Heat stress is affecting summer reproduction at much lower THI than expected.

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According to a recent article written by Katie Kelly and Donna M. Amaral-Phillips, Ph.D. titled Effects of Heat Stress Dairy Cattle Reproduction.

Heat stress is having a significant impact on summer reproduction levels. Fertility begins to be affected at a THI of 50. Much lower than a THI of 68 when production begins to be affected. 

"Reproductive failure is one of the main reasons that dairy farmers cull dairy cattle. With lower reproductive performance, cows have longer days open, longer lactations, reduced milk production per day of life, and often produce less profit for their owners. Heat stress has a big effect on the cow's reproductive performance. While most farmers know that the effects of heat stress on milk production start at a temperature humidity index (THI) of 68, the effects on fertility can start at even lower THI with some scientists suggesting it may even start at a THI of 50. When heat stress occurs, conception rate can decrease by 53%. Understanding the causes for this decrease in fertility during heat stress and ways to decrease these effects are discussed in this article."

Heat stress affects the cow and her calf:

  • Estrus expression is reduced. During periods of heat stress, cows are less likely to show signs of estrus or heat, which is related to decreased amounts of blood hormones. Estrus events are shortened and not as intense as during the winter months. Without these visual signals, the farmer doesn't know that a cow should be bred and when a cow does not get semen in her, the cow cannot conceive a pregnancy.
  • Developing follicles and sperm. This decrease in hormones in the blood also prevents the normal development of ova. Without this proper amount of hormones, the cow will experience longer follicular waves and underdeveloped dominant follicles. Additionally, heat stress inhibits sperm development and reduces libido in bulls. Heat stress will cause lower sperm concentrations, lower mobility, and more deformities in the sperm. These service sire effects will decrease the conception rate in a natural serviced herd.
  • Embryos can be heat stressed too. An embryo in heat stressed cows can die within 17 days after fertilization because the early embryo does not have proteins that help it combat heat stress. When a cow is heat stressed, the cow's core body temperature rises. This increased heat inside the body is what affects the embryo and without heat shock proteins, the embryo will die, decreasing pregnancy rate. Heat stress also retards the growth of young follicle which results in the weakening of the signal that tells the cow that she is pregnant. Without this signal from the embryo, the cow will stop secreting the hormones needed to maintain pregnancy.
  • The fetus and heat stress. Not only does the cow have fertility issues with heat stress, but the developing fetus can also have fertility issues in the future. Calves from a dam that expressed high levels of heat stress when pregnant are more likely to require more services than calves whose dams had heat abatement systems. Additionally, these calves are born with lower birth and weaning weights and produced less milk in the first lactation."


This article shows us that Heat stress has a negative impact, not only on milk production but also on reproductive performance. It reduces the duration and strength of estrus and decreases embryo survivability. How can the effects of heat stress be minimized or even better be prevented? By installing a CowKühlerZ system. 

CowKühlerZ is a fully automated system that provides precision heat stress prevention by cooling and maintaining cows' core body temperature. Cows only sweat about 10% of what humans do. They cannot self-regulate their core body temperature the way humans can. As the THI rises, CowKühlerZ injects an ultra-fine droplet that acts as an imitation sweat. As the droplets land on her coat, her body heat and the airspeed from the KühlBlu fan evaporate it. The evaporation causes a chemical reaction to take place, which creates a chilling effect. The blood flowing close to the surface of her coat is chilled; the cooled blood is pumped to her core and works to maintain her core body temperature. 

The CowKühlerZ system works efficiently and effectively to keep her core body temperature optimized, avoiding heat stress and keeping her in her thermal comfort zone.

The system works automatically to adjust itself to the current climatic conditions in your barn. The temperature settings and timing of the droplets can easily be adjusted on the controllers to suit your geographical area. Around 10°C/ 50°F the fans come on at 30%; as the temperature rises, the fan speed increases. Once the temperature reaches somewhere around 26°C/ 78.8°F and the fan is running at 100%, the real difference between CowKühlerZ and "Just a Fan" becomes massive. A fan alone can no longer offer a cooling effect. It is just moving the hot air around. The intuitive CowKühlerZ controller monitors the temperature-humidity index (THI) and injects an ultra-fine timed mist, which acts like "imitation sweat" to maintain Core Body Temperature and keep the cows within their thermal comfort zone. 

Maintaining CBT and minimizing the threat of heat stress proactively with CowKühlerZ means that farmers can maintain reproduction levels throughout the summer. We have numerous customers that tell us that improved summer reproduction has had the most significant impact on their operation. If you would like more information or to speak to a customer, please fill in the contact form below, call 1-844-GET-KUHL or email nancy@kuhlerz.com

This article is based on ifomation from the previously mentioned publication. To  read the fullarticle https://afs.ca.uky.edu/content/effects-heat-stress-dairy-cattle-reproduction