Heat Stress Affecting Cows' Fertility, Study Finds

Since the mid 1980s, the reproductive efficiency of dairy herds has fallen sharply, despite advances in genetics and the management of herds. Many factors affect the fertility rate of cattle, and in this latest study, scientists from the Autonomous University of Barcelona in Spain studied the impact of heat stress on conception rates.

Heat stress occurs when the environmental temperature, radiant energy, humidity and wind speed create conditions that are hotter than those of the temperature range of the animal's thermal neutral zone. The scientists analysed data from over 10,000 inseminations from four herds over a three year period. Using climate data from a nearby weather station, they worked out levels of heat stress in the days immediately before and after each insemination.

They found that high levels of heat stress at key points reduced the likelihood of a successful conception. For example, high heat stress three days before insemination reduced fertility, probably due to ovulation failure. Ovulation failure was 3.9 times higher in cows inseminated during the warm period (May to September) than the cooler part of the year.

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