Development of a heat load index and risk map for grazing sheep

ABSTRACT

A total of 5500 measurements of the respiration rate of 123 sheep with and without access to shade were obtained on 40 days in the summers (January–April) of 2022(Year 1) and 2023(Year 2). Models to predict respiration rate (breaths per minute (BPM)) based on temperature, humidity, wind speed, and solar radiation were calibrated to the Year 1 unshaded dataset (R²=0.35, RMSE=39 BPM) and validated with the Year 2 unshaded dataset (R²=0.58, RMSE=40 BPM). Respiration rate increased 6 BPM for each 1°C increase in temperature. The shade treatment decreased respiration rate by 24 BPM and the shaded model for respiration rate had a validation RMSE of 47 BPM. The between sheep variability in respiration rate was described by a standard deviation of 18.9 BPM. The probability of open mouth and tongue out behaviours both increased linearly from 200 BPM. We constructed a daily Heat Load Index based on the cumulative respiration rate greater than 200 BPM. Climate data and sheep density information were used to construct a heat stress risk map for New Zealand from October 2018 to March 2022. Heat stress risk was concentrated in January–February and was primarily located on the east coast of the North Island.

KEYWORDS:

• Sheep
• respiration rate
• temperature
• shade
• heat load index
• risk map

Disclosure statement

No potential conflict of interest was reported by the author(s).

Acknowledgements

We also gratefully acknowledge the New Zealand Institute of Water and Atmospheric Research (NIWA) and Trevor Carey-Smith (NIWA) who provided the Virtual Climate Station Network (VCSN) data for this project.

Additional information

Funding

This work was funded by the New Zealand Ministry for Primary Industries.