All animals, including dairy cows, have four possible routes of exchanging heat energy with their environment. These are conduction, convection, radiation and evaporation. The first three are referred to as sensible routes of heat loss, and all require a thermal gradient. In other words, the cow must be hotter than the environment for heat to flow out of the cow into the surrounding environment.
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Bob Collier

The last route, evaporation, works on a vapor-pressure gradient, and it is the relative humidity that is critical to the rate of water evaporation from a surface. These two factors, environmental temperature and relative humidity, are incorporated into the temperature-humidity index (THI), which is why this measure is most effective when estimating the thermal environment animals are facing. Recently, the THI was revised to reflect increased sensitivity to heat in high-producing cows. This is shown in Table 1. The level of heat stress is indicated by color coding, and it has specific physiological responses associated with each level of stress.

Click on the image below to view the full table.

THI for high-producing cows

Sensible heat loss

As stated earlier, sensible heat loss requires a thermal gradient, and this is also true for moving heat from the interior (core) of the cow to the skin surface. Research shows the skin surface needs to be below 95ºF in order for heat to flow from the core to the skin. Since the core temperature is 101ºF, that provides a gradient of 6ºF for heat to flow out to the skin. When skin temperatures are above 95ºF, there is insufficient gradient for the heat to flow out and the cow begins to store heat, resulting in a rising body temperature. This is shown in Figure 1 below.

Figure 1, rectal temp and respiration rate

This has several negative consequences in feed intake, milk yield and reproduction. A second reason for keeping skin surface temperature below 95ºF is that skin temperature has major impacts on respiration rates, also shown in Figure 1. Elevated respiration rates alter blood acid/base balance, leading to urinary bicarbonate loss and eventual metabolic acidosis. Elevated respiration rates also cause cows to stand in order to respire more effectively, leading to less time lying down and reduced time chewing cud.

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Evaporative heat loss

Evaporative or insensible heat loss occurs from the respiratory tract (panting) and the skin surface (sweating). As shown in  Figure 1, panting increases sharply above a skin surface temperature of 95ºF. The same is true of sweating rate, as shown in Figure 2. However, cattle are poor sweaters, and there is considerable genetic variation between cows in their ability to sweat. Horses, for example, which have the same type of sweat glands that cows have, can sweat at 10 times the rate of a dairy cow. There is considerable genetic opportunity to increase sweating rate in cattle, which would dramatically improve their ability to withstand a thermal load. Given the increasing costs of electricity and water for mechanical cooling of dairy cattle, this is an area of opportunity for the dairy industry in the future.

Figure 2, sweating rate and surface temp

An additional environmental factor that regulates evaporative heat loss is relative humidity. Even under conditions of high relative humidity, it has been demonstrated that increasing wind speed can improve cattle cooling.  

The speed at which air flows over the surface of water affects the rate at which the water evaporates from a skin surface. As the wind blows, it sweeps away airborne water particles in the air. The humidity of the air in the region of this evaporation is reduced, which allows more water molecules to dissipate into the air. Wind can also change the vapor pressure by moving air about rapidly, thereby causing it to expand. This process creates room for extra water vapor, and evaporation will continue to occur while the wind is blowing. Thus, if the air temperature is below 95ºF, there is opportunity to increase evaporation from the skin surface in cattle by increasing wind speed. A minimum wind speed under these conditions would be 3.5 to 5 mph in order to achieve maximal evaporative heat loss.

Key indicators of heat stress

When evaluating your cows for heat stress, there are four key indicators to check:

  1. Determine the THI in the barn, which should be below 68.  
  2. Determine how many cows are standing. If more than half the cows are standing, they may be experiencing some degree of heat stress. 
  3. Check the skin surface temperature of cows by using an infrared gun, which is readily available online from a number of manufacturers. If the skin temperature is above 95ºF, they will be at some level of heat stress.
  4. Count the respiration rate of several cows. If the respiration rate (breaths per minute) is above 60, the cows are heat stressed.

Solutions

Cooling strategies for cows should be based on cost-versus-return estimates. The cost can be estimated using the summer production average divided by the winter production average, or the summer-winter ratio. That ratio should provide an estimate of the level of milk income lost each summer per cow. Reproduction costs can also be estimated using the summer-winter ratio on conception or pregnancy rates. Once this is established, a producer should work with a consultant to provide a plan for investment in cooling that provides a reasonable return.  end mark

PHOTO: There are four possible routes for cows to exchanging heat energy with their environment: conduction, convection, radiation and evaporation. Photo by Peggy Coffeen.

Bob Collier

  • Bob Collier

  • Emeritus Professor
  • School of Animal and Comparative Biomedical Sciences
  • University of Arizona