Many dairy farms rely on water, usually in combination with fans, to cool cows in hot weather. Dr. Jennifer Van Os, assistant professor and Extension specialist in animal welfare, Department of Animal and Dairy Sciences, University of Wisconsin-Madison, said although cooling cows with water is helpful, it shouldn’t be the primary cooling mechanism.

Van Os explained the role of soakers in cow cooling: “Respiration rate and core body temperature are associated with the cow’s posture. While the cow is lying down, there’s an increase in her core body temperature and respiration rate.” The theory is that when cows stand up, more of their body is exposed, resulting in heat loss through air flow. Soakers can aid in more efficient heat loss.

However, when soakers are available in hot weather, cows’ behavior may change. “Cows are shifting where they’re standing,” said Van Os. “Instead of standing in stalls they’re standing at the bunk. We know that can be a risk factor for lameness.” Standing on concrete with soakers running can compound the effects of heat stress that leads to lameness.

Before adding water spray for cooling, Van Os suggested examining the existing mechanical ventilation system. “From a welfare perspective, we know high-speed air over the resting area is the key to promoting lying time,” she said. Multiple studies show decreased lying time in heat-stressed cows. “As the temperature-humidity index increases, we see a decrease in daily lying time.”

Van Os described misters and foggers as similar in that both use high water pressure. High pressure generates fine droplets that are injected into the air around the animal. The droplets evaporate and lower the temperature of the microclimate around the animal so they’re inhaling cooler air. This indirectly cools cattle and works best in lower humidity climates, because in high humidity, the air lacks the capacity to take on additional moisture. Misters alone are probably not the best option for the continental climate of the Northeast.

Soakers, also known as sprinklers or showers, distribute water through a low pressure (30 psi or lower) spray nozzle which delivers a variety of droplet sizes. The smaller droplets evaporate before landing on cows, functioning similarly to misters, creating a microclimate with a lower air temperature.

“The large, coarse droplets land directly on cows, which allows the energy from their body heat to evaporate, which provides direct cooling,” said Van Os. “This cooling effect can be enhanced when combined with high-speed air.” In open lot areas, wind is the source of high-speed air; in barns it’s mechanical.

The goal is to directly wet the cow’s back to soak through the hair coat to the skin but with no water running down her sides. “There’s a reason for this recommendation,” she said. “You want to wet the cow enough because evaporative heat loss or latent heat loss does not rely on a temperature gradient between the animal and the environment. It’s okay if the environment around the animal is hotter than her body temperature – the water will still evaporate.”

However, in terms of the evaporative mechanism, a lot of common wisdom comes from research that focuses on the evaporation that occurs after the water is turned off. “The idea is you only want to wet the cow ‘enough’ because it’s the evaporation after you spray her that matters,” said Van Os. “The second reason is people are worried that if there’s dripping water coming off a cow, it could lead to mastitis. But no studies have found a direct link between soaker use, dripping water and mastitis.”

She said the often-higher incidence of mastitis and somatic score in summer are not necessarily related to heat abatement systems.

Regarding water that drips from cows after wetting, Van Os cited studies showing that dripping water can help cool cows. Fluid convection occurs when water is cooler than the temperature of the cow, so as water drips from the cow’s body, excess heat is removed.

“After a single short spray, immediately we see a reduction in skin temperature and respiration rate,” said Van Os, “even while evaporation is still occurring. When there’s a long single session of cooling, we have a reduction in body temperature, and this is before the coat is dry.”

With soakers that cycle on and off, how often should cows be soaked? Studies show it takes about 15 minutes for the coat to dry, regardless of how much the coat is wetted. Another consideration is that cows dry faster in warmer temperatures or in high wind speeds. “In hotter conditions, we need to spray more frequently,” she said. “Spray every 15 minutes or more frequently to provide consistent cooling throughout the day.”

The goal with water cooling is to use water efficiently – a balance between helping cows cope with heat while using water responsibly. “Use enough water,” said Van Os. “If you don’t use enough water, water is wasted because it isn’t effective for cooling cows and is not an efficient use of that resource. After a certain point, adding more water doesn’t make the cow cooler in any one session.”

Van Os addressed the myth of not using small droplets: “There’s a popular belief that small droplets form an insulating barrier on the top of the hair coat that will trap heat and make heat stress worse,” she said. “It’s a misunderstanding of the fact that when droplets evaporate from the tips of the hairs, that’s indirect cooling, and not as effective as when cows are wetted all the way through to their skin surface which allows for heat transfer directly from their body.”

A study using nozzles with the same flow rate but different droplet sizes showed no difference in cooling as long as the system included low-pressure, large droplet soaker nozzles.

Water should be secondary to mechanical ventilation. Van Os said if all mechanical ventilation is good, consider supplemental cooling in the form of soakers at the feed bunk, in the home pen or holding pen or in combination.

“Soakers can be a very effective tool, but they are not enough to promote adequate resting time,” said Van Os. “From an animal welfare perspective, before I would install soakers, I would make sure mechanical ventilation is working properly over the stalls so cows have adequate resting time.”

by Sally Colby