Whether you dairy in Texas, Minnesota, New York, California, or somewhere in between, it’s been one long, hot summer.
A growing body of research shows that heat stress not only affects cows. It also can impact the fetuses being carried by close-up dry cows. Two studies recently published in the Journal of Dairy Science explore the impact of heat stress on the babies-on-board of near-term cows, and possibly even the next generation.
- Effects of heat stress on birth weight, and comparison of environmental versus genetic parameters. This German study evaluated data from 171,221 Holstein cows to assess the effects of heat stress on calf birthweight. Specifically, the researchers sought to determine whether phenotypic responses or genetic breeding for heat tolerance were transmitted more prominently via calf birth weight when dams were under heat stress in their last 8 weeks of pregnancy.
Prenatal heat stress – measured via daily temperature-humidity indices (THI) -- reduced calf birth weight by a range of .66 to 1.38 pounds per calf. The researchers concluded this decline in birth weight was most likely caused by maternal permanent environmental effects versus genetics.
As a result, they recommended that dry-cow cooling efforts are more important in protecting fetal growth than using genetic evaluations for heat tolerance. But they also noted that the overall impact of maternal heat stress on fetal birth weight was relatively small from a practical perspective.
- Changes in skin and hair characteristics in offspring of heat-stressed dams. This study, conducted at the University of Wisconsin-Madison, looked at the influence of heat stress in the pregnant dam on skin and hair development of the offspring.
They noted that the fetal germline in the developing reproductive organs of the fetus (F1) will eventually give rise to the next generation (F2), and also experience heat-stress insult. Thus, heat stress has the potential to impact the dam (F0), the fetus she is carrying (F1), and the eventual offspring of that fetus (F2).
Looking specifically at hair and skin adaptations triggered by in-utero heat stress, the researchers noted that heat stress in the last 56 days of gestation did, indeed, influence skin and hair characteristics in the granddaughters of those dams. The F2 offspring of heat-stressed dams had shorter and thicker hair, thinner skin, and more but smaller sebaceous glands.
Similar skin and hair adaptations in other animals species have been shown to aid in dissipating heat from the skin’s surface. Whether the same effect will occur in cattle remains to be investigated.
