At one time, dairy barn ventilation was simple: leave the doors at the end of the barn open in summer and close them in winter. Although this method is simple and cheap, it doesn’t solve the problem of ridding the barn of excess moisture while keeping cows comfortable.
Years of research have shown that cows not only need fresh air but that animals thrive and deliver superior performance when air is delivered in the best way possible. Most new dairy barns are designed for superior airflow, but is it possible to adequately ventilate older barns?
Dr. Nesli Akdeniz, assistant professor and livestock controlled environments Extension specialist, University of Wisconsin, described a steel and fabric barn that houses 180 cows.
“It has natural light,” she said. “There are no ventilation fans, just circulation fans to replace the air inside. It has east-west orientation, so it captures the prevailing wind, the air quality is exceptional and it’s very quiet.”
There are several reasons this barn design works well. First, it’s on top of a hill with no other buildings parallel to it nearby. “Adjacent buildings cause downwind effects up to 10 times their height,” said Akdeniz. “Nearby buildings can block the airflow rate – then natural ventilation doesn’t work.”
The barn also has a steep roof, with 6/12 pitch. Lower pitch results in slower air movement and trapped warm, moist air against the cold roof. Air has a more difficult time leaving the building. Although construction costs are higher for a steeper roof pitch, airflow rate will improve. In some cases, poor fresh air distribution at cow level in cold weather can be a problem.
The ridge design is another reason this barn is working and comfortable in winter. Ridge openings are one to 1.5 inches per 10 feet of barn width. Although this barn doesn’t have a covered ridge, the small openings contribute to less rain entry.
The farmer reported that in winter, the ridge is often covered with ice, which keeps out rain and snow. Ridges are placed eight to 12 feet from each end of the barn to minimize downdrafts.
The sidewalls are 12 to 16 feet with adjustable sidewall inlets. The curtain system is adjustable according to weather. In summer, circulation fans and sprinklers help during hot, windless weather.
However, not all barns are naturally ventilated and require additional devices such as soakers and fans to ensure good airflow and adequate cow cooling.
Akdeniz cited numerous studies showing the value of providing water droplets via feed bunk sprinklers or soakers. However, the challenge can be keeping feed dry while allowing water droplets to cover the cows’ skin for a cooling effect.
There are tradeoffs with both mist and sprinklers – in heavy wind, mist may be blown away and not reach the cow at all, while sprinklers can increase the relative humidity inside the barn.
One concern with a barn on a hilltop with no surrounding buildings is that birds may be more of a problem. Recent concerns with the transmission of highly pathogenic avian influenza from wild birds to dairy cattle highlights the need for bird exclusion screens.
While many buildings are naturally ventilated, design is the key. Akdeniz said the minimum standard should be one foot of sidewall opening on each wall per 10 feet of building width.
“If natural ventilation doesn’t work, the alternative is tunnel ventilation,” said Akdeniz. “Ventilation fans are placed on the end wall, and if there isn’t enough space on the end wall, some of the fans can be placed on the side wall.”
Fans move air in one direction, through the barn to the other end. In tunnel ventilated barns, ridge vents serve as air inlets. Small circulation fans placed over stalls enhance airflow over cows.
A combination of ridge vents, side curtains and building placement in this new barn provide plenty of fresh air to cows. Photo by Sally Colby
In barns that don’t have a steep peak, cross ventilation works well. This system directs air into cows’ resting areas but is only suitable for mechanical cross-ventilated barns. As air moves, baffles push it downward over tops of cows. While baffles aren’t especially attractive, they make a significant difference in airflow rate.
If air movement will be enhanced with fans, Akdeniz cautioned dairy farmers to be aware of the ventilation efficiency ratio (VER), a measure of how much air is moved per watt or energy consumed. High volume, low speed (HVLS) fans can help circulate air throughout a barn. HVLS fans are typically large, up to 24 feet in diameter, and move slowly.
Dairy barns with AMS (automated milking systems or robotic milking) are becoming more popular. However, these systems can present ventilation challenges. Akdeniz said airflow can be hampered in the space over the commitment pen where animals wait to enter the robot. Heavy traffic near the robot, as well as the square robot structure itself, may also obstruct inlets and airflow.
Fans placed near the AMS unit and adjacent to heavy use areas help distribute air. Some barns that have been reconfigured for AMS may need new air circulation systems.
For any ventilation system, Akdeniz said the most important factor is adequate air inlets. While it might seem practical (and easy) to simply open the garage doors to allow fresh air to enter, birds usually find such openings quickly.
“Small air inlets increase static pressure and reduce fan efficiency,” said Akdeniz. “Aim for 40 to 60 air exchanges per hour in summer and four to 40 air exchanges in winter. Use variable speed ventilation fans and adjust fan speed according to weather conditions.”
Ventilation fans should be on the east side of a building to avoid pressure from stronger west winds.
Keep fans clean to maintain efficiency. Dirty fans can reduce efficiency by 40%. In AMS barns, fans should not block air inlets. Automatic fan controls can help operate fans at the appropriate times.
Slight alterations in barn configurations can greatly influence air movement. New barn design should allow for maximum ventilation to prevent animal health and air quality issues. Barns may require a combination of fan types to most effectively move air.
by Sally Colby
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