With the heat of summer on tap, keeping cows cool will be a priority on dairy farms.

Veterinarian Dr. William Prokop, director of operations at Dairy Innovations LLC, discussed the combined use of moisture and fans to maintain cow comfort and production when temperatures rise.

Prokop described the temperature/humidity index (THI) as the threshold above which heat takes a toll and results in losses. The THI threshold of 68 for dairy cows doesn’t account for wind or solar radiation.

Cows are far more sensitive to heat than humans, who feel heat stress at around 80º F and 40% humidity. Prokop said a cow in such conditions is likely already experiencing heat stress.

“Her respiratory rate is about 75, she’s probably lost the equivalent of about six pounds of milk and her rectal temperature has gone up,” said Prokop. “While 68º is the number at which we have a problem, the warning starts at 65º. If we aren’t implementing strategies to mitigate heat stress, we’d better do it before it’s too late.”

Research has proven the most effective heat mitigation is obtained with a combination of water and fans. “If we add just fans, we get a slight reduction over time in respiratory rate as a surrogate for heat stress,” said Prokop. “That’s over 90 breaths per minute. Soaking for one minute, using 0.35 gallons of water every 15 minutes, drops the respiratory rate below 90, but it’s still fairly elevated.”

By soaking at 15-minute intervals along with fans, there’s further drop in respiratory rate and additional cooling.

By soaking every five minutes without fans, you achieve about 80% of the effect with fans, said Prokop. “We can’t always soak as often as we want and can’t always have as many fans as we want owing to availability of electricity. Soaking is a function of water availability and how we dispose of water.”

Water plus fans equals cool cows

The goal of efficient evaporative cooling is large droplets of water that completely soak the cow followed by airflow provided by fans. Photo by Sally Colby

As a cow’s temperature increases, she becomes more dependent on evaporation from her skin surface. “Respiratory evaporation can account for 25% of heat dissipation under the most extreme conditions,” Prokop said. “Heat dissipation from conduction radiation and convection is only about 10% or 15% at those extremes. That’s where evaporation from the skin can be a critical part of cooling.”

Nozzles deliver water but require adequate flow rate to be effective. Cows should be wet across the entire surface of their backs. “The more area we soak, the greater the evaporation and loss of heat from the skin,” said Prokop. “We want to spray the cow and get her soaked in about 20 to 30 seconds. In order for this to happen, we have to have check valves in the lines so the line never depressurizes – it always maintains water. Also, water should not be dispersed beyond the cows’ tails onto the ground.”

Ideally, the cow’s body evaporates 80% of the water that’s applied before re-soaking her. “That’s the optimum for getting the most cooling for a water application,” said Prokop. “We also want large droplets of water for the most effective soaking.”

In the presence of cross breezes that can lead to water drift, there’s a risk of wetting feed and subsequent secondary fermentation and spoilage. Sensors can be used to ensure soakers are turned on only when cows are present to receive water.

Two important considerations for fans include throw (how far air is projected) and the width of airflow. “We want a minimum amount of airflow, otherwise we won’t get effective evaporation,” said Prokop. “We’re only going to get cooling if we move enough air to get evaporation.”

Controller valves on soakers should be set to complete soaking in 30 seconds, with an “off” interval allowed for the amount of time it takes for 80% of water to evaporate from the skin – a function of how much air is moving over the cow.

“Evaporation duration decreases as the air velocity increases,” said Prokop. “The more wet/dry cycles per hour, the more heat transfer away from the cow.”

In the holding area, where cooling is critical, cows should be walking into the airflow as it moves away from the parlor. “It doesn’t matter which way the prevailing winds are,” said Prokop. “You want hot, humid air from the holding area moving away from the parlor.”

Large water droplets for cooling in the holding area help minimize excess drift. Coverage should be uniform, with check valves to keep the lines full to eliminate on/off time. Timers should turn off water as soon as cows are soaked. Without high velocity airflow in the holding area, the only result is excess humidity.

Return lane soakers are a tremendous asset for heat mitigation whether they’re homemade or commercial units. “The important part is the top-down application and, if possible, the spray should come on when the cow’s head is past the first spray jet because cows don’t like being sprayed on the head,” said Prokop. “Single lanes are most effective because the cow behind pushes the cow ahead of her. A wide lane soaker may result in a traffic jam because cows aren’t forced to move.” Return lane soaking should be designed to avoid moisture on udders.

Prokop summarized the goal of efficient evaporative cooling: to soak body surfaces as quickly as possible with large water droplets, which should be accomplished in 20 to 30 seconds. The more skin soaked, the better the cow is cooled. Once cows are soaked, increased airflow, either natural or mechanical, is key to aiding evaporation that results in cooling.

“Remember, the goal is to create a microclimate around the cow, not (change) the ambient temperature,” said Prokop. “We’re changing what she experiences, not what’s around her.”

by Sally Colby