Understanding the changing nature of heat stress in Australian feedlots

Summary

​The heat load index (HLI) and accumulated heat load units (AHLU) are calculated from weather data and used by the livestock industry to assess heat stress impacts on cattle. This work examined the changing nature of heat stress using these metrics both historically and in the future, for five major feedlot regions of Australia.  Changes in heat stress over the five decades from 1963 to 2012, as well as changes for future years 2030 and 2050 have been evaluated and are presented in this report.
The historical analyses show wide ranges in heat stress levels between regions, with central Queensland experiencing the most heat related stress (37 high risk days per year for sensitive cattle) and southwest WA experiencing the least (4.2 days per year). The scenarios for the future years vary considerably between the regions. The number of annual high heat stress days affecting highly sensitive cattle in central Queensland may increase 6-13% by 2030 and 16-19% by 2050. Southwest WA is projected to experience lower increases in high heat stress days (for highly susceptible cattle) with a 7-10% increase in 2030 but a 15-19% increase by 2050.
Various management options exist to reduce heat stress, or to enhance the cattle tolerance, and understanding the historical trends and likely future scenarios of heat stress will inform future feedlot management strategies.