Rising temperatures reduce resting time for dairy cows

MADISON, Wis. — Ventilation systems in dairy barns should be designed to impact the cow.

“Cows cool when they stand and they accumulate heat when they lie down,” said Nigel Cook, chair of the Department of Medical Sciences at University of Wisconsin-Madison’s School of Veterinary Medicine. “While lying down, cows accumulate heat at about one degree per hour, and when they stand, they cool at about half that rate.”

“That’s why we’re talking about ventilating not only the barn, but the space the cow occupies, including the pen and stall microenvironment,” said Cook during a webinar organized by Hoard’s Dairyman.

“Dairy cows have changed over the last few decades. When you compare the heat output of a 40 pound milk per day cow to a 120 pound milk per day cow, we’ve doubled the heat output,” Cook said.

“Older barns that used to work quite well with lower producing cows tend not to work as well with high producing cows because those cows are generating more heat and that means they’re more sensitive to heat stress,” he said.

There are many physiological consequences of heat stress, including the impact on immune function, fertility and GI tract.

“But my main focus has been on the behavioral consequences of heat stress because we think this is at least as important or perhaps more important,” Cook said.

“Data shows as temperature increases we lose about three hours per day of resting time and we think that is a pretty big deal,” he said.

“That loss in lying time comes with a really frustrating behavior that drives dairy producers nuts,” he said. “They build a big, beautiful barn and the cows decide to live in half of it.”

Cook identified several critical priorities for ventilation design:

1. Target air speed in the resting microenvironment.

2. Sufficient air exchange.

3. System should work well across all seasons.

4. Cost effective.

Wisconsin researchers set up a study in two test pens with three treatments — no fans, fans at low speed and fans at high speed. Eight groups of 16 cows were monitored for a week in a naturally ventilated barn.

The air speed was measured at 1.5 feet above the stall. The low-speed fans were at least 200 feet per minute and the high-speed fans above 400 feet per minute.

“Our control cows lost lying time and when we applied fan treatments we improved lying time about one hour,” Cook said. “And the fan treatments yielded two to three pounds more milk.”

It is expected that electricity costs will increase when going from 60% to 100% fan speed.

“But I was amazed it was more than double the cost,” Cook said. “We get most of the benefits at the 60% level, so we have to do an economic analysis to decide if the extra air speed is worth it.”

When designing a dairy barn, Cook said, it is important to achieve goals for air movement.

“I prefer to measure air change per hour,” he said. “It should be four to eight ACH in the winter and from 40 to 60 ACH in the summer.”

The inlet speed is measured to make sure incoming air is mixing adequately with air in the barn.

“It should be 500 to 800 feet per minute,” Cook said.

A dairyman’s design choice for a barn will be a function of many factors.

“Those factors include the farmer’s tolerance for cleaning and maintaining fans, the climate, barn layout, economics, social influences and the performance of systems,” Cook said. “There are better choices that fit certain climatic, social and economic circumstances.”

Natural ventilation is a good option for many situations, Cook said.

“It is economically viable and it works in single barns when we’ve got adequate spacing between the barns,” he said. “It works for narrow barns up to six rows of stalls with ideal topography and good access to prevailing winds.”

However, natural ventilation doesn’t work well when it gets really hot and humid, but it is good for regions where electricity cost is high or in milk markets where the cows must have access to pasture.

“A positive pressure tube works in relatively short distances to bring fresh air in from the outside and it works good in variable climates for smaller barns with pen sizes of 100 cows,” Cook said. “It uses very little electricity and is good for closed barns where fresh air delivery is a priority.”

A cross-vent barn traditionally uses baffles to create air speed in the resting area, Cook said.

“But the disadvantage of the baffles is they block the air in the winter so some barns have retractable baffles that are moved up in the winter,” he said. “This is suited to wide body barns with 10 rows of stalls, where electrical costs are high and it can be adapted to use of evaporative cooling systems.”

In some cross-vent barns, fans replace the baffles to get air over the resting space.

“This is good for producers who don’t like baffles,” Cook said. “There are more fans so if electrical costs are high, that’s a concern for this design.”

It is important for dairymen to have a discussion about fan performance when designing a system.

“For fan efficiency, the cubic feet per minute is how much air flow you’re getting per watt of electricity,” Cook said.

“You must have a conversation about operating costs because you can get a difference of $20 per cow per year depending on what type of fan you pick,” he said.

And don’t forget, Cook said, every fan that is installed needs to be cleaned and maintained twice a year.

“You’ll loose 24% ore more of the efficiency of a fan by buildup on the louvers,” Cook said.

“There are a wide variety of options to keep cows cool in the summer and to exhaust contaminated air in the winter,” he said. “But they have to be designed well, appropriate to the climate, economically viable and effective all year around.”