Adapted by Dr. Anna Denicol - UC Davis, Department of Animal Science
With high temperatures averaging 98-103°F during July (2018) across the Central Valley and other areas of California, the topic of heat stress (and all the problems it brings to dairy cows) is again at the center of every conversation. As temperatures go up, we watch reproductive efficiency of our herds go way down, with strategies for heat abatement only partially taking care of the problem.
Although the exact numbers are different in every dairy, studies looking at the effect of season on fertility found that compared to winter, there is about a 50% drop in fertility during summer and a slow recovery during the fall months until “normal” fertility returns the following winter. Infertility during heat stress is multifactorial and affects multiple parts of the reproductive tract and other body systems. Direct effects on reproduction include altered secretion of estradiol and progesterone and decreased quality of the pre-ovulatory follicle. However, these effects do not entirely explain why there is such a delay in the return of fertility. To better understand the long- term effects of heat stress on cow fertility, we designed a study to evaluate responses of the early stages of ovarian follicles to
heat stress.
The ovary of a cow (and females in general) is full of tiny follicles called primordial follicles that can only be seen under a microscope. These follicles must grow into primary and secondary follicles before turning into antral follicles that respond to synchronization injections for ovulation and insemination. It takes around 90-100 days for a primordial follicle to grow to the antral stage, which means that when a cow is inseminated, the egg, that will hopefully generate a calf, began growing three months prior. We isolated primary and secondary follicles from cow ovaries and individually cultured them in the laboratory under two conditions: control (seven days at 101.3°F) or heat stress (101.3°F for 16h and 105.8°F for 8h, daily, for seven days). These temperatures and times were chosen to mimic what cows may experience during one week of high air temperatures. Exposure to heat stress decreased the proportion of viable follicles by 10%, and growth rate of heat-stressed follicles was 50% lower than control’s growth rate; moreover, there were fewer follicles actively growing in the heat-stressed group after one week. Interestingly, primary (smaller) follicles seemed to be more resistant to heat stress compared to secondary (larger) follicles. Even more interesting, some follicles were able to remain viable and grow under heat stress very similarly to the ones cultured under normal temperature. At this point, we don’t know whether these follicles would have the necessary quality to generate a pregnancy.
We concluded from this study that heat stress impairs early growth of ovarian follicles, which contributes to the delayed return to fertility. One of our current goals is to find strategies to protect early ovarian follicles from the effects of heat stress, so fertility can be recovered faster after the hot season ends. Another avenue, however, that will likely give us a more permanent solution, is to investigate what makes some follicles more resistant than others. We are investigating these “heat tolerant” follicles in search for cellular and genetic adaptations that allow for maintenance of growth and viability during heat stress.
Link to PDF Version: Long-Term Impact of Heat Stress on Reproduction of Dairy Cows