MADISON - Naturally ventilated barns have served dairy farmers well for decades.
They feature open ridges, open eaves and a slope that assists the wind flow in pulling air out of the barn. But there can be challenges with wind shadows, barn orientation or the wind simply doesn't blow.
"Then we're left with minimal ventilation in a naturally ventilated facility. That's quite a challenge to keep cows cool during the summer," Dr. Nigel Cook, D.V.M., University of Wisconsin Madison, told listeners during the April Hoard's Dairyman Webinar.
During "A Breath Of Fresh Air - Ventilating Barns", the internationally acclaimed ventilation expert observed that as dairy facilities have gotten larger, cows are producing more milk and summers have become warmer, producers are looking for different options to ventilate their barns.
Unfortunately, the scientific recommendations are outdated with animal heat estimations based on studies done in the 1950s. Companies are going it alone, and confused producers have lots of questions about the best system for their farm, Cook said.
One thing is certain: heat has a serious impact on dairy cows. One recent study conducted during a heat stress spike that lasted six days found cows lost four hours per day in lying time.
Deliberate ventilation systems
The selling points of a mechanically ventilated facility are more control of the air flow, ease of placement, and more cows in a smaller footprint. There's also less cow bunching and fewer concerns with flies and birds.
Whether cross ventilated, tunnel ventilated or a hybrid of the two, Cook said a mechanically ventilated barn must have an adequate ventilation rate to remove moisture, noxious gases and heat generated in the barn.
The industry has three main ways of specifying a mechanical barn: air speed, heat transfer or air changes per hour (ACH).
In recent years, Cook noted, fan air speeds have been increased from 2.5 mph to 5-6 mph over the cross-section of the barn. Heat transfer is usually quoted around 1,000 CFM per adult cow, while air changes per hour (ACH) is traditionally aimed at 4, 15-20 and 40-60 ACH in winter, transition and summer, respectively.
Issues and challenges
Tunnel barns have problems with air flow because the fickle wind wants to flow down the center alley . "Generally, wherever a cow is living, that's where we see much slower air speeds, " Cook noted.
Producers have learned, however, that bringing air through the side of the barn provides fast-moving air that penetrates through the barn, moving directly across whole pens of cows.
"In newer facilities, we're seeing temperature and humidity controlled curtains on the sidewalls and end walls, separate from the feed lanes which were traditionally and historically used as inlets," Cook noted.
Ceilings are also being lowered to about 18 feet tall. That creates a need for winter inlets, which are being situated high up near the roofline where the air warms as it enters the barn.
Cross-ventilated barns have the advantage that air moves perpendicular to the feed lanes, pulling across the barn. "Many use baffles, particularly over a head-to-head platform of stalls, to redirect and get that nice fast-moving air in the resting space," Cook said.
However, it has become obvious that the focus of many facilities has been on air speed below the baffles and not on air changes per hour, he added. The solutions include changing the height of the baffle, fitting the barn with retractable baffles, or using overhead fans instead of baffles.
"In both types of facilities, we have gravitated back to ensuring that we have fast-moving air in the resting space...using fans," he pointed out.
The best of both naturally ventilated and mechanically ventilated barns come together in a hybrid barn, which is mechanically ventilated in the summer and transitional periods, and naturally ventilated in the winter.
Such a barn uses a low roof and low pitch with cupolas and fans to manage and draw air out of the ridge. Side wall curtains are used along the length of the barn.
While there is an increase in construction costs for a hybrid barn over 24 /7 mechanically ventilated barn, Cook noted the hybrid offers flexibility with the valuable option of letting the cows go outside.
Cook boiled ventilation design down to three critical elements. There must be fast-moving air in the resting microenvironment. There must be sufficient air changes per hour to remove heat and moisture from the barn, and the system should work as well in the winter as it does in the summer.
Cows prefer fast-moving air when they're hot, he said, recommending a target speed of about 400 feet per minute. To achieve that, he recommends typical 48-52 inch fans over all rows of stalls every ~24 feet, activated at ~65 degrees.
"If we ensure that we have fast-moving air over the resting space, we just need to provide sufficient ACH for the barn as a whole with the ventilation system to remove the heat and moisture," he said..
Most mechanical ventilation operating system cost about the same in temperate climates of the U.S., at 2-2.4 times the cost of a naturally ventilated barn, he noted.
Wide body cross vent barns are the cheapest build and operate, but they have operating issues with adequate ventilation and the solutions have not all been realized, he noted. Hybrid tunnels are more expensive to build and operate, but they have the most flexibility.
In one cost comparison, the hybrid tunnel barn cost $121 more per cow per year than a wide body cross vent barn, which Cook figured was about two pounds of milk per cow per day.
The systems can be fine-tuned a bit. For example, turning the fans over stalls on at 68 degrees instead of 72 degrees costs about $5 more per cow per year. It would cost about $5 more to reduce it to 65 degrees, Cook noted.
Be very careful with fan selection, he underlined, since costs can vary significantly. "The choice of fan can impact your operating costs by $30 per cow per year or more! That's really a strong message to focus on the efficiency of these systems and make the right choice," Cook said.
Variable speed fans may provide about $8/cow/year savings, while exponential vs. linear ramping can mean another $4/cow/year savings. "An ideal system could save about $12/cow/year in operating costs, but how much do these systems cost to implement?," he questioned.
The economic opportunity for biofeedback is small when electricity is cheap, he observed, but the beneficial effects on the cow may be great.
if location and orientation allow, Cook believes natural ventilation is still a great option. Fans can and should be used to optimize the fast- moving air in the cows' resting space.
Mechanical ventilation facilities are viable solutions, he said, particularly where the site is compromised and where wind shadows are likely. However, pay close attention to the three criteria: fast-moving air in the resting space, adequate air exchange rates and efficient transitions.
Cross, tunnel and hybrid barns have both strengths and weaknesses. "I think we need to look at the farmstead situation, look at what we need to provide for the cows, and design the facility that best fits that given situation," Cook said.