The affects of high temperatures on livestock reproduction

By Dr Sue Street - Senior Land Services Officer, Livestock

With the hot weather well and truly upon us, it is worth keeping in mind that heat stress can affect reproductive performance in cattle and sheep. By understanding how heat affects reproductive performance, we can take management steps to limit the effect of heat on the reproductive ability of our stock.

Key points

Before and during joining, things to consider include:

  • Seasonal timing for joining in your enterprise – can I get better results if I join earlier or later?
  • Introduce more trees into the landscape;
  • Manage bulls and rams prior to joining – manage nutrition and carry out the five “T’s” 7 weeks before joining;
  • Male condition – ensure rams and bulls are in good condition and have a body condition score (BCS) 3.5 (rams) and fat score 3 (bulls);
  • Female condition – ensure ewes and cows are in good condition and are meeting their liveweight and condition/fat score targets (3 to 3.5 BCS for ewes and fat score 3 for cows);
  • Shade – ensure animals have adequate shade available;
  • Surveillance - continue to look out for any signs of ill health, especially excess panting; and
  • Water - ensure adequate, good clean water is available.

Reproductive performance is impacted in heat by a decrease in sperm production and changes in sperm morphology, decreased oocyte quality, and decreased embryo success at conception and implantation. Anecdotal evidence from across the Central West region in 2018/19 has shown that in some cases, pregnancy scanning rates were decreased, especially when average temperatures were above 38°C during the months of January and February.

What is heat stress?

Heat stress is when body temperature rises above the animals’ set point temperature. External temperature (ambient temperature, humidity, solar radiation and hot surfaces) and endogenous (digestion and cellular metabolic activity) heat production (Refshauge, 2020) contribute to body temperature increase.

How does heat stress effect reproductive performance?

Heat stress impacts reproductive performance both directly and indirectly. Heat stress directly effects the production of gonadotrophin-releasing hormone (GnRH), reducing levels of this hormone. Important for reproduction, this hormone is responsible for sperm production, sexual activity, the development of follicle stimulating hormone and the production of ovulation (Refshauge, 2020).

Heat stress indirectly effects reproduction by decreasing feed intake. Reductions in feed intake may result in animals not meeting their recommended nutrient requirements, causing them to go into a negative energy balance. This negative energy balance then causes stress on the body and reduces the GnRH reproductive hormone.

Heat stress tends to occur when temperatures exceed 32°C for long periods or during short periods of high temperatures (41°C +) and, in particular, when overnight temperatures exceed 26°C (Refshauge, 2013).

Rams/Bulls

High summer temperatures have been shown to decrease semen quality in both rams and bulls. Temperatures over 32°C for long periods or short bursts of very high temperatures (above 38°C) may affect sperm production in bulls and rams (Crawford, 2013). Bulls and rams that are heat stressed before joining could still successfully serve ewes and cows for a period of time, with sperm produced and stored prior to the heating event. This period would be approximately the first 2-3 weeks of the mating window. However, it will then take 7 weeks to produce new viable sperm (Crawford, 2013; Refshauge, 2013).

Ewes/Cows

In field studies, results indicate that high ambient temperatures (>32°C) during the mating period adversely affects ewe fertility (van Wettere et al., 2019). This includes the length of the oestrous cycle, duration of oestrus, quality of the ovum and its ability to fertilise as well as the survival of the embryo, particularly during the early stages of development (van Wettere et al., 2020). Heat is most destructive is when high temperatures (above 38°C) occur 3 days before, and 3 to 10 days after joining.

Like sheep, studies in cattle found that when subjected to heat stress, reproductive efficiency declined. This was due to a reduced duration and intensity of oestrus, altered follicular development and impaired embryo development (Lees et al., 2019). Conception rates were found to be influenced by a heat load event during the month preceding breeding, to two weeks following breeding (Morton et al., 2007) and was found to be more pronounced in Bos taurus cattle compared to Bos indicus cattle (Torres-Junior et al., 2008).

If you would like more information on joining in the heat, please contact your nearest Local Land Services Ag advisory team member https://www.lls.nsw.gov.au/regions/central-west/contact-us/services-and-staff-contacts

References and further reading

Ashwood A (2012) Fertility and infertility. Australian Brahman Breeders’ Association, June # Issue 175 [Online]. Available at: http://www.brahman.com.au/technical_information/reproduction/fertilityInfertility.html (verified 27 November 2019).

Crawford M (2013) Heat may affect ram fertility - Natural Resources Eyre Peninsula [Online]. Available at: https://www.naturalresources.sa.gov.au/eyrepeninsula/news/151217-heat-ram-productivity (verified 27 November 2019).

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