van Wettere, WilliamLeu, StephanLowe, Alyce (formerly Swinbourne, SARDI)Lewis Baida, Bobbie Emilia2025-07-222025-07-222025https://hdl.handle.net/2440/146225Heat stress presents a major challenge to the sustainability of the Australian sheep industry. Although controlled studies have demonstrated its adverse effects on the fertility of ewes and rams, as well as on early embryonic and fetal development, the impact of heat stress under extensive grazing conditions remains poorly understood. Furthermore, the role of individual variation in thermal thresholds and its influence on reproductive performance and productivity has not yet been defined. This thesis aimed to quantify heat stress at the individual animal level and investigate its relationship with reproductive outcomes and behaviour in grazing sheep. The objectives of the first two chapters (Chapters 1 and 2) were to: (1) review the effects of heat stress on sheep productivity, and (2) evaluate technologies for the automated collection of heat stress data in sheep. Chapter 3 aimed to design and validate a minimally invasive, indwelling, and automated method for monitoring core body temperature in ewes. Vaginal temperature proved to be a reliable indicator of true core temperature under intensive conditions, showing a sound correlation with rectal temperature. This study also demonstrated that the handling and restraint required for manual rectal measurements induced stress-related hyperthermia, potentially confounding subsequent assessments of core temperature. Chapter 4 classified ewes based on individual thermal thresholds under grazing conditions and evaluated associations between heat stress responses, reproductive outcomes, behaviour, and lamb growth. Continuous vaginal temperature measurements showed greater repeatability during hot conditions, particularly among high-responding ewes. High-responding individuals exceeded their normal temperature range at lower ambient temperatures, travelled greater distances, and were in lower body condition during early gestation. Although there were no significant differences in measures of reproductive output between groups, these findings highlights the complexity of assessing heat stress effects under extensive field conditions, where multiple environmental and animal-level factors interact. These findings reinforce the challenges of detecting clear physiological signals in real-world settings, even when using continuous, high-resolution temperature data. The final study (Chapter 5) examined axillary and scrotal thermoregulation in rams and its association with male fertility traits under grazing conditions. This study assessed the capacity of juvenile Merino rams to maintain thermal homeostasis during summer and explored relationships with semen quality, as well as maternal thermal tolerance. Rams with superior scrotal thermoregulation displayed some improved semen characteristics, and maternal heat tolerance was associated with enhanced thermoregulatory capacity in offspring. Consistent with findings in ewes, repeatability of both scrotal and axillary temperature was higher under hot conditions and significantly greater in high-responding individuals. In conclusion, low and high-responding animals can be identified under grazing conditions with sound repeatability. Individual ability to regulate body temperature under heat load has partial flow-on effects to behavioural decisions and reproductive outcomes. Additionally, some transgenerational patterns indicate the potential in incorporating thermoregulatory traits into breeding programs and developing management strategies to enhance heat resilience in sheep, thereby supporting the industry's sustainability amid changing climatic condition. Future research should further refine individual temperature thresholds, exploring genetic resilience, and leverage long-term sensor data to inform adaptive management of heat stress.enHeat stressfertilitythermoregulationphysiologybehaviourovinesheepramsewestemperatureThesis