages the oocyte once it has been released from the follicle at ovulation.
- The early embryo is also sensitive to heat stress until about day 3 of pregnancy when it becomes resistant to maternal hyperthermia.
What this means is that one cannot solve the effects of heat stress by limiting inseminations to the cooler part of the day. By the time of insemination, the follicle has already been damaged. Moreover, if conception does occur, the resultant embryo will be susceptible to heat stress for the first two to three days of gestation.
Nonetheless, one can reduce the magnitude of heat stress effects on fertility by providing cooling for a few days around ovulation (Figure 6). Therefore, under situations where it is not feasible to provide intensive cooling throughout the dairy (for example, in grazing dairies), it might be advantageous to cool cows subjected to timed artificial insemination protocols from about three days before insemination (when cows on Ovsynch receive prostaglandin) until three to four days after insemination.
Figure 6. Effectiveness of providing cooling for a few days before and after breeding (d.p.b = days post breeding) on pregnancy rate per insemination. The numbers at the end of each bar are the number of cows pregnant/number of cows inseminated and the pregnancy rate.
Emerging Ideas in Cow Cooling
New engineering approaches to mitigate heat stress are continually being developed. All are based on increasing heat loss via either conduction, convection, evaporation, or some combination of the three. In some cases, new technology is adopted by dairies before scientific data have been obtained to rigorously scrutinize the effectiveness of a new cooling device or system. In addition, existing housing is sometimes retrofitted so that the cooling system is not engineered to produce optimal results. Therefore, the expected increase in cooling from adoption of a new cooling technology does not always occur. Consider one example shown in Figure 7. This figure shows average rectal temperatures of over 11,000 cows in Florida housed in 10 separate facilities. Data were collected between 2:00 p.m. and 5:00 p.m. during the summer and have been adjusted for differences in milk yield, parity, stage of lactation, and THI. Note that rectal temperatures for cows in the two barns that were cooled using tunnel ventilation were similar to two barns on the same farm and six barns on other farms in which cows were cooled with fans and either sprinklers or misters.
Figure 7. Average rectal temperatures of over 11,000 cows in Florida housed in 10 separate facilities. Data were collected between 2:00 p.m. and 5:00 p.m. during the summer and have been adjusted for differences in milk yield.
In a tunnel ventilation barn, air is more likely to move across areas where there is little impedance, such as alleyways, rather than where cows reside. Also, frequent movement of scraping and feeding equipment into the barn disrupts air flow. These problems are reduced in cross-ventilation barns, in which air is moved across the width of the barn. Some cross-ventilation barns have baffles installed to facilitate movement of air into areas where cows are located. Despite the apparent advantages of the cross-ventilation barn, such barns are also expensive to install and operate. No data exist to compare the effectiveness of cross-ventilation barns as compared to other systems for cooling cows. It is important to consider net profit to be expected from various cooling systems given the high and rising costs of energy.
One of the new ideas for heat abatement is called conductive cooling. The idea is to increase the loss of heat by the cow to the floor when she is resting in a stall. This is achieved by installing water-cooled heat exchangers buried under the stall. The costs for operating a conductive cooling system should be low, but it is not yet clear whether conductive cooling should be used as a supplement to or replacement for other cooling systems.
It is important to consider cow behavior when designing systems to reduce heat stress. A recent paper from California (Legrand et al., 2011) indicates that cows vary greatly in preference for evaporative cooling when given free access to an overhead shower activated by floor pressure. Cows made an average of 23 visits to the shower each day and spent an average of three hours total in the shower. However, the range of time in the shower varied from 0 to 8.2 hours. One implication of these results is that optimal cooling using sprinklers or showers is obtained when cows are not given the choice to experience wetting.
Conclusions: The Take Home Message
Monitor the body temperatures of your cows so that you can determine first hand which cows cannot cope with heat stress and when during the day cows need extra cooling. In very hot climates, cooling alone will not completely alleviate the effects of heat stress on reproduction, but a well-designed system can improve fertility and milk yield.
Author Information
Peter J. Hansen1, Serdal Dikmen2, Miki Sakatani3 and Geoffrey E. Dahl1
1Department of Animal Sciences, University of Florida, P.O. Box 110910, Gainesville, FL 32611-0910; Hansen@animal.ufl.edu
2 Department of Animal Science, Faculty of Veterinary Medicine, University of Uludag, Bursa, 16059, Turkey; serdal@uludag.edu.tr
3 Kyushu-Okinawa Agricultural Research Center, National Agriculture and Food Research Organization, Kumamoto, Japan; msaka@affrc.go.jp
Acknowledgments
The Dairy Heat Stress Roadshow and proceedings were supported by Agriculture and Food Research Initiative Competitive Grant No. 2010-85122-20623 from the USDA National Institute of Food and Agriculture. Original research described here was supported by the same grant and by funds from the Southeast Milk Inc. Milk Checkoff Program.
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