Using milk tanker pickup and weather station data to quantify the impacts of heat stress on milk production in Australia                                                                                                                        

Conference Proceedings

Dairy cows are more sensitive to heat stress than other livestock. As heatwaves become more frequent and severe, the impacts on milk production, farm income and milk supply are increasingly important.

Researchers from the University of Melbourne have estimated the sensitivity of milk production to heat stress on Australian farms. On-farm milk tanker collection includes recording data on volume and composition of milk. In this study, milk tanker data from 2000 to 2017 was paired with weather station data based on the postcode.

The temperature-humidity index (THI) is a measure of heat exposure that combines temperature and relative humidity. Daily, 3-day, 5-day, and 7-day minimum, average, and maximum THIs were calculated from the weather data. Linear mixed effect models were fitted to the data for three regions: Gippsland, Murray, and southeast Queensland (QLD) - northeast New South Wales (NSW). The THIs calculated over 7 days provided the best fit in all three regions for both milk volume (L) and milk solids (kg). This supports previous findings of lag effects of heat stress and impacts of prolonged heat. Daily minimum THI averaged over 7 days was the best performing THI metric for volume at all sites and for milk solids in southeast QLD - northeast NSW. The best performing THI metric for milk solids in the Gippsland and Murray regions was average THI over 7 days. The improved performance of  the minimum and average THI over the maximum THI suggests that cool night-time temperatures are important in alleviating the impacts of high daytime temperatures. Results suggest milk production is more sensitive to heat stress in Gippsland than the Murray region. This could be due to the sensitivity of cows in this relatively cool region, a lack of management interventions, or a combination of both.