Reproduction is reported to be the most sensitive component of sheep production to heat stress. Reports from the 1940s suggest Australian sheep producers were aware that summer-mated flocks had lower reproduction rates, with lay observations reporting an association between high ambient temperatures and lower flock reproduction. Recent anecdotal evidence of low pregnancy scanning percentages following mating during heat wave conditions supports these observations. However, large-scale scientifically robust on-farm data that describes the mechanisms and extent to which heat stress impacts sheep reproduction and subsequent farm profitability are lacking. This is a significant issue given the context of a rapidly changing Australian climate.
Potential drivers of reproductive failure from heat stress include alterations to behaviour and nutrition as the animal attempts to reduce its body temperature as well as direct effects on reproductive cells and tissues due to the increase in body temperature. The metabolic consequence of heat stress in the mammal is increased production of reactive oxygen species (ROS). When the production of oxygen radicals exceeds antioxidant capacity, damage is incurred. Sperm, oocytes, corpora lutea, early embryos, placental blood flow, neonatal survival and milk production are affected.
Little research has examined solutions for the seasonal problem of heat stress impairing sheep reproduction. As animals become more productive, their set-point body temperature increases, narrowing the gap between the sheep’s normally high body temperature and hyperthermia. Breeding for tolerance is not directly possible because there are no genetic correlations estimated between productivity traits and heat tolerance for Australian sheep. The industry currently operates independently of the potential effects of selection for production and a rapidly changing climate and a future offering more intense and frequent heat stress events.
This review examines the impacts of heat stress on sheep reproduction and discusses the modifications that need to be considered in the context of hotter and longer summers.