When it comes to heat abatement, most dairy farmers are likely to focus on their lactating cows. At the 2023 Pennsylvania Dairy Summit, Dr. Geoffrey Dahl from the University of Florida shared why this may not actually be the best method. He spoke about which cows to cool, when to cool and why cooling dry cows could have a positive economic impact.

Cooling methods and adverse effects of heat stress 

Dahl said the best way to keep cows cool is with a combination of fans and soakers. When it comes to soakers, the key is making sure cows are being soaked to the skin. He shared the results of a Kansas State University (KSU) study that determined fans alone have very little cooling power, especially when it comes to deterring heat stress. In fact, at 15-minute intervals, soakers alone had a greater effect than the fans did. But the real results came with the combination of the two. Dahl said the most effective cooling in the study was from soakers at five-minute intervals combined with fans. 

“We know what happens when lactating cows are heat stressed. They’re going to decrease their dry matter intake and decrease the amount of milk that they produce,” Dahl said.

Preventing heat stress is key to optimal milk production, but the use of soakers also uses a lot of water – which is a growing concern in many areas of the U.S. Because soakers at the feedbunk can lead to an increase in water loss, Dahl performed a study of feedbunk occupancy. He found that there were only cows under the soakers at the feedbunk 25%-30% of the time, leading to a high volume of wasted water. One method to reducing water waste could be utilizing systems that monitor when a cow is actually at the soaker – rather than just using a timer.

Dahl supported this point with data from a number of studies completed by KSU. These studies were completed at different phases of the lactation cycle. As expected, the cows with heat stress always decreased in milk yield. However, they discovered something else: the cows that were stressed during midlactation did not bounce back to normal production once they were cooled, but the early and late-lactation-stressed cows did.

What this means for heat stress and dry cows

After this study, Dahl posed some thought-provoking questions.

“What if they aren’t producing any milk at all? What about cows in their dry period? Will heat stress in these animals have any lingering effects on their next lactation? Will it have an impact on their calves, in utero, that are also experiencing heat stress?”

Dahl completed multiple studies to help understand how heat stress during the dry period impacts both the cow and her calf. During the studies, the cows were either heat stressed or cooled during the entirety of the dry period. Right off the bat, the cows that were cooled during the dry period had improved milk production once they began their lactation. Animals that were heat stressed had a reduction of 8 to 10 pounds per day.

In addition, the more severe the heat stress, the more milk production decreased. Dahl said this is due to a decrease in secretory epithelial cells in the mammary gland, which simply reduces the cow’s milk-making capacity. This fact also translates to springing heifers. Dahl saw a reduction in milk production of first-lactation animals that were heat stressed during the last two months of their gestation.

During a field study, Dahl also found there were adverse health effects for heat-stressed dry cows as they entered their lactation. The heat-stressed cows had increased mastitis, respiratory diseases and retained placentas when compared to the cooled cows. This is supported by research that he has completed regarding the immune system function of heat-stressed cows. They not only have acute effects on their immune status but also long-term ones. 

These negative effects can even carry into the reproductive system. Cows that were heat stressed during their dry period had an increase in days in milk to pregnancy of about one week.

So, do cows really need to be cooled for their entire dry period? After evaluating the data, Dahl said he believes so. One of his studies found that cows that were cooled during only part of their dry period still had adverse effects.

“Any heat stress during that dry period is going to have a negative impact,” Dahl added. “And in this case, if we heat stress them for half of the dry period, we’re going to see the same response as if they have been heat stressed the whole time.”

Impact for generations

Calves born to heat-stressed dams are often born at a lighter birthweight due to the shorter gestation length that heat-stressed dams experience. This smaller size continues through calfhood, as they are lighter at weaning compared to the calves of cooled dams. This can even carry past weaning age. A study completed by Dahl found that these heifers were smaller throughout the first year of their life.

Additionally, just like their mothers, calves of heat-stressed dams have an impaired immune status. These calves have a lower amount of circulating immunoglobulins (Ig), even after receiving colostrum equal to that of a cooled dam. This suggests their ability to absorb Ig from colostrum is impaired by heat stress. The cause of this decreased absorption is a more rapid closure of cells lining the intestine that usually allow for the absorption of Ig. However, as they close more quickly in heat-stressed calves, they are not able to absorb larger molecules such as Ig. As a result, there is a long-term effect on their immune system.

“You can’t manage your way out of that once the calf is born,” Dahl said. “The only way we can manage that is to get the dam cooled off during late gestation.”

This carries through to survivability as well. Dahl found that fewer heifers born to heat-stressed dams made it through their first lactation. While there are animals that do make it, they found they spend about a year less in the herd, on average, compared to calves with cooled dams. These poor survival rates are even passed on from these animals to the granddaughters of the cows that were heat stressed during late gestation. Dahl said they also have poorer survival rates.

Beyond survival, these calves can also experience a decrease in milk production not only in their first lactation but throughout their future. Dahl said in their first and second lactations, these calves made 5 pounds less per day than their counterparts with cooled dams. This gap then rises to about 10 pounds for future lactations.

The true economic impact

While the implications of heat stress on animal health and milk production are apparent, Dahl also shared the economic impact of heat stress.

“The total estimate is that the dairy industry loses about $1.5 billion for not cooling dry cows versus getting them cooled off,” he said.

In any area of the U.S. where there is potential for heat stress in two or more months throughout the year, Dahl said that cooling dry cows could have a significant economic impact on your bottom line.