Cows cool themselves primarily by evaporating water from their body surface and their respiratory tract during hot weather. The body evaporation process in cattle is in a vapor form emanating from pore-like structures. The cow can sweat only to a limited degree, primarily in the brisket area. As heat stress increases, cows increase their respiration rate to the point of panting to help increase evaporation from the respiratory tract. As the air temperature surrounding the cow approaches body temperature, sensible heat (body warms the air) loss becomes minimal.

To help cattle evaporate water, fresh, dry air must be provided by the barn ventilation system. This drier air is obtained from outside the barn by use of a ventilation system.

The cow increases her respiration rate dramatically when temperature rises under still air conditions. Moving air past the cow at high velocity (400 to 600 feet per minute) increases the evaporation rate and the convective heat loss rate when the air temperature is below that of body temperature. These increases in heat transfer rate can help reduce heat stress when air temperature is between 70 to 90ºF. However, as air temperature approaches body temperature, the effect is only minimal. In fact, increasing air velocity above 530 feet per minute (six miles per hour) has little benefit to improve respiration rate.

Intentionally increasing air velocity past the animal is accomplished by the use of fans by one method or another. Brouk et al. placed a cow suffering heat stress in a barn and provided no relief. Her respiration rate remained high (100 breaths per minute) and unchanged for 95 minutes. When air was blown at 700 cubic feet per minute onto another cow suffering from the same heat stress conditions, her respiration rate declined to about 90 breaths per minute after 95 minutes. Heat stress is not significantly relieved until the respiration rate has been reduced to 60 breaths per minute.

Hillman, Lee and Willard observed cows exposed to heat- stressed conditions averaging 88.7ºF dry bulb temperature and 61.1 percent relative humidity which is a temperature humidity index (THI) of 82.4. When lying in freestalls cows were exposed to air velocity averaging 256 feet per minute. They found cows’ vaginal temperature rose at the rate of 1.06ºF per hour under these conditions. This indicates her heat stress was not being relieved.


When cows stood in an airstream, their vaginal temperatures did not change; thus, they were in thermal balance with their environment. Exposure of larger portions of their body surface by standing helped to reduce the internal temperature build-up.

Hillman et al. observed a core body temperature of 102ºF as a trigger above which cows would stand to seek cooling conditions. Cows would lie down when their core body temperature reached 101ºF. Cows experiencing heat stress exhibit several behaviors including:

•less time lying in freestalls and more time standing
•increased water consumption and standing around waterers
•reduced feed intake
•increased respiration rate to the point of panting and tongue hanging out
•seeking shade or darker areas
•movement into areas where air velocity is higher and THI is lower
•movement into areas offering sprinkling with water

Water sprinkled onto cows is evaporated largely by the heat of the cow’s body. The high heat of vaporization of water makes it a very good cooling method. The evaporation rate is also influenced by relative humidity of the ambient air and air velocity past the body surface. Best results are obtained when the animal’s hide is wet.

Brouk et al. found the best heat stress relief while sprinkling cows at five-minute intervals with a one-minute sprinkle time and a fan blowing on the cow delivering about 700 cubic feet per minute. During their study, they sprinkled the cow without air velocity and found only sprinkling had a larger benefit than air velocity alone. Increasing the sprinkling frequency always improved heat stress relief with and without high air velocity. Hillman et al. showed that when cows elected to stand under a feedline spray with circulating fans, their vaginal temperature dropped at a rate of 1.35ºF per hour compared to no change in core temperature when standing with fans only. Thus, the spray was contributing to heat stress relief.

Natural ventilation with circulating fan cooling
The design parameters for natural ventilation include barn ventilation features and location features. Barn features are often emphasized, including:

•continuous open ridge measuring 2 inches clear for each 10 feet of building width
•eave openings of half the ridge opening for coldest days
•roof slope is 4 inches of rise for 12 inches of run
•sidewalls openable to some height based on barn width and animal housing density

A minimum of 10 foot sidewall height is commonly accepted, but recommended values of sidewall opening vary by author. The opening size may be influenced by frequency of heat stress conditions based on location within the United States. Those in warmer climates recommend tall openings. Bickert et al. suggest a minimum opening of 8 feet with larger openings being needed to overcome high animal density or wind obstructions.

Recommendations for location include a high elevation and oriented with the long wall perpendicular to the prevailing summer wind to capture as much wind velocity as possible. There should be no obstructions in the direction from which the summer winds prevail. Example obstructions to wind include: solid sided barns and sheds (with two-story barns being most problematic), tower and bunker silos (including silo bags), tall crops like corn, woods and trees, hills and bluffs.

The location of a naturally ventilated barn during new construction makes farmstead planning of paramount importance to assure new and future facilities will not block the wind. When considering retrofitting existing facilities to natural ventilation, consider the existing obstructions to summer wind movement. Frequently, existing barns cannot be made to use natural ventilation because of existing obstructions. In this case, the best choice is some form of mechanical ventilation. A rule of thumb most people remember is to locate naturally ventilated barns at least 100 feet from the nearest obstruction to wind when it is coming from the prevailing direction in summer.

Stowell and Bickert found several freestall barns exposing large amounts of sidewall opening per cow provide a suitable environment with little variation in temperatures within the barn. A wind approach of 90º provides maximum natural ventilation compared to any other approach angle.

There is some minimum ventilation rate below which the temperature and humidity within the barn will be appreciably higher than outside. Bickert et al. suggest a minimum of one air exchange per minute and 470 CFM per cow for summer ventilation. Stowell, Gooch and Bickert state, “In some cases, rates twice this amount (470 CFM per cow) are being considered the minimum acceptable for new facilities.” In a 4-row freestall barn of 98 feet in width and 12-foot wall height, one air exchange per minute of the building volume below the eave height is 1176 CFM per cow. In a 6-row barn of 116 feet in width and 12-foot sidewall height, one air exchange per minute of the volume below the eave height is 928 CFM per cow. In tunnel-ventilated barns, Gooch and Stowell recommend the minimum air exchange of 1000 CFM per cow to minimize the build-up of air temperature, humidity and odors at the fan end of the barn.

In naturally ventilated barns, fans are added to circulate air past the cows at high velocity to help relieve heat stress. Brouk et al. found heat stress was best relieved when fans are located over the feedline and over the freestalls when there was sprinkling over the feedline. Bickert et al. and others have recommended fans be spaced no more than 10 times the fan diameter and no more than 40 feet apart. They should be aimed at the floor at a location below the downstream fan.

The lowest point of the fan should be above where cows, workers and equipment can contact them. Such an arrangement provides air velocity in the space around where cows congregate and encourage them to use these areas. However, their benefits for individual cows are influenced by obstructions to this air flow. Obstructions can include other animals upstream of the cow, waterers, walls around waterers, etc. Cows standing and lying perpendicular to the direction of air flow are the most frequent obstructions.

Tunnel ventilation
Tunnel ventilation is a form of mechanical ventilation where fans are installed exhausting from one endwall and the other endwall is opened as an inlet. All other openings must remain closed along the length of the barn to avoid short-circuiting. The tunnel system is designed to provide ventilation (air exchange) and air velocity throughout the barn. The system is primarily for summer use. Operating the system during cold conditions draws low temperature air into the barn on one end, exposing the cows to drafts and freezing the manure.

Only a few fans will be used under these conditions, resulting in poor quality (elevated temperature, high moisture, odor and pathogen level) air at the fan end of the barn. For these reasons, an alternative ventilation system is needed for the cold season.

Design example – 6-row barn
Assume a 6-row freestall barn is 114 feet wide, has 12 foot sidewalls and a roof slope of 4-to-12. The cross section area of the barn below the eaves is 1392 square feet (116 feet by 12 feet). The cross section of the vaulted area of the roof is 1102 square feet (0.5 feet by 116 feet by 19 feet). The total cross section is 2494 square feet.

Using an average velocity of 600 feet per minute, the ventilation rate becomes 1,496,400 CFM. This is the fan capacity needed at about 0.15 inches of water static pressure.

Fans studied by Gooch, Timmons and Karszes with diameter of 48 inches had a delivery rate of 19,200 CFM at 0.15” SP. To achieve the total ventilation rate, 78 of these fans are needed. However, only 53 fans can be accommodated on the end of a 6-row barn. Expense can be saved for fans and the electricity to operate them if the number of fans can be reduced.

The air flowing in the vaulted space above the eaves does the cows no good. If it could be eliminated from the ventilation need, about 44 percent of the ventilation rate could be saved. Installing a ceiling at the eave height could eliminate the need to move air through the vaulted space. However, this would be expensive and eliminate the use of natural ventilation throughout the rest of the year when tunnel ventilation is not needed.

Installing vertical baffles at 20- to 30-foot intervals in the vaulted space and perpendicular to the ridge line could force air to flow below the eave line and still allow natural ventilation when needed. These baffles could be installed by fastening plastic sheets or ventilation curtain material on the trusses. When this is done, the ventilation rate needed is 835,600 CFM (1392 square feet by 600 feet per minute). Forty-four fans of 19,200 CFM per fan would supply this ventilation rate. The ventilation rate will serve a barn of fewer than 836 cows with a ventilation rate of at least 1000 CFM per cow.
When designing a tunnel- ventilated barn, care must be taken to close all openings that are not the designed inlet at the opposite end of the barn.

Potential openings can include:

•doors to access the parlor
•doors providing access to feed and manure-handling equipment
•ridge and eave openings used in natural ventilation
•manure discharge ports with direct access to the outside

The closer these inlets are to the fan end of the barn, the more impact they have for the rest of the barn.

The inlet area may be a limiting factor in tunnel ventilation. The system design is based on 400 to 600 feet per minute of velocity. The flow path area (barn cross-section) is used to establish the design value. The openings at the inlet end of the barn are frequently less than the cross- section of the barn flow path due to door framing and other barn features.

An effort must be made to provide opening area close to that of the barn cross-section. If it is appreciably less, the inlet velocity will rise and cause a friction loss at the inlet. This friction loss will affect the static pressure the fans must overcome and this will reduce their air delivery rate.
Often doors are used as part of this inlet area. These doors are often located at the end of an alley. This gets air started down the alleys. Thus, openings over the freestall areas should also be provided to increase the likelihood of air moving over the freestalls. Since waterers are often located at the ends of freestall rows, they can divert some of the inlet air around and above the freestalls.

Stowell et al. measured the temperature in paired naturally ventilated freestall barns with circulation fans and tunnel-ventilated barns at one site in New York and two sites in Ohio. They found the natural ventilated barns averaged 0.72ºF warmer than the tunnel-ventilated barns. They concluded “... naturally ventilated barns with supplemental cooling fans and tunnel-ventilated barns produce similar convective conditions within the barns.” They also state “air speeds at cow level are similar.”

These authors observed, “Air speeds in the center of these barns (tunnel) and higher off the floor were usually greater than in the corresponding freestall areas. Evidently, airflow was channeled toward the areas providing the least resistance to airflow and away from areas offering more resistance due to blockage (cows and freestalls) or confinement (shorter height).”

Gooch and Stowell state, “Measured air speeds within the lower portion of many tunnel-ventilated barns were noticeably lower than the design air speed. Measured air speeds in the central areas, like drive-through alleys, and higher off the floors were usually greater than in the corresponding freestall areas and other occupied cow spaces. This shows air naturally channels toward those areas with least resistance to air movement and away from areas offering more resistance due to blockage (cows and freestalls). Longer barns appear to have a more pronounced airflow channeling effect, resulting in little air movement at cow level.”

With tunnel barns, the reduced velocities in the freestalls and higher velocities in the alleys will likely encourage cows to stand more compared to circulating fan-cooled freestalls to find a cooling environment rather than spend more time resting in freestalls during extremely high temperatures. Gooch has observed air velocity in the freestalls located next to the wall in tunnel barns is often so low that cows are not comfortable in those stalls. He has recommended opening (2 to 4 inches) the bottom of the walls over the length of the barn to direct ventilation air over the cows in the stalls. This could have the effect of reducing the velocity of air at the inlet end of the barn, but it should improve resting use of stalls.

Tunnel ventilation can help keep cows cool when natural ventilation cannot be used effectively. Examples of situations where tunnel ventilation should be considered over natural ventilation include:

•a barn is being retrofitted and its location does not allow natural ventilation
•space is limited for new construction and natural ventilation design criteria may not be met
•site conditions do not allow barn orientation to optimize natural ventilation

As producers increase their barn sizes, they note the large number of fans needed to provide circulation in naturally ventilated barns. As they consider tunnel ventilation, they note fewer fans may be needed for a tunnel design compared to natural ventilation with circulation after some barn size is reached. PD

References omitted but are available upon request at

—Excerpts from 9th Annual UW Arlington Dairy Day Conference Proceedings