Thermal environment is the largest single stressor affecting the development, growth and reproduction of sheep, especially those managed in extensive pasture-based systems typical of Australia and other countries including China, India, Nigeria, Sudan and Iran. The greatest impacts of temperature on sheep performance and wellbeing occur above 25 – 31°C, when thermoregulatory mechanisms and thus homeothermy are seriously challenged, and the wellbeing and productivity of sheep is impaired. Due to seasonal variation in pasture availability and fertility, most Australian sheep flocks are mated between October and March (late spring to early autumn). However, this leads to critical stages of reproduction occurring during the hottest months, with mean daily maximums regularly approaching or exceeding 32°C in southern and sub-tropical regions. Consequently, it is extremely common for cycling and pregnant ewes, and rams, to be exposed to heat stress conditions.

A detailed review of available evidence from climate-controlled and field-based studies indicates that heat stress affects reproduction in multiple ways. Heat stress during the six to ten days surrounding mating reduces both fertility and fecundity of ewes, by shortening the duration of oestrus, impairing oocyte quality and reducing fertilisation rates and embryo survival, particularly during early stages of development. In two Australian studies, each utilising data from more than 50 Merino flocks, ewe fertility (% of ewes lambing) and lambing rate (lambs born per 100 ewes mated) correlated negatively with exposure to heat stress during the mating period. For each additional day ≥ 32.2°C during the week of mating, ewe fertility and lambing rate decreased by 2.7% and 3.5% respectively^.In rams, heat stress near mating also reduces spermatogenesis and sperm quality, with a reduction in fertility occurring between nine and 50 days after the heat event.