When the weather is warm, cows need to be cool. A variety of factors contribute to overheating, and it takes a combination of measures to help cows dissipate heat.

Dr. Tom Tylutki, president, Agricultural Modeling and Training Systems LLC, began a presentation on heat stress by explaining that cows inherently have several natural heat sources including fermentation, metabolic heat production and internal insulation such as fat and muscle.

Cows also have external insulation, which Tylutki said changes how we think about wetting cows and cooling systems. “It isn’t really the hair coat that’s insulation,” he said. “It’s the dead air space that’s captured within the hair coat and skin surface. With a thick hair coat, there’s a lot of dead air space for a high insulation level.”

If an animal with a hair coat becomes wet, dead air space is disrupted and water goes directly to the skin. Evaporative cooling allows more heat loss because water is going from a liquid to a gas. There’s some evaporative heat loss from sweat, but most evaporative heat loss is from respiration.

While most humans are comfortable in temperatures up to 80º, cows begin to feel discomfort at around 68º due to their greater mass, larger surface area and heat production. In the past, farmers initiated heat stress mitigation at 70º; now they begin taking measures to manage heat stress at around 65º.

Cows under heat stress have lower feed intake, lower milk volume, lower milk fat, lower reproductive efficiency and reduced immune function.

“We see sole ulcers start to increase about two months after heat stress starts,” said Tylutki. “There are always more foot problems – one reason is the cow being unable to cool herself so there are probably some metabolic shifts that make them more susceptible. We know cows stand more under heat stress, and previous temperature impacts heat metabolism.”

Animals can withstand short-term (three to four days) heat stress if the humidity is low and nights are cool. “This explains why cows in high deserts do well,” he said. “The cow soaks up heat during the day, and when it cools down at night, her body can dissipate the heat and cool down.” However, if nights are not cool, animals crash because they can’t dissipate heat.

Citing robotic system data, Tylutki said robot visits and milk production drop during heat stress with a two-day lag from when heat stress was initiated. He said this is due to cows not being able to dissipate heat in addition to the several days it takes to heat the concrete mass of buildings.

“This is why in the longer term, when it cools down, animals don’t start to act more normal until three to seven days after the heat stress period ends,” said Tylutki. “It takes that long for buildings to cool down and become a more comfortable zone for living.”

In longer term heat stress conditions, controlling moisture (humidity) is a major factor. “Air movement is critical,” said Tylutki. “In the housing area, we need 40 to 60 air changes per hour through natural ventilation or fans. Placement of fans and sprinklers becomes critical.”

When cows lie down, their respiration rate and body temperature rise. Cows often choose to not lie down during heat stress because they get hotter – their core temperature increases about one-half degree for every hour they’re lying down.

“We want them to lie down but they’re going to get hotter,” said Tylutki. “There’s more surface area touching where they’re lying and they can’t dissipate heat.”

Respiration is a factor in heat stress, and begins at greater than 60 breaths/minute as the cow rids her body of heat and moisture. “She may exhale greater than 15 liters per day,” said Tylutki. “That’s a lot of water.”

The science behind heat stress

Fans should be properly placed, clean and fully functioning to provide adequate cow cooling. Photo by Sally Colby

Water trough availability and cleanliness is critical, along with fans. Fans must be fully functioning and clean for effective cooling. “You can lose 30% to 60% of airflow if the fan screen shroud is dirty,” he said. “For soakers, are settings correct? Are they angled correctly and is there adequate water flow? If animals will be outside, do they have shade?”

Cows often bunch in barns during heat stress. Bunching can be due to flies, but on farms with good fly control, cows often continue to bunch.

“Watch where cows go,” said Tylutki. “They’re getting away from bright light because they sense bright light, which equals sunlight, which equals solar radiated heat gain. They’re trying to find shade to minimize that.”

During heat stress, cows eat less. Some dairy farmers increase concentrate feeding to help manage metabolic changes due to heat stress, but Tylutki said that strategy doesn’t work well. “You may get a bit of production response,” he said, “but there’s still a drop in intake and an increase in heat load.”

Cows under heat stress often calve two to six days earlier with smaller calves. Early research shows calves born to heat-stressed dry cows grew less and yielded less milk in their first and second lactations.

“Being exposed in utero to heat stress has a lifelong negative response on productivity,” said Tylutki. “Cooling dry cows has a (positive) long-term effect.”

One potential metabolic issue during excessive heat is increased risk for sub-acute rumen acidosis (SARA). The rumen is generating heat, feed intake is lower and feed passage rate is lower. Sodium and potassium levels are lower overall and in the rumen because animals are sweating. These factors lead to increased risk for SARA, so increasing concentrate isn’t helpful.

“If we decrease forage levels, we have to keep NDF levels where they belong,” said Tylutki. “We have to do what it takes to keep the rumen healthy, otherwise cows are at risk for acidosis. It’s much more important to maintain a healthy rumen during heat stress. She’s battling metabolic insults, and if we throw on another related to SARA, it’s a case of dead cow. We have to be thoughtful about maintaining rumen health and at the same time, implement good cow cooling strategies.”

Tylutki advised dairy farmers to not go overboard with heat stress diets. “We need to follow good nutritional strategies as part of the heat mitigation plan,” he said, adding that improving fiber digestibility is one easy measure.

“The most important part of dealing with heat stress is farm management. Farmers have to be willing to invest in appropriate cooling systems, try to not overcrowd cows in heat-stressed areas and pay attention to details to keep girls eating, milking, breeding and surviving.”

by Sally Colby