A review of the impact of heat stress on reproductive performance in sheep - Stage 1 &2

Summary

Thermal environment is the largest single stressor affecting the development, growth and reproduction of sheep, especially those managed in the extensive pasture and pastoral based systems typical of Australia. 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. As such, the purpose of this review was two-fold:

1. To determine the impact of heat stress on ewe and ram fertility, as well as the ability of ewes to rear a lamb.
2. To identify to what extent the reproductive performance of the national sheep flock is affected by heat stress under current climatic conditions and to what extent it will be affected in the future should temperatures rise by 1°C or 3°C relative to a recent climatological baseline.

The project team utilised Australian Gridded Climate Data, in conjunction with management data, from 26 NRM sites across Australia, to determine the impact of heat stress on reproductive performance. Key findings from the review include:

• Ewes experiencing heat stress during the 6 – 10 days around the time of joining experience reductions in oocyte quality, fertilisation and embryo survival.
• Ram fertility reduced during the 9 – 50 days following a heat stress incident.
• The number of ewes lambing and the number of lambs born per 100 ewes joined correlated negatively with the number of days per week during the mating period when ambient temperatures were ≥ 32.2 °C.
• 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.
• Heat stress during gestation is likely to retard conceptus development, particularly for flocks mated in spring/summer, with the potential to reduce lamb birthweights by 0.6 – 1.4 kg, and decrease survival to weaning by 20 – 30%.
• The current national cost of heat stress is estimated to be between $97 - $168 million annually increasing to $166 - $278 million annually at +3oc; based on a lamb price of $6/kg.
• The reduction in profit per ewe in the national flock due to heat stress averages varies considerably by region, from $0 - $15.50 under a current climate.
• It is estimated that 40% of the flock has a cost of $1/ewe or less and another 40% of the flock has a cost of $10/ewe or more.

The need to develop effective, and commercially adoptable strategies to alleviate the impacts of heat stress on the Australian sheep flock is clear. Based on this need, the project team in consultation with a producer/consultant focus group have developed four key research priority areas which been ranked in order of urgency, practicality and likely time to outcome delivery. These are:

• 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.