Heat Stress: Impacts on Sheep Reproductive Performance with Dr William van Wettere

This week on the podcast we’re discussing heat stress during mating and pregnancy with Dr William van Wettere.

Heat stress is classed as anything over 32°C, which can disrupt fertility in ewes, semen quality in rams and embryonic development in lambs, so it has an overarching effect on all aspects of reproduction.

Mark and Will start by discussing the effects of heat stress on ewes. Trials show that heat stress affects ewes most in the five days leading up to ovulation and the first five days after ovulation; while heat stress after day eight of ovulation did not affect fertility. Heat stress also reduces the duration of oestrus, with the impact depending on the timing of the heat stress. Trials show that heat stress can shorten the length of oestrus by five to eight hours. Heat stress can also impact the cycle length, lengthening it by nearly two days in some cases.

One trial showed that for every additional day above 32°C, lambing percentages decreased by around 3.5%. A lot of this is comes down to disruption of the ewe’s follicle development. Because of this, and poor placental development, lamb birth weight and survivability can be affected. Heat stress at different times throughout the pregnancy can also have an effect. 

Mark and Will also discuss the impact on ram fertility, which can be affected at any time in the 60 days leading up to mating. Heat can cause not only a reduction in the amount of sperm but also cause issues with motility and along with structural abnormalities. 

Fortunately there are solutions to mitigate heat stress. Research has shown that sheep that have to look for feed are more likely to succumb to heat stress, so confinement feeding with adequate shade could be an option. Another solution is to increase the ram per ewe ratio. There is also research looking into the role of melatonin in mitigating heat stress effects. 

This is a great factual podcast that will arm you with knowledge about what goes on with your sheep when you’re mating during the peak of summer so that you can make a positive change for future matings. 

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