Dairy farmers know it’s a good idea to cool dry cows during periods of heat stress. Cooled dry cows simply calve in better and perform better after calving than uncooled dry cows.

But farmers might not be aware that heat stress can have a huge impact on unborn calves that reaches well into their adulthood. “Cooling is pretty important for the fetus,” says Geoff Dahl, a dairy physiologist with the University of Florida.

Unborn calves heat stressed dams will have lower birth and weaning weights, and might produce 2,500 to 3,000 lb less milk in their first lactation.

One study shows birth weights differ about 12 lb. Holstein calves from uncooled dams averaged 80 lb. while calves from cooled dams averaged a more normal, 92 lb. And that difference continues through to weaning, with calves from uncooled dams averaging 148 lb while calves from cooled dams average 172 lb.

The calves from uncooled dams did eventually catch up, and there was no difference in body weight at calving or at the end of the first lactation. But here’s the kicker: Calves from uncooled dams produced about 10 lb. of milk less per day through their first lactation than calves from cooled dams.

Cooling has a number of effects on dry cows. First, cooled dry cows eat more, or more precisely, their dry matter intakes are not as depressed as they approach calving as uncooled dry cows. The difference might be just three or four pounds per day, but that can have a huge effect on how many nutrients are available to the cow and her developing fetus.

Second, cooled dry cows have more IgGs to pass on to their calves in colostrum. Total IgGs can be 50% higher in calves from cooled dams, plus the apparent efficiency of IgG absorption is 15% higher.

Finally, heat stress also likely affects survivability, both short and long term. In one study, no calves died on arrival (DOA) when their dams were cooled but three of 44 calves from uncooled dams were DOA.

Of the calves from cooled dams, 85% grew to complete their first lactation. Only 66% of the calves from uncooled dams completed their first lactation.

Cooling dry cows

Cooling dry cows has a whole host of benefits to the cow herself, says Dahl. She eats more, she milk more, she stays healthier and she breeds back sooner.

As mentioned above, cooled dry cows will consume three to four more pounds of dry matter than uncooled cows. Heat stressed dry cows can lose body weight during the dry period. “Cooled dry cows lose more bodyweight early in lactation, but they have more body condition to mobilize in early lactation,” says Dahl.

Cooled dry cows also are propagating more epithelial cells in their mammary glands than uncooled cows. As a result, cooled dry cows have a greater capacity for milk production after calving.

Plus, cooled dry cows have much greater numbers of lymphocytes, which help them fight off disease challenges. Retained fetal membranes, mastitis and respiratory problems are all higher in cows that are dry during the summer versus those dry in winter. And, it almost goes without saying, reproductive performance also suffers.

It all translates to milk production. In the Florida study, cooled dry cows increased milk production 8 to 10 lb/cow/day over uncooled dry cows, even though all lactating cows were cooled once they calved in, says Dahl.

The study was also confirmed by nine other trials from across the country dating back to 1982, where the average increase was 8 to 10 lb of milk.

Dahl admits Florida is a literal hotbed of heat stress, where 70% of dry periods occur during hot weather. But states in the South, West and Midwest can experience heat stress 25% of the year or more when temperatures and humidity are combined. Even a state such as Pennsylvania can experience as much heat stress as Arizona because of the humid weather in the Northeast.

Just about every state experiences heat stress two months each year. As a consequence, cooling dry cows can pay benefits for just about every dairy in the country.