Minimize the Adverse Affect of the Dog Days of Summer on Reproduction

Posted: June 5, 2006

Dairy cows are at risk of becoming heat stressed more than most animals because with genetic selection for milk yield the cow has a high internal heat production.

This has presented the dairy producer with a constant challenge to maintain high
production and good reproductive performance during the dog days of summer. In lactating Holsteins, the critical limit when hyperthermia occurs is approximately 78°. Even in Pennsylvania periods of high temperature decrease reproductive performance. It is real!

Researchers at Penn State analyzed over 1 million insemination records from Pennsylvania cows serviced during 2000- 2004 by a Genex Cooperative, Inc. technician to determine the effect of ambient temperature on 90- day non-return rate. Non-return rates by month of insemination are presented in the table below. Non-return rates are expected to be inflated by approximately 15-20% relative to true conception rate because there is no way to identify cows that were culled after insemination, serviced by a herd bull, serviced by a non-Genex technician and several other factors. However, the relationship of temperature with 90-day non-return rate will be very similar to the relationship of temperature with conception rate. It is clear that conception rates in Pennsylvania are lower during the summer months, particularly July and August. The effects of heat stress are beginning to become evident as early as June and continue through September.

Heat and Insemination rate

Two major problems occur during hot weather. The duration of estrus and expression of estral signs decrease and there is a significant reduction in the number of cows that conceive and maintain pregnancy. Reduced rates of estrous detection are likely due to overall reduction in cow activity during warmer weather and lower circulating levels of estradiol. Estradiol from the follicles is important for the onset and intensity of estrus. Quite possibly during heat stress lower estrogen might reduce uterine blood flow and restrict the cooling mechanism for the uterus. Some researchers suggest that luteinizing hormone (LH) may also be reduced. This hormone is important in the process of ovulation, oocyte maturation and formation of the corpus luteum.

Most cells in the body produce Heat Shock Proteins in response to heat stress that limit the damaging effects of elevated temperature on cell function. Unfortunately in cattle, around the time of ovulation, the oocyte and /or the resulting early embryo are unable to produce such proteins. Consequently, embryo viability is compromised resulting in lower conception rates. Collaborative research from Penn State and the University of Florida demonstrated that as embryos
develop beyond the first 48 hours they become more resistant to heat stress. Why is this important? It demonstrates that there is a narrow window of time
when embryos are very susceptible to hyperthermia. Most any effective heat abatement strategy should have a significant payback due to improved reproductive performance and maintenance of high milk production. For example, it has been shown that an increase in uterine temperature of only 1.0°F on the day of insemination will reduce conception rate approximately 12%.

In addition to implementing heat abatement procedures there are management strategies that can be implemented to improve reproductive performance during the summer. These include the following:

  • Observe for estrus more frequently and during the cooler periods of the day. Use heat detection aids.
  • Since high temperatures reduce the expression of estrus, consider implementing a timed breeding program.
  • Review feeding management practices with your nutritionist so that energy intake is not severely depressed and adequate levels of potassium and sodium are consumed.
  • Provide enough watering stations so water intake is optimized.
  • Since overcrowding can aggravate heat stress conditions, adjust animal density especially for cows in the early lactation and breeding groups.
  • An increase in the incidence of uterine infections, retained placenta and mastitis can be expected if precautions are not taken to provide clean, dry and well-ventilated transition cow and calving pens. Clean and bed these areas more frequently.
  • Bull management - dairy and beef producers using natural service should be aware that prolonged high temperatures can severely reduce the bull’s ability to maintain optimal testicular temperature. It only takes an increase of a few degrees in testicular temperature for several days to suppress sperm production and cause production of abnormal sperm cells. Since spermatogenesis is such a long process, the effect on pregnancy rate will not be seen immediately and may continue into the cooler months.

The key to minimizing the effect of heat stress on reproduction is to develop a plan now.

Michael O’Connor and Chad Dechow, Professor of Dairy Science and Asst. Professor of Dairy Cattle Genetics, Dairy and Animal Science Extension