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1009 PD: Can you spit into the wind?

David R. Bray Published on 29 June 2009

Is your cow cooling system up to the task of keeping your cows cool?

There are many components to cooling cows; the first is spraying water on the cows’ backs and the second most important component is air movement to evaporate the water off the cow to cool her.



Here is a test
For proper air flow, stand halfway back from your fans and spit at the fan. If the spit hits the fan, you don’t have enough air flow. If it blew the chew out of your mouth, you have enough air flow. When we design a cooling system for an open barn it is usually done by “rule of thumb.” A fan blows ten times its diameter, or a 36-inch fan blows 30 feet. A 48-inch fan blows every 40 feet, etc. This means that in most cases, you should have enough air flow to dry the cow off and take the heat with it.

Fans need to be placed over the feed face. You also need fans over the freestalls to provide air movement, and they will help dry the beds out. If your barn is not at least 16 feet at the eaves, has a 4:12 roof pitch and a 3-foot-wide ridge opening, it may need more fans to ventilate the hot air out of the barn and evaporate the water off the cows’ backs. Many tall barns can get by with one fan over the freestalls. Not-so-tall barns need two fans over the freestalls.

Should I be green?
Being green can be good or bad. You can be “green with envy” or green like Kermit the Frog. Or just be smart about what you do. Fans are rated for air flow at different static pressures. As a general rule, the least expensive fan will be the least efficient. Some of the old 36-inch fans we have used are very inefficient but some are very efficient. There are several tests that are done for efficiency. The secret is to make sure the testing was done by an independent testing service. Fan manufacturers may do their own testing but the results may be biased.

Cubic feet per minute (CFM) per watt is a good standard to judge a fan because it standardizes any size fan. The higher the CFM per watt rating, the more efficient the fan, so you can compare 36-inch fans to 48-inch fans for efficiency. Fans can lose 40 percent of their efficiency by being dirty. Belt-driven fans can lose up to 40 percent of their efficiency because the belts are not tightened properly. Along with the losses of being dirty you’ll have bigger losses. Fans with plastic shutters are often more efficient because metal shutters may have linkages that corrode and cause the shutter to not open fully.

Saving green
Effectiveness of fans varies by a factor of two. The annual cost of operating most 36-inch fans for 240 days at $0.10 per kilowatt electricity rate is about $300 for the most efficient fans and about $600 for the most inefficient fans. In a 600-foot-long open four-row freestall barn, with 36-inch fans spaced 30 feet down the barn and fans over each row of freestalls and each feed face, you would need 120 fans for the 600 cows.


The inefficient fans will cost $72,000 per year to operate; the efficient fans will cost $36,000 per year to operate. This is a savings of $36,000 per year for this 600-cow barn! If you have three of these barns for a total of 1,800 cows this would be a savings of $108,000 per year for 1,800 cows. If you had to pay $200 more for an efficient fan this would be $28,000 for the 120 fans in each barn, or you could pay for the fans in less than a year and have $8,000 left to take your wife to Hawaii.

Keep your fans clean, belts tight and as fans die, replace them with more efficient fans. Look for CFM per watt as a guide, no matter what size fan you decide on. PD

—Excerpts from University of Florida Dairy Update, Vol. 8, No. 3

David R. Bray
Dairy Extension Agent – Institute of Food and Agricultural Sciences
University of Florida