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