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Cooler Heifers Have Healthier Calves

Maureen Hanson

June 9, 2022

Intriguing research in recent years has shown that calves born to dams suffering heat stress during the dry period have lower passive transfer of immunity from colostrum, compared to calves born to cooled dams.

But what about first-calf heifers, who don’t technically have a “dry” period before calving? Historically, they have been perceived to be more heat-tolerant than older cows, but that might not be completely true, particularly for the calves they are carrying.

Geoffrey Dahl, Professor of Animal Science at the University of Florida and pioneer of some of that original, hallmark research on dry-period cooling, now has explored the impact of heat stress on first-calf heifers and their offspring.

Dahl and his team recently published the results of a study that examined if late-gestation heat stress in first-calf heifers had any impact on postpartum milk production or the immune status of their offspring. Graduate student Brittney Davidson led the study.

The researchers predicted that given the exponential growth of the mammary gland in late gestation – and the fact that first-calf heifers still are growing their own bodies during their first pregnancies – the additional toll of heat stress would be significant. And it turns out they were right.

In the study, 30 Holstein heifer in their first pregnancies were randomly assigned to two groups of 15 heifers each during the last 60 days of gestation. One group was cooled with soakers, fans, and shade, while the other only had shade. Both groups were housed in sand-bedded freestalls, and were combined as a single management group after calving.

The trial was conducted from June through November, in typical hot, humid Florida environmental conditions.

Prior to calving, the cooled heifers had significantly lower respiration rates a rectal temperatures, confirming that the cooling methods provided them with heat stress relief. After calving, the effects of cooling included:

  • Longer gestation length – The heifers with access to active cooling averaged nearly 4 days longer in pregnancy, which the researchers said was associated with subsequently higher milk production.

  • Higher milk production – Milk yield in the first 15 weeks of lactation was significantly higher for the cooled heifers. They yielded an average of 8.6 pounds per day more than the heat-stressed heifers. There was no significant difference in milk composition between the two groups.

  • Healthier offspring immune status – Calves born to cooled dams had higher IgG transfer via colostrum compared to the offspring of the heat-stressed heifers. The researchers noted this higher immune status indicated greater potential for survival of the calves from cooled dams.

Dahl noted that while the future milk production potential of heifer calves born to heat-stressed dams in this study is unknown, previous research in multiparous cows has shown that heat stress in-utero has resulted in significant reduction of offspring milk production when those heifer calves eventually enter lactation.

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