Develop a Plan to Minimize the Adverse Affect of Heat Stress on Reproduction

Even in Pennsylvania periods of high temperature decrease reproductive performance. It is real! 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 summer. In lactating cows, the critical limit when hyperthermia occurs is approximately 78°.

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 reduced 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. Alan Ealy, at Penn State, and his colleague Peter Hansen at 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%. Furthermore, during heat stress blood flow to the placenta is reduced. Florida researchers showed that heat stress during the last trimester of pregnancy effected placental development and calf weights were lower. Minimizing heat stress is also critical for dry cows.

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.
• Since high temperatures reduce the expression of estrus, consider implementing a timed breeding program.
• In anticipation of a prolonged period of hot weather, develop an “AI Blitz” program this spring so that a high percentage of cows eligible to be inseminated are bred before stressful conditions occur.
• Review feeding management practices with your nutritionist so that energy intake is not severely depressed and adequate levels of potassium and sodium are consumed.
  
• Since overcrowding can aggravate heat stress conditions, adjust animal density especially for 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 sevely 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. Discuss this plan with your nutritionist, veterinarian, AI consultant and employees before the hot weather arrives. BE PREPARED!

Michael O’Connor, Dairy and Animal Science Extension