It wouldn’t be a Wisconsin summer without heat and humidity. On the dairy farm, periods of heat stress necessitate special care to mitigate its adverse effects on calves, cows and people. The thermo-neutral zone, or the environmental temperatures when cattle don’t expend additional energy to stay warm or cool, ranges from 41 degrees to 77 degrees F. When the temperature is above this zone, cattle start to respond negatively.

University of Wisconsin-Extension has created a library of resources on managing heat stress on dairy farms, located online. 

Along with affecting milk production, heat stress impacts dairy cattle reproduction. Effects include decreased expression of estrus activity, reduced conception rates and an increase in double ovulation that can increase the incidence of twinning. Embryonic development can also be impaired during periods of heat stress, which has implications for the use of embryo transfer as a mitigation strategy.

New consequence of heat stress

We are now learning more about a new aspect of heat stress’s negative consequences on reproduction: potential effects on the developing fetus — specifically, the impact of heat stress on the fetus in late gestation coinciding with the dam’s dry period. Such effects drive us into epigenetics, which is the study of heritable alterations in phenotypes and gene expression that occur without changes in the DNA sequence.

In a 2020 Journal of Dairy Science paper, Laporta et al. studied the effect of actively cooling (shade, fans and water soakers) vs. shade only coinciding with late-gestation/46-day dry period in Florida. Resulting offspring were tracked through their first three lactations. Additionally, granddaughters of cooled and shade-only cows were identified and tracked for similar analysis.

Results found no statistically significant differences in stillbirth rates, percent of daughters surviving to weaning and percent surviving to first breeding. Reproductive performance was also similar between groups, as measured by average days to first breeding, age at first calving and conception risk as cows. However, survival to first calving did show a statistical tendency for a treatment difference, with daughters of cooled dams having a greater percentage remaining in the herd until first calving.

Where things get interesting are overall longevity and milk production. There was a remarkable difference between the daughters of cooled vs. shade-only dams. On average, daughters of cooled dams remained in the herd 356 days longer (birth to moment leaving the herd) than daughters of shade-only cows.

Productive life in the milking herd, as measured by the number of days between first calving and when the cow left the herd, was 4.9 months greater for the daughters of cooled dams.

Milk production differences were observed for the first three lactations, with the biggest differences showing in the third lactation. Milk production was compared for the first 35 weeks of each lactation. The difference was 4.8 pounds of milk per day in the first lactation, 5 pounds of milk per day in the second lactation and 14 pounds of milk per day in the third lactation. While there were fewer granddaughters than daughters with milk production records to analyze, indications are granddaughters of heat-stressed granddams may also experience a reduction in first-lactation milk yield.

We often analyze the payback on investments in cooling and cow comfort in the immediate lactation. Sometimes, this means dry cows are left out of the picture. However, knowing the potential impacts of late-gestation heat stress on resulting offspring and the dams coming into lactation can help herd managers make an informed decision on the economic impact of future investments in dry-cow cooling.

Sterry is the Extension agriculture agent in St. Croix County, Wis.