Mechanical Ventilation Options for Dairy Barns
Madison - Like an old-time milking stool, ventilating a dairy barn stands on three legs.
"The critical elements of design are fast moving air in the resting space, sufficient air changes per hour, and a system that works in the winter, as well as in the summer," Dr. Nigel Cook, University of Wisconsin Madison dairy facility ventilation expert, said during "Mechanical Ventilation Options For Dairy Barns".
His Feb. 15 presentation was the second segment of "Breathe Easy With Ventilation", a World Class Webinar presented by Professional Dairy Producers of Wisconsin.
Ventilation is the provision of fresh air to the building space which displaces heat, moisture, noxious gases and airborne pathogens.
The current mechanical choices include cross-ventilated, tunnel and hybrid barns. "They're the talk of the town. There's a lot of interest in them and that has led us to further investigate these options," Cook noted.
System specifications for cross-vent barns can key off interior air speed, which has increased over time from 2.5 to 5-6 mph, or off body mass of the cows and heat transfer required, typically about 1,000 cfm per adult cow. Air changes can also be used, running from 4 per hour in the winter to 15-20 during transitional periods, with summertime rates of 40-60 per hour and even higher.
Such negative pressure systems usually locate inlets opposite the fans, generally sized so incoming air is delivered around 500 feet per minute to allow good mixing and flow.
Cross-vent barns are becoming common, particularly for larger facilities. They usually have 8-16 rows of stalls, a fairly low roof pitch and are sealed for deliberate mechanical ventilation 24/7, 365 days a year.
" I think the design really lends itself well to the use of baffles over stalls to create that fast-moving air in the resting space that is one of our main requirements," Cook said. The barns are also considered ideal for sand bedding.
Cross vent barns are popular because they offer total control; the barn can be oriented however desired; and the air moves perpendicular to the feed lanes into the cow space. Cows don't bunch much, there are few fly concerns and the cost of construction is relatively low.
However, some have issues with performing as well in the winter as they do in the summer. There can be relatively low air changes per hour, condensation, moisture buildup and fogging.
The air can also be unruly and unpredictable, following the paths of least resistance and sweeping into alleys and cross lanes. "We start to question how wide, truly, we can make these barns and make them functional," Cook observed.
Issues with air getting trapped up between the baffles might be solved by making the baffles retractable in the winter, an approach Cook considers promising and worth further investigation.
Other ideas include secondary inlets to bring in more fresh air, perhaps through the ridge and closable cupolas; managing inlets with fans pushing air into the barn; and playing with the height of the baffles.
Cross- vent barns can also be built without baffles, shifting the focus to ventilation rates based on body mass or air changes per hour, which generally means more fans on the barn.
Tunneling the wind
Tunnel ventilation barns, first on the dairy scene some 20 years ago, are attracting renewed interest. They draw air along the length of the barn with fans that are usually located on the northeast ends with inlets on the southwest end.
The rule of thumb is that air quality gets a little nasty when barns exceed 500 feet in length, Cook noted. They run from 4-row to 8-row with two feed lanes and are specified on air speed, which is usually 5-6 mph.
Farmers like tunnel barns because they offer control, sufficient air changes per hour, can be oriented as desired and placed closer together. Cows tend not to bunch and there are few fly concerns.
Some, known as hybrid barns, can retain a natural ventilation system and some allow cows to go outside.
Managing the inlets is important. Barns are now being built with managed inlets on the sidewalls to help deliver brisk air directly where the cows are living, rather than into the feed lane where they are not. "I think that's an important adjustment of tunnel barn design," Cook said.
Lots of tunnel barns have been built with lower false ceilings, usually around 18 feet, while other farmers have installed baffles, adjusted roof heights or changed the pitch.
In the winter, Cook advised bringing air in high over the cows so it has a chance to warm, such as a crack at the end of the barn over the center feed alley. Barns can also opt for poly carbonate sidewalls which are cheaper, easier to maintain and last longer than curtains.
However, Cook pointed out, none of these strategies - false ceilings, roof pitches or baffles - have solved the problem of getting fast-moving air in the cow's resting spaces.
The answer is locating fans right over the cows, much the same way as in naturally ventilated barns. "We are now back to fans again," Cook said.
For the best of both worlds, hybrid barns are now being built that use mechanical ventilation in the summer and transition periods, and assisted natural ventilation in the winter. They have a manageable ridge opening, sidewall curtains and increased construction costs.
Hybrids are being built with a low roof pitch, fans to draw the air out of the ridge to facilitate the flow of air up and out, and fans at the far end.
If there is fast-moving air over the resting space, all that's needed is sufficient air changes an hour for the barn itself to ensure enough heat transfer and exchange." I don't think we need to be living at 80-90 air changes per hour," Cook said. "I think it's between 40 and 60 in most of these facilities."
He questions the wisdom and efficiency of barns pushing air changes per hour to force more air into the cow pens. "I think focusing on adequate heat transfer and air changes per hour and using fans to make sure we've got the air where we want it to be, in the cow resting space, is a more efficient way to go," he said.
It is important that a dairy barn functions well year-around. "We see a lot of barns focused on optimal summer performance and forgetting the need to get back down through the transition period and winter," Cook observed.
There are other issues. "I think it would be a mistake not to mention some of the other considerations that go along with this," Cook said.
As dairy farmers optimize the environment in their cow space by switching to a tunnel ventilation system, they need to take fan noise, vibration and odor into account. "The more fans we have on the outside of the barn, the more the issues, not only internally, but also if you are close to neighbors," he pointed out.
At issue is the switch from a system that leaks out of the whole area of the barn to a system that expels contaminated air at high speeds through a very confined area.
Plan for more time spent maintaining fans and take aesthetics into account. "This is a current concern of mine," Cook said. Under current USDA guidelines for organic dairies, there is ongoing discussion about letting cows outside daily. Although that is not the current rule for conventional milk, it could be up the road.
"How do you do that with some of these 24/7 mechanical ventilation systems?", Cook asked. "That leads me to making you think about being flexible, of creating environments that we can justify, that are great for the cow and good for you economically, but keeps our consumers comfortable with what we're doing with our cows and our barns."
For more information on welfare-friendly cow housing, visit www.thedairylandinitiative.vetmed.wisc.edu.