RiskWi$e: Some tools to examine the historical odds of the autumn break
Peter Hayman and Dane Thomas, SARDI Climate Applications 5 June 2025
Most farmers and their advisers are focussed on this season and will have limited interest in long term records. Nevertheless, some have asked how this season differs from the long term record and whether breaks are getting later.
Defining the autumn break using rainfall
There is no standard definition of the break of season, but a common definition is 25 mm in 3 days used by Pook et al. (2010) for Victoria and Smith (2021) for NSW. Some other commonly used definitions are 20 mm in 7 days used in some Agricultural Production System sIMulator (APSIM) modelling (Fletcher et al. 2020, Nidumolu et al. 2025). After consulting grain growers in southern cropping regions in 2021, the Bureau of Meteorology included 15 mm in 3 days and 25 mm in 3 days as part of the Forewarned is Forearmed suite of products available on their Climate Outlook website Overview—Summary - Climate Outlooks
Historically, the autumn break was primarily an opportunity for weed germination. As more options for in-crop weed control became available, the break became the start of sowing activity. Now, with dry sowing becoming widespread in South Australia, the break represents the date of crop germination. These changes in farming practices revise meaningful definitions of the break of season. The amount of rain required for germination and emergence of a dry sown crop is less than what was previously required as an opportunity to sow a crop or knockdown weeds and then sow a crop. After a long wait to the end of May in 2024, dry sown crops in SA emerged with less than 10 mm on light textured soils. This is a credit to the operational skill of growers with managing their farming system including setting up their sowing equipment and knowing their paddocks. Some paddocks with heavier soils did not respond to the end of May rain in 2024, which highlights the need for a range of rules depending on soil type.
