The immediate effects of heat stress are noticeable; a drop in milk output or a reduction in feed intake are measurable. The more researchers study heat stress, the more we learn about other consequences we might not see right away that can affect dairy cattle over the long run.

Really, damage from heat stress starts in the womb. Years ago, research showed that calves born to heat-stressed dams weighed less at birth compared to calves born to cooled dams. As time goes on, we are discovering more problems that arise when calves are exposed to heat stress before birth, and Geoffrey Dahl, a professor at the University of Florida, detailed some of this research during the April Hoard’s Dairyman webinar.

One study found that calves born to heat-stressed dams had lower immunoglobulin concentrations compared to calves from cooled dams, and this deficiency persisted through the first month of life. These calves were fed colostrum shortly after birth that was collected from their own dams. The immunoglobulin (IgG) level in the colostrum from heat-stressed cows versus cooled cows did not differ, which led Dahl and his team to believe that the apparent efficiency of IgG absorption was reduced in calves born to heat-stressed cows.

In two follow-up studies, calves were born to heat-stressed dams and cooled dams. All calves received quality colostrum from another source, not their own dams. Meanwhile, the colostrum collected from the heat-stressed and cooled cows was fed to another group of calves born under cooled conditions.

What they found again was that calves born to heat-stressed dams had lower immunoglobulin uptake relative to calves born to a cooled dam. Despite receiving IgG from the same source, something influenced the capacity of calves experiencing in utero heat stress to take up the IgG in the colostrum they were fed, Dahl noted. On the other hand, there was no difference in absorption efficiency in the calves fed colostrum from cooled cows compared to the heat-stressed cows.

So, when it comes to the ability to absorb IgG after birth, “This is an effect on the calf itself,” Dahl shared. Other studies have shown that, in addition to a lower birth weight, calves born to heat-stressed dams are lighter and shorter at weaning.

Looking further down the road, Dahl pointed to data showing dramatic reductions in first lactation milk yield in animals that had been heat stressed in utero compared to animals that were cooled prior to birth. These heifers had been managed the same way after birth, fed the same diets, and born the same time of year. And yet, this difference in first lactation milk yield occurred.

What’s more, this reduction in milk production became more apparent in the second and third lactations. Dahl explained that animals heat stressed in utero do not remain in the herd as long, either. This reduction in longevity carries on to the granddaughters of cows that were not cooled when in utero.

Dry cow heat abatement — or a lack thereof — can have long-lasting, detrimental effects on a herd. To learn more about the research described here, watch the Hoard’s Dairyman webinar titled “The long-term consequences of heat stress in dairy cattle.”