Heat stress and its effects on reproduction in cows

Heat stress is perceived as a major factor contributing to low fertility of dairy cows inseminated in the late summer months. The decrease in conception rate during the hot season can range between 20-30% as compared to the results obtained in the winter months (Wolfenson et al., 2000; Rensis et al., 2003).

In recent years substantial increases in milk yield has further aggravated the summer infertility syndrome. This is due to increased metabolic rate and metabolic heat production. The upper ambient temperature limit at which lactating dairy cows can maintain a stable body temperature (upper critical temperature) is as low as 25-27ºC. The problem of heat stress is not confined to tropical regions of the world and imposes a considerable cost on the dairy industry.


There is a proven carry-over effect of the summer heat stress on the fertility performance in the autumn months (Wolfenson et al., 1997; 2002). This negative effect on reproduction persists over the first 1-2 autumn months even though the cows are no longer exposed to the heat stress. It is thought that this is a result of the effect of heat stress exerted during the hot months on antral follicles, which will develop into dominant follicles 40-50 days later (Roth et al., 2000; 2001; Wolfenson et al., 2002).

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