Challenges of naturally ventilating cow barns

Carole Curtis
Research continues to underline the importance of adequate ventilation when housing for adult dairy cattle in Wisconsin is designed, built or improved.

Madison — Thousands of naturally ventilated dairy cow barns have been built, but as farms have gotten larger, summers tend to be warmer and cows are producing more milk, challenges with naturally ventilated barns have emerged.

Bringing fresh air into a structure to displace heat, moisture, noxious gases and airborne pathogens is a critical concept of ventilation, Dr. Nigel Cook reminded listeners during "Challenges of Naturally Ventilating Cow Barns".

The Jan. 11 presentation was the first segment of "Breathe Easy with Ventilation," a two-part World Class Webinar being presented by Professional Dairy Producers of Wisconsin.

One reason that some dairy farmers struggle to keep their cows cool in naturally ventilated structures during the summer is that cows are producing more heat. "High producing cows generate a lot of heat," he pointed out. "A higher producing cow produces twice as much heat as a lower producing cow."

Dairies are also expanding into multiple, larger facilities. Space is at a premium as farmers try to fit as many cows into a farmstead as they can, but that concentration of buildings can cause serious problems with wind shadows.

Barn orientation is a significant issue, but even when oriented correctly east/west to take full advantage of prevailing winds, multiple barns can create wind shadows when located south and west of existing freestall facilities.

Midwest building tables recommend some 200 feet between naturally ventilated barns, but, in practice, it is very common to find the spacing closer to 100 feet, Cook said. The result is wind shadows.

"As facilities have expanded with multiple barns, I think we have really created a challenge, particularly during the summer, for natural ventilation," Cook said. "In some situations, it fails us. It really does."

Heat stress

Research on heat stress has moved forward dramatically in the past few years. New studies shows heat stress on dairy cows kicks in around 72-73 degrees, Cook said, lower than the previously thought and much lower than humans.

Farmers need to be making steps to reduce heat stress challenges because of their impact on milk production, reproductive performance and other health issues such as lameness.

Behavioral studies show that cows can become too hot to lie down.

Cook explained that when a cow settles down to rest,  she accumulates heat relatively quickly, about 1 degree per hour. She will stand to dissipate, or blow off, the heat  by thermal panting. "

"They have to do that or they're simply going to die. They're going to accumulate so much core body temperature that we get to life-threatening conditions," he said.

The heat stress insult to resting times can be huge. In one study of a seriously hot week, researchers found the cows' lying times dropped from 10 hours to six hours as the duration of each lying bout fell from 50 minutes down to 30 minutes over the six-day period.

"This is a massive insult," Cook said. "We don't need to look to acidosis or other mechanisms for the lameness we suffer in September and October. Losing four hours of rest in a day will do it just fine."

Bunching is another hot weather behavior that frustrates dairy producers as cows suddenly start living in half the barn. It is more prevalent in barns oriented north/south, Cook observed; caused by the sunshine coming in from the west all afternoon.

The bunching behavior is part of a dairy cow's grazing mentality, sparked by heat stress and fly worries. Cows equate "hot" with light and will seek the darker area of the barn, even though it may be hotter and will surely get hotter as the cows congregate.

"They are not seeking cool; they  are seeking dark," Cook observed. "These bunching cows are screaming at us to do something about ventilation and heat abatement."

Meeting the challenges

To keep cows cool, dairy farmers need to provide fast moving air in the resting space and enough air changes per hour to remove the heat and humidity from the barn.

Designing effective housing for adult cows starts with ventilation "When we're constructing buildings now, the reason we're so interested in ventilation designs is because it's really driving everything else that we put into a barn now," he said.

Over the past year, research has settled on three main criteria. Dairy farmers need to provide fast-moving air in the resting space, which is the key to keeping cows lying down in their stalls.

The second critical element is sufficient air changes per hour to remove heat and moisture.

The system should also work as well in winter as in summer. "Do these three things and it will work out just fine," Cook promised.

Moving air

It's a fact that cows like fast-moving air when they're hot, but just how fast is unclear. "You'd think we'd know the answer to that, but we don't," Cook said. "We're still searching for it, but until proven otherwise, we suggest 4.5 mph."

Fast-moving air can be provided fans, tubes or baffles.

Air leaves a fan in a cone, slowing as it moves away from the fan. Not taking that "throw distance" into account leads to problems with fans up too high, not angled enough or spaced too far apart.

In many cases, horizontal fan spacings are too far apart for effective air speeds. "The fans may get rid of the cobwebs on the purloins, but they're not going to optimize the environment for our cows," Cook observed.

He recommends 48 or 52-inch fans be positioned 24-feet on center over all rows of stalls and activated  around 65 degrees. The higher capacity fans that throw air further can be positioned closer to 60 foot apart.

Fan placement needs to work around roof supports, but should optimize fast-moving air over the cows' resting space. "The trend is for more fans closer together," Cook said. "The results are impressive. Cows lie down longer."

Cook has found tubes, a positive pressure air delivery system, ideal for the holding area because they thrust air directly down on the cows, instead of over them. "We have retrofitted quite a few holding areas in barns, piping in air from outside," he noted.

Currently, there are limitations on using tubes to drive fast moving air into a barn's lying area. At present, about a 100-stall pen is the maximum.

Cook will discuss baffles during his next presentation on Feb. 15, as well as mechanical ventilation options for cow barns. He will review the pros, cons and comparative economics of the different systems.

For more information or to register for the webinar, call 800-947-7379 or go online to