Your Reading List

Dairy Corner: Twofold impact of heat stress

Reproduction setbacks may take longer to correct than milk production problems

dairy cattle in a pasture

This summer in the eastern Prairies seemed to be hotter and definitely more humid than in previous years. I noticed this year in particular had many August nights when the humidex barely fell below 25 C.

In talking to dairy producers, some had the same experience and also noticed their heat-stressed cows’ milk production as well as reproduction performance fell hard with this rising humidex. While I am confident milk production recovers as we approach cooler days, dairy reproduction often takes a little more time to bounce back and might be a significant problem for some dairy barns well into the fall. Fortunately, dairy producers can take some post-summer steps to finally get their cows bred.

That’s because failure to get a heat-stressed cow pregnant with a calf creates “holes” in even the best dairy breeding programs. The literature is littered with studies that show the effect of heat stress upon dairy conception rates and upon their unborn fetuses. Here are few cited examples:

  • University of Wisconsin (1999) showed conception rates among dairy cows fell steadily from 40 to 15 per cent as April spring weather progressed into heat in July. The researchers also found recorded conception rates steadily increased starting in August, but did not return to over 40 per cent until November of that year.
  • University of Barcelona (2008) analyzed 11,000 inseminations in four dairy herds over three years. They recorded the THI (re: a temperature-humidity index of above 72 is generally considered to cause heat stress in dairy cattle); seven days before and three days after insemination. Specific results showed that high THIs recorded three days before insemination yielded lower conception rates. Ovulation failure was nearly four times greater during hot weather compared to cooler periods. When temperatures fell below 20 C, conception rates increased.
  • University of Florida (2011) reported calves born to heat-stressed dairy cows had a higher death rate at birth (4.1 per cent versus zero per cent), higher rates of leaving the herd as replacement heifers (20.5 per cent versus 4.9 per cent) and even had a higher failure rate to complete a first lactation (57.1 per cent versus 77.8 per cent) compared to calves born to artificially cooled mothers.

Evidence in the first two citations is backed by the facts that dairy cows have two to three follicular waves during an estrus cycle and heat stress may retard growth and impair their function. It seems the number of follicles in heat-stressed cows does not vary, but less follicles are ovulated due to less essential hormones being released to cause ovulation. There is also earlier emergence of the dominant follicle (contains the egg or ovum), which causes further negative changes in ovarian and follicular function.

Other study dictates once the cow’s egg is fertilized, a developing embryo is still vulnerable to heat stress until it is 40 days old. That’s because a heat-stressed cow redistributes blood flow away from her body core, which includes her uterus carrying the embryo, in order to maintain a constant body temperature. As a result, if the uterus temperature rises to +40 C (normal body temperature is 38.6 C), dairy cow conception rates fall on average 10-15 per cent because of early embryonic death.

Take steps to reduce heat

In looking back to our own summer, the ramifications of such important heat stress evidence indicate one should have done everything possible to cool cows during heat stress. This means providing plenty of cool drinking water, reformulating dairy diets to improve dry matter feed intakes, and improving barn conditions through better sanitation, ventilation and possibly setting up a sprinkler system.

As the hot weather ends, it is important to make frequent barn walks. Look for any signs of active estrus in supposedly pregnant cows. Such observations (e.g scratched tail head, restlessness) are signals that cows bred during heat stress and could have not caught, suffered an early embryonic death, or even aborted later on. I also think dairy producers should review seasonal breeding records from years gone past. Some producers might find some of the same cows with chronic reproductive problems after a tough summer.

Kansas State University (KSU) suggests dairy producers go one step further and use estrous synchronization techniques on their cows during the summer and early fall for successful breeding program. KSU showed heat-stressed anestrous cows (cows that did not show visible signs of estrus) assigned to a timed scheduled breeding program had a 40 per cent improvement in pregnancy rate at 27 days after insemination.

Such success is promising for dairy producers struggling to get their heat stressed cows pregnant by 90-100 days post-partum, so they can maintain a desired 12.5- to 13-month calving interval. Failure to get fertile cows bred for one or two more cycles due to late summer heat disrupts the predictable flow of newborn calves and early lactation milk production.

Combating heat stress that impacts otherwise good dairy cow reproduction should not end once the days become cooler. The burning effects of our hot and humid weather upon dairy cow reproduction may be still be with us, so sound strategies should be put into place.

About the author


Peter Vitti is an independent livestock nutritionist and consultant based in Winnipeg. To reach him call 204-254-7497 or by email at [email protected]



Stories from our other publications