Humans feel comfortable when the air temperature hovers around 22°C/71°F and the relative humidity sits at just 50%, that weather equals a temperature-humidity index (THI) of 68. (THI is a combination of temperature and humidity.) Until recently, a 72 THI was considered the point when heat and humidity began to stress dairy cows. University of Arizona researchers have found a 68 THI is a better indicator.
According to research done by the University of Kansas and published in an OMAFRA factsheet Jan14/2020
Kansas State University researchers looked at the effectiveness of cooling with sprinklers and fans. They measured cow respiration rates and core body temperature for eight different cooling strategies.
The average air temperature during the research was 31°C/87°F at 57% relative humidity. The THI during the study was 80, similar to what is typically experienced during Ontario summers. The London area, for example, averages 58 to 60% relative humidity at 4 p.m. during July and August, while the Ottawa area averages 53 to 55%.
With no soaking or fan, the cow respiration rate stayed around 100 bpm, and core body temperature rose slightly to 40°C/104°F. Using fans without sprinklers reduced respiration to 95 bpm after 90 minutes, and body temperature declined slightly.
As researchers decreased the interval between sprinklers use, they saw greater reductions in core body temperatures and respiration rates. Using fans with sprinklers improved results further. Sprinklers running one minute out of every five with the fans running reduced the cows' core body temperature to 38.7°C and respiration rate to 50 bpm. Sprinkler systems wet a cow's hair coat on the back to the skin with water droplets. Fans blow air over the cow's body causing evaporative cooling on her skin and hair coat, and heat from her body causes moisture to evaporate. (This is the basic principle behind the CowKühlerZ fully automated system)
It doesn't take much of a heatwave to start stressing your herd, impacting high-production cows first. Having a well-ventilated barn provides the foundation for relieving the stress, and sprinkler or misting systems provide additional cooling relief to maximize cow comfort and production. CowKühlerZ combines a high-velocity fan, an intuitive controller and a fine timed mist to bring cool relief to your cows 24/7 effectively and efficiently.
To read the full article follow this link. http://www.omafra.gov.on.ca/english/livestock/dairy/facts/stressthres.htm