P.PSH.1292 - Investigating heat stress in ewes - reproductive performance

Summary

Extreme heat events increasingly threaten sheep productivity and welfare in Australia’s extensive grazing systems. While the impacts of heat stress on reproduction are well documented, almost all research has been conducted under controlled conditions, leaving a critical gap in understanding real-world paddock responses. This project addresses that gap by investigating how sheep use microclimatic resources, such as shade, vegetation, and topography, to mitigate heat stress during mating. Using advanced sensor technologies and environmental monitoring, the study identified key behavioural and physiological strategies that enhance thermal resilience. These insights inform practical management tools and breeding programs to improve welfare and reproductive performance under warming climates.

Objectives

The project addressed three objectives: 

1. Quantify the effects of heat events on sheep reproduction, thermoregulation, behaviour, and wellbeing through long-term data collection across diverse climatic conditions and production systems. 

2. Collect and integrate physiological, behavioural, and environmental data to provide a comprehensive understanding of heat stress impacts under real-world conditions.  

3. Inform on mitigation strategies to reduce heat-stress related reproductive losses, and allow for further avenues for investigation into management options.

Key findings

Merino sheep reproduction is strongly influenced by radiant heat load, interacting with timing of shearing relative to joining, shade availability, and adaptive behaviours that mitigate heat stress. Heat load also has lasting negative effects on ram sperm, which can be alleviated using long-acting melatonin. The project delivered robust GPS, rumen temperature, and accelerometer technologies, alongside advanced data pipelines and significant postgraduate and international researcher training.

Benefits to industry

The project demonstrates to the sheep industry that classical indicators of heat stress, such as THI, are not the most reliable index to predict the impact of head load on the welfare and reproduction of sheep. Our data strongly suggest that the next generation of heat load index should include a component related to the solar radiation load. Also, the time of shearing and the exposure to solar irradiance can impact head load and can be detrimental to reproduction, and access to shade when solar radiation is extreme can mitigate this effect. The negative effects of heat load on the rams can be mitigated by using antioxidants and pre-joining management. The project has also provided the industry with knowledge and novel technology that could be used to monitor or select animals that are better adapted to a hot climate.

MLA action

As part of the program of work ‘The impact of Shade and Shelter on sheep reproduction and welfare’, we presented at seven WALRC events, reaching more than 1,200 attendees and presented to SALRC, developed a project specific webpage that on average had 63 views per month, produced a literature review that was published in Animal Production Science, featured on Landline, and regularly had articles published in the Countryman and Farm weekly as well as two articles published in the MLA Feedback Magazine plus articles in the MLA newsletter. The project had a high profile on Twitter and also had radio interviews and podcasts.

Future research

Further research is needed to clarify the impact of heat-load on sheep reproduction, adaptation, and resilience. Proposed studies include multi-year time of shearing trials which would be highly suitable for an integrated research PDS. The trial needs to run for at least two years because of the inherent variability in weather; we need variability in the number of cloudy days during joining, and similarly on the number of hot days joining, to dissect the effect of each on the heat load of the animals and their reproductive outcomes. An experiment to investigate the impact of radiation load on the body temperature, rumen temperature, and skin temperature of Merino sheep, shorn and unshorn should also be conducted. A series of controlled experiments on commercial or experimental farms to test the effect of the history of heat load and the physiological response and reproductive responses of rams to heat waves. Our data suggest that it is necessary to characterise the conditions that are conducive to the adaptation of sheep to heat load over multiple reproductive cycles in commercial settings. Importantly, an extension of the present project would be an investigation of the genetic and epigenetic basis of adaptation to heat load in sheep using the blood database that was collected during this project. Our project has raised questions on the relevance of the current heat load index; it becomes essential to conduct a critical review of the heat load indices to define the most biologically relevant index. To further our understanding of the capacity of sheep to respond and adapt to heat waves, it will be necessary to investigate the utilisation of shrubs and/or supplementation with antioxidants to improve the resilience of sheep production systems and promote carbon sequestration. Lastly, an investigation into the role of the microbiome would help to integrate the physiological adaptation of sheep to heat load, the reproductive performance and the greenhouse gas emissions.