How does heat stress affect sheep reproduction?
Case study: Effects of heat stress and predicted climate change on reproductive performance of the Australian sheep flock.
The thermal environment is the largest single stressor affecting the efficiency of sheep production, especially those managed in the pasture and pastoral based systems typical of Australia.
The problem
The thermoregulatory mechanisms of sheep are seriously challenged when temperatures rise above 25 to 31°C, and the physiological and behavioural adaptations that allow sheep to maintain homeothermy negatively impact growth, welfare and reproduction.
From late spring to early autumn (November to March), cycling and pregnant ewes, as well as working rams, are commonly exposed to ambient temperatures which challenge homeothermy, wellbeing and reproductive function.
Based primarily on observed and projected increases in temperature associated with anthropogenic climate change, exposure of the Australian sheep flock to thermal stress will occur more frequently and for longer periods.
The exercise
The vulnerability of the Australian sheep industry to the impacts of heat stress stimulated Meat and Livestock Australia to call for tenders for a review of the impacts of heat stress, and climate change scenarios on the reproductive performance of the Australian sheep flock.
This review was conducted as a collaboration involving scientists from a range of disciplines within the University of Adelaide and SARDI.
The outcomes
Figure 1A: Effect of heat stress around mating on ewe and ram fertility.
1. Heat stress and reproductive function
Ewes experiencing heat stress during the 6 to 10 days around the timing of joining experience reductions in oocyte quality, fertilisation and embryo survival, resulting in a reduction in the number of lambs born and the number of ewes lambing. Ram fertility is reduced during the 14 to 50 days following a heat stress incident. Heat stress during pregnancy reduces lamb birthweight by 0.6 – 1.4 kg, decreases the proportion of lambs born alive by 30% and causes approximately 25% more lambs to die after birth (These effects are summarised in Figures 1 and 2).
2. Impact of heat stress on the Australian Sheep Flock
Based on the sites investigated and their current seasonal joining patterns, it is estimated that 2.1 million potential lambs are lost due to heat stress under the current climate, with this number likely to increase to 2.5 and 3.3 million should a 1 °C or 3 °C increase in Australian temperatures occur.
The current national cost of heat stress is estimated to be $97 million annually (based on lambs not born / mated ewe) or $168m when the impacts of low birthweight on lamb survival were also considered (lamb price $6/kg)., which increases up to $166 million or $278 million annually in a climate scenario +3 °C.
Figure 2: Effect of heat stress during pregnancy on ewe and lamb reproduction.
3. Research priorities to protect the sheep industry from heat stress
- Establishing mitigation strategies which promote homeothermy and reproductive function of ewes and rams under thermal strain (i.e. nutrition, shade, wool cover, mating protocols)
- Understanding the impact of ambient conditions on the behaviour, resource use and fertility of sheep under typical production systems, and identify differences between individuals (behavioural, physiological, molecular) in their ability to thermoregulate and maintain reproduction under thermal strain.
- Modifying the environment to reduce the severity of thermal strain (use of shade, establishing cooler micro-climates, establishing THI thresholds, adoption of containment housing systems)
- Selection for physiological and behavioural adaptions which promote heat resilience without impairing productivity.
Contributors
University of Adelaide
- School of Animal and Veterinary Sciences
- Research Centre for Reproductive Health
- Faculty of Engineering, Computer and Mathematical Sciences
South Australian Research and Development Institute (SARDI)
- Dr Dave Kleemann
- Dr Alice Weaver
- Dr Jen Kelly
- Dr Simon Walker
- Dr Dane Coleman
- Dr Peter Hayman