A research group from the UAB Faculty of Veterinary Medicine gives that summer heat stress is a main factor related to low conception rates in high producing dairy herds. The group studied the impact of several climate variables on conception rates in high producing dairy cows in northeastern Spain , by examining 10,964 inseminations.
The environmental temperature, radiant energy, relative humidity, and wind speed all contribute to the degree of heat stress. Heat stress may be defined as any combination of environmental variables that give rise to conditions that are higher than those of the temperature range of the animals thermal neutral zone. The temperature-humidity index (THI) incorporates the effects of both ambient temperature and relative humidity (RH) in an index. This index is widely used in hot areas worldwide to assess the impact of heat stress on humans and dairy cows.
We analyzed data derived from 10,964 inseminations in 4 herds during 3 years. Climate data such as daily mean and maximum temperature mean and minimum relative humidity, and rainfall were obtained from a meteorological station located less than 6 km away within the herds. Mean and maximum THI were calculated during 7 days before insemination to 3 days postinsemination. Fans are activated when temperature arise at 23-27šC.
Results indicated that high THI during 3 days before insemination, specially the Day 3 before, reduced fertility, probably due to ovulation failure. Ovulation failure was 3.9 times higher in cows inseminated during the warm period (May-September) compared to the cool period (October-April). THI on the day of insemination is also important for fertility, probably because heat stress could affect oocytes, spermatozoa and embryos.