A growing number of research studies show that heat stress during the dry period impacts milk production in the subsequent lactation, and not in a good way. That means cow cooling during late gestation is valuable, but do cows need to be cooled the whole dry period to reap the benefits?

During the April Hoard’s Dairyman webinar, University of Florida professor Geoffrey Dahl shared results from a recent study that split dry cows into four treatment groups. One group was cooled the entire dry period. The second group had shade but no other heat abatement. The third group was cooled during the first half of the dry period and then not cooled the second half. The final group experienced heat stress the first half of the dry period and then was cooled for the second half.

Dahl said cow’s rectal temperatures and respiration rates responded just how they expected, with elevated temperatures and respiration in cows during periods of heat stress. What was not anticipated was the reduction in gestation length, and therefore the dry period, for the two switch groups as well as for the heat-stressed group.

This showed that heat stress at any time during the dry period has a negative impact on gestation and dry period length, which Dahl said is an initial indicator of issues with placental function. This affects both the calf and the cow.

As for milk production, Dahl said cows that were cooled the entire dry period made more milk than any other group. Cows that were heat stressed the full dry period made less milk, and both of the switch groups produced less milk as well.

“Our interpretation of that is any heat stress during the dry period is going to have a negative impact on animal productivity in that next lactation,” Dahl summarized. “From a management perspective, we need to cool animals the entire dry period to realize some of the benefits.” To learn more about the benefits of cooling, watch the Hoard’s Dairyman webinar titled “The long-term consequences of heat stress in dairy cattle.”