Heat Stress In Dairy Cows

Increased ambient temperatures & humidity during the recent heatwave can have a negative effect on cow fertility. We explain the affects of heat stress in dairy cows

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Increased ambient temperatures and increased humidity during the recent heatwave can have a negative effect on cow fertility.

Pregnancy rates may be reduced by up to 30% and these effects can last well into late Summer/Autumn as early follicles developing during times of excessive heat will be ovulating two months later.  These effects are mediated through the ‘stress’ region of the brain, the hypothalamus, which is directly linked to ovarian activity.

Effects Of Heat On Fertility

Increases temperatures can impact fertility in many ways:

  • Reduced expression of oestrus meaning that fewer cows are served.
  • Reduced appetite and dry matter intake (we often don’t feel hungry when it’s hot). This can lead to a longer and more severe period of negative energy balance after calving with a subsequent increase in the calving to conception interval.
  • Reduced follicular activity, together with poor egg and embryo quality.

And lets not forget the bull – excessive temperatures can have a detrimental effect on semen quality which will last for approximately 70 days.

Cow Cooling Strategies

Various systems have been tried to cool cows down including shade, fans, air conditioning and sprinkler systems.  These systems seem to have some beneficial effects on fertility however, they do not appear to improve it enough to reach pregnancy rates achieved in the winter.

Other strategies to try and improve fertility include:

  • Heat detection aids – these are useful when cows are out grazing anyway (note: they won’t help if cows aren’t actually expressing signs of oestrus at all though).
  • Provide a high quality forage throughout the summer to maximise dry matter intakes.
  • Hormonal strategies such as Ovsynch or CIDR program together with fixed time AI may help in cases where normal cyclicity after calving has not been resumed.

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