Dairy farmers who provide a combination of fans and wetting systems for cows often believe they’ve taken care of heat issues for the herd, but that may not be the case.
Dr. Jennifer Van Os, assistant professor and Extension specialist in animal welfare, Department of Animal and Dairy Sciences, University of Wisconsin-Madison, has spent over 10 years conducting heat stress research from an animal welfare standpoint and said individual cows should be determinants of heat stress and subsequent abatement.
Van Os explained the temperature-humidity index (THI), which incorporates air temperature and relative humidity to account for the fact that when the relative humidity is higher, it feels hotter. Van Os said heat stress for cows begins at a THI of 68º.
Because every farm has different conditions, the challenge is to plan appropriate cow cooling for each cow. “If we’re just looking at outdoor conditions, that isn’t enough information to know how well cows on a given farm are able to cope with their environment,” said Van Os. “In order to trigger heat abatement system, there’s usually a threshold that’s based just on air temperature – it doesn’t capture relative humidity, air speed or solar radiation. It’s a single point in the barn and doesn’t capture the variety of microclimates cows experience.”
Individual cows in the same environment show unique responses and cope differently to heat stress. “This can depend on breed, current production level, pregnancy, health status, coat characteristics (color),” she said. “More dominant cows will have access to important resources, so in times of heat stress, they could be dominating access to the water trough, taking advantage of the stalls with a cooler microclimate or standing directly under other sources of heat abatement. We have to think about the individual animal and what it means for her to be thermally comfortable.”
Cows begin to feel hot before they reach their upper critical temperature and before they become hyperthermic. Even in thermoneutral conditions, the cow can be uncomfortable and experience heat stress from an animal welfare perspective.
As cattle evolved, they sought shade as part of their natural behavior, and modern studies continue to prove cows are motivated to access shade and are willing to work hard and make trade-offs with other things important to them. “If cows don’t have shade and you provide other resources for dissipating heat such as soakers, that actually limits the effectiveness of other resources,” said Van Os. “Heat gain from solar radiation is counteracting heat dissipating.”
In visiting 12 dairy facilities, Van Os noted variations in lying time for individual cows. “It comes back to the fact that welfare is at the individual animal level,” she said. “They aren’t all having the same experience – some cows are spending less than half their day lying down. We think it’s because within a facility, air speeds can vary among stalls.”
Angling fans downward directs the air cone more evenly over the whole stall platform and improves uniformity of airflow. Positioning fans at stalls at each row rather than the feed bunk also helps because cows need to spend significant time lying down.
“Consistency is key,” said Van Os. “We’re looking to reduce variation in air speed among the stalls. We want to make sure all the stalls get above the target rate … at cow resting height. When we see greater variation in air speed, there’s greater variation in individual cows’ lying time.”
Evaluating a heat abatement system means observing indicators of how cows are coping. Measuring responses in cows is the only direct indicator of how well the heat abatement system is working. “This allows you to assess whether additional intervention is needed,” she said, “even for specific individuals or the herd on average.”
Cows express thermal discomfort in several ways, including lethargy. “Cows decrease their activity level in hot weather, and that’s an attempt to reduce body heat production,” said Van Os. “They also seek shade and water. Cows often bunch around the water trough whether they’re drinking or not, or will seek out cooling resources. They’ll stand at the feed bunk, not eating, but under the soakers.”
Van Os suggested watching for three signs that indicate heat-stressed cows: drooling, open mouth panting and elevated respiratory rate.
Drooling cows have visible strings of drool rather than the typical foamy saliva from rumination. Sometimes panting includes the tongue sticking out. “Panting is useful [as a heat stress indicator] because it’s very conspicuous,” said Van Os. “It’s easy to walk by a pen and tally the number of cows panting.”
However, one of the limitations of panting is it’s a late, and serious, sign of heat stress. When panting is observed, Van Os recommended noting individual cows that are panting, then measuring and recording their respiration rate. While 60 breaths/minute is normal, 80 to 100 is panting. Check the cow later and record her respiration rate again, and if she’s still panting, the heat abatement strategy might require troubleshooting.
Respiration rate is an earlier and more sensitive thermoregulatory response. Multiple cows with elevated respiration rates are sentinel animals – they’re already starting to struggle and more animals may start to show signs of heat stress.
“I would err on the side of intervening sooner,” said Van Os. “There’s inconsistency in what cows are experiencing. Some cows with better access to stalls under the fans are the ones coping better on any given day. Make sure there are fans over each row of stalls, and make sure they are aimed correctly and delivering adequately high-speed air.”
If fans are in place over the stalls, determine whether all stalls receive at least 200 feet/minute at 20 inches high, which would hit the cows broadside when they’re lying down. If that isn’t the case, start troubleshooting. In a cross-ventilated barn, are the baffles positioned correctly, or should they be moved or added? Determine whether fans are placed correctly, then adjust the fan angle so the air cone is hitting cows properly.
“It’s good to check the weather forecast and consider conditions in your region so you can plan when to monitor cows,” said Van Os. “If it’s going to be a very cool day or extremely hot, that isn’t the best day to monitor.”
by Sally Colby