Keeping Cool - Set Up Supplemental Fans Now to Comfort Cows Next Summer

Talking about heat stress may seem strange when the temperature is falling outside, but if you plan your cooling system now, you'll have more comfortable cows producing more milk next summer.

The combination of rising temperatures and high humidity can cause heat stress in dairy cows. This reduces feed intake, leading to decreased milk production. After stretches of hot, muggy weather, milk yields commonly drop 20 or 30 per cent. Heat stress also causes poor reproductive performance, and can make cows more susceptible to mastitis and other diseases.

Cows have four primary ways to get rid of body heat:

  • Conduction. The transfer of heat between two bodies in contact, both at different temperatures;
  • Convection. Heat loss between two objects, one of which is moving, such as wind or water;
  • Radiation. The ex-change of thermal energy by electromagnetic waves;
  • Evaporation. When heat is given off as water changes from a liquid to a vapour.

The most effective ways to improve cooling for dairy cows is to increase the convective heat transfer rate, increasing the air speed over the cow and increase the evaporative cooling rate by wetting the cow, or the air around her.

Most free stall dairy barns are naturally ventilated, and will remain reasonably cool and fresh as long as there is enough wind to provide air movement. Researchers have determined that air should move past a cow at 200 to 430 feet per minute (fpm)-2.3 to 4.9 miles per hour (mph)-to provide desirable rates of heat and moisture movement through and from their hair coats during warm weather. Often during muggy weather there is not enough wind to produce these desired air flow rates. The solution is to speed up the air flowing over the cows using supplemental cooling fans.

Kansas State University researchers conducted studies on fan and sprinkler placement in a free stall barn in the summers of 1999 and 2000. The barn used was a four-row free stall barn with stalls placed head-to-head.

In 1999 the researchers studied three different configurations (Figure 1) .One had a row of fans over each row of free stalls (28) .A second arrangement had one row of fans over each outside row of free stalls and a row over each feed alley (F&S). The third had a row of fans over each row of free stalls and one over each feed alley (F&2S). All configurations used 36-inch-<diameter fans spaced 24 feet apart. A sprinkler line was located over feed alley.

Cows cooled with the F&S system produced 2.2kilograms more milk than the 2S system ( Table 1) . Strangely enough, cows cooled with the F&2S system produced more than the 2S system (1.2 kg) but less than cows cooled with the F&S system.



Fan Configuration




Average Milk (kg)




Dry Matter Intake (kg)




Change in Body Condition




Table 1 1999 Study Results

Fan Arrangement

Figure 1: Fan Arrangement in 1999 Study

The extra row of fans did not appear to add any benefit. Dry matter intake was essentially the same for all. Cows in the 2S system had a greater increase in body condition score. The researchers concluded that this increase was likely due to a dry matter intake similar to that of the other two groups, combined with lower milk production.

The 2000 study compared two fan arrangements (Figure 2) .In one, the fans were only placed over each feed alley (F), and in the other, fans were placed over each feed alley and over the two outside rows of free stalls (F&S). As in 1999, this study used 3&- inch-diameter fans spaced 24 feet apart and a sprinkler line over each feed alley.

Cows cooled with the F&S layout produced 2.6 kg more milk per day than with the F arrangement (Table 2). Feed intakes were slightly higher with the F&S configuration. Respiration rates were lower on average for the cows cooled with the F &S system when compared to the F system.

Fan Arrangement

Figure 2: Fan Arrangement in 2000 Study


Fan Configuration



Average Milk (kg)



Dry Matter Intake (kg)



Respiration Rate (breaths/min)













Table 2 2000 Study Results

The researchers concluded that in a four-row, head-to-head free stall barn, the best fan arrangement is a single row of fans over the outside row of free stalls and over the feed alley (F&S). There did not appear to be any benefit to adding a second row of fans over the free stalls.

Tips for Selecting the Right Fan

Axial-flow or propeller-type fans are the fans usually selected for cooling. Typically, these fans are 24 to 48 inches in diameter and operate with one- quarter to 1 horsepower (hp) motors. They can generate high air flow rates when operating at little or no static pressure, roughly 10,000 cubic feet per minute (cfm) per one-half hp.

Choose fans that move air in a relatively tight cone. Typical fans from a reputable supplier effectively maintain air flow at a distance of 10 times the fan diameter. For example, a two-foot-diameter fan will throw air 20 feet.

Also select fans for their capacity and the air velocity they produce. They should provide at least 500 cfm to 1000 cfm per cow. Air velocities should be in the range of 220 to 500 fpm (2.5 to 5.5 mph).

Mount fans high enough to provide clearance for any equipment used in the alleys and about eight feet above the free stalls to keep cows from damaging them. Fans should be placed to blow air in the direction of the prevailing winds and spaced 10 times their diameter apart. Tilt fans downward to aim their air flow at a point directly below the next fan in line (Figure .3).

Working Your Fans

Feed intake for Holstein cows typically starts to drop off at temperatures above 21 degrees C (70 degrees Fahrenheit). At 32 degrees C (90 degrees F) it has been reduced by 20 per cent. Set your fans on thermostats to operate above 21 degrees C.

It's a good idea to have manual overrides so you can selectively operate or idle rows of fans at night. Cows have some capacity to store heat during the day and release it at night when temperatures fall. Allowing fans to run into cooler evening hours on hot days lets cows release this heat faster. Be careful when running fans at night - the day-to- night temperature swing could become so great that cows become stressed and more susceptible to pneumonia.

Three Key Steps

Follow a three-step process to reduce heat stress in free stall barns:

  1. Ensure the natural ventilation system is working effectively by reducing any barriers to natural air flow around buildings and checking the wall design. Some screening used to keep birds out can greatly reduce air flow.
  2. Provide adequate water space and volume. Water consumption increases as temperature increases. It's important to have an unrestricted water supply when and where cows need it.
  3. When natural ventilation cannot handle the heat load, supply supplemental cooling over the cows. This can increase their convective cooling rate significantly. If heat stress is still a problem, sprinkler or fogging systems can be used to increase the evaporative cooling rate.

Placement of Fans

References: Brouk, MJ., JF. Smith, and JP. Harner; III. 2000. Freestall Barn Design and Cooling Systems. Heart of America Dairy Management Conference, p. 87-94, St .Joseph, MO

Brouk, MJ., J.F. Smith, and JP. Harner; III. 2001. Impact of Fan Configuration on Heat Stress. American Society of Agricultural Engineers International Meeting, Sacramento Ca.

Stowell, RR 2000. Heat Stress Relief and Supplemental Cooling. Dairy Housing an Equipment Systems Conference, p. 175-185, Natural Resource, and Engineering Service, Camp Hill, Pa.

This article appeared in the December 2002 Ruminations column of the Ontario Milk Producer magazine

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