Natural ventilation is designed for six rows of free stalls or less. It is a good option in many situations and an economically viable choice under variable climatic conditions.
Natural ventilation is designed for six rows of free stalls or less. It is a good option in many situations and an economically viable choice under variable climatic conditions. PHOTO COURTESY OF THE DAIRYLAND INITIATIVE
WATERTOWN, Wis. – Looking at the big picture of overall airflow throughout a barn is deceiving. Rather, farms need to bring their analysis down to the stall level. How much air is a cow receiving in her microenvironment or resting space? This is where the true effectiveness of a ventilation system is revealed.
The stall is considered the cow’s microenvironment – the place where she spends most of her day resting. By measuring air speeds in the resting space as well as cow respiration rates, farms can determine if their heat abatement systems are doing their intended job.
“My philosophy is to build from the cow up,” said Dr. Nigel Cook, DVM, University of Wisconsin-Madison School of Veterinary Medicine. “Whatever we do needs to impact the cow space not just the barn space. When ventilating a barn, you need to think about that. You must ventilate the pen and the space where cows live and lie down, which is at the stall level.”     The Dairyland Initiative at the University of Wisconsin-Madison put on a workshop entitled, “Assessing Adult Cow Barn Ventilation” at Rosy-Lane Holsteins near Watertown Aug. 24. The workshop was one of three held on Wisconsin farms this summer by Cook and his colleagues.
The afternoon session offered hands-on activities for attendees, who had the opportunity to measure airflow in the barns.
Poor ventilation can cause problems in both summer and winter. Subpar air quality during cold weather can create an elevated risk for pneumonia, while heat stress during the summer can impact feed intake, milk production, fertility and health. Systems should be designed to keep animals healthy and productive through all seasons.
Heat affects a cow’s ability to rest. Cows lose approximately three hours per day of lying time when the temperature is between 70 and 82 degrees.
“That loss is like going from a sand-bedded stall to concrete,” Cook said. “That is how much comfort they’re losing. Three hours is a lot.”
A cow’s body temperature goes up when she lies down. But when she stands, her body temperature decreases. According to Cook, cows gain about one degree per hour when lying down. When standing, they cool by about half of a degree per hour.
A study at UW-Madison showed that air speed at cow resting height positively influenced daily lying time, body temperature and milk yield under conditions of heat stress. Cow groups with fans produced 4 pounds more milk per cow per day versus the control group, and they also rested longer.
Cows with fans operating at a speed over 400 feet per minute averaged 14.3 hours of resting time per day, and cows with fans operating at a speed of 200 to 400  feet per minute averaged 13.9 hours of daily lying time. The control group had a lying time of 13.2 hours per day. Cook said that, globally, the industry averages 10.5 hours of resting time per cow daily. Cook did note these were small sand-bedded freestall pens that were not overstocked, and cows were only out of the pen milking for about 20 minutes per day.
“Cows are very sensitive to air speed, and these results indicate the need to provide a minimum cooling air speed in the cow resting microenvironment,” Cook said.
The minimum cooling airspeed is defined as a minimum of 200 feet per minute or 2.25 mph measured at a resting height of 18 inches.
There are six common ventilation choices – natural ventilation with fans over stalls, positive pressure hybrid ventilation with fans pushing air into the barn from the side walls, tunnel ventilation with fans over stalls, tunnel hybrid ventilation with cupola fans and curtains, cross ventilation with baffles over stalls, and cross ventilation with fans over stalls. Positive pressure is the newest approach and a practice done commonly in buildings designed for people. However, negative pressure is more common in agricultural facilities.
Natural ventilation is designed for six rows of free stalls or less. Electrical cost is low, and fan maintenance is minimal. However, air exchange is not always a given in natural ventilated barns.
“When the wind blows, it’s great,” Cook said. “But when it doesn’t, natural ventilation doesn’t achieve the air exchange rates needed in the summer.”
According to Cook, the wind does not blow 20% of the time in Wisconsin in July. Furthermore, as farms get bigger and expand, they block the airflow needed for a natural ventilation system to be successful.
“Natural ventilation is still a good option in many situations and an economically viable choice under variable climatic conditions,” Cook said. “But if you have to build in a north-south orientation, don’t build a natural ventilated barn as most wind comes from the south or southwest.”
Positive pressure hybrid systems are ideal for 4-row barns and offer seasonal flexibility combined with low electrical cost. However, fan maintenance is hefty, and install cost is high.
A hybrid system is a combination of mechanical and natural ventilation. An example would be using exhaust  fans with curtains. In summer, the farm would close the curtains and the ridge and turn on the fans.
A tunnel system is great for predictability of airflow, whereas a naturally ventilated barn can be challenging because it does not have predictable air speed. A tunnel system or tunnel hybrid system can be installed in barns with eight rows or less but comes at a high electrical cost and requires considerable fan maintenance. This type of system can be challenging in the  winter at low air speeds, and  barn length is a restricting factor.
“Tunnel barns are great in the summer but can freeze manure in the winter,” Cook said. “Moving small volumes of air through very big buildings and avoiding freezing can create a challenge.”
Barns with eight to 10 rows may consider a cross baffle system. This is attractive, offering low electrical cost and low fan maintenance, but retractable baffles are a must to avoid trapping air between the baffles at low air speeds in the winter. Cross-ventilated barns without baffles use fans to create the air speed in the stalls, but this increases costs.
“The use of baffles to create the required air speed in the resting area creates an operational advantage for a cross-ventilated barn, but they need to be retractable in the winter,” Cook said.
In barns containing more than 10 rows, a cross-ventilated system with fans might be considered. However, this system comes with a high electrical price tag and a lot of fan maintenance and is preferred for wider body cross-ventilated barns.
In a mixed climate like that of Wisconsin, where it is hot in summer and cold in winter, the costs per cow for natural ventilation are between $20-$25 per year. It is double that for most mechanical ventilation systems, with cross baffles being the cheapest and hybrid systems the most expensive to run of the mechanical options. The study was based on an electricity cost of $0.10 per kilowatt-hour in an 800-cow barn.
To ensure effective ventilation, barns should be designed to include sufficient air exchange to remove heat, dust, noxious gases and moisture. The system must also be economical, considering what the cost per cow is per year to run the system and deciding if that number is compatible with the losses incurred from the heat.   
In Madison, there are 77 days where the temperature is greater than 68 degrees Fahrenheit. The calculated marginal cost of heat stress is $123 per cow per year factored on a $19 milk price.
“The benefits of these ventilation systems more than outweigh the cost,” Cook said.  
Practical design recommendations suggest a sufficient air change per hour of 4-8 in the winter and 40-60 in the summer with tunnels being on the lower end at around 40 and cross vents being around 50. Air should be moved at a minimum of 1,500 cubic feet per minute per adult cow.
“Sufficient air exchange per unit of body weight in the summer is absolutely crucial because we have stocking rates of 1.5 cows per stall,” Cook said. “Air change doesn’t get it done. It just adds all that heat. You need air exchange per animal.”
Cook recommends installing fans with a variable frequency drive.
“That’s a no-brainer to me and one of the biggest and best choices a farm can make,” he said. “Variable frequency drives are an absolute win for our industry. These fans are able to operate at less than full speed, giving the farmer flexibility.”
Operating at 60%, fans with a VFD can cut electrical costs in half.
“You run them at a real low speed in winter just to move air around,” Cook said. “In one year or two years tops, you will have it paid for.”
Each type of ventilation system has pros and cons. But when systems are designed from the cow up, the likelihood for success is much higher.