Better Frost Decisions September 2022

Frost in spring of 2022 is best understood as a random process

Frost risk matters in every season, but it is difficult to imagine a season where the stakes are as high as 2022.

The extremely high input costs can only be offset if grain is harvested and sold for potentially good prices. Even urea prices of $1300/tonne can be profitable with wheat prices at $400/tonne, providing the 40 to 50% of the nitrogen fertiliser is recovered in the grain.

A badly frosted crop has low fertiliser recovery as the price received from the reduced yield won’t balance the high input costs no matter what happens to grain prices.

Table 1 (page 2) shows the number of nights colder than ≤2 degrees C in the last two years. We have used four sites with long term records so that we can get a sense of the ranking of the last two years.

The six months from May to October shows the rankings. Firstly May, June and July have been grouped and then the business end of the season that includes August, September and October. The ranking and the long term average is based on 66 winters and 65 springs for Yongala, Kyancutta and Mildura (since 1957) and 53 winters and 52 springs for Ararat (since 1970).

Yongala in the Upper North of South Australia is the coldest site followed by Ararat, Kyancutta and then Mildura.

Local experience will point out that Yongala is in an elevated valley and can be bitterly cold. As expected, there are more frosts in the three months of May, June and July than August, September and October. In 2021, at all four sites, May to July had fewer cold nights than average and this was most likely due to the very wet July last year.

The 11 cold nights at Ararat was less than half the average of 24 and the second lowest number since 1970.

Only Kyancutta on EP had more cold nights in August to October, the other three sites experienced average or less frosty finish to the season.

The start of this winter cropping season (May, June and July) has recorded more cold nights at Kyancutta, the average number at Yongala and fewer than average at Mildura and Ararat.

An obvious question is what will happen in the important months of August to October when frosts can severely impact yield.

A reasonable question is whether there is any relationship between the number of cold nights early in the season (May June July) and later in the season.

In other words, should we expect some persistence or memory in the seasonal frostiness whereby seasons that start frosty tend to stay frosty.

It is not unreasonable to expect patterns arising from persistence in weather and climate. Some seasons seem to start raining and other seasons just seem to miss out.

Unfortunately using the 65 years at Kyancutta, Yongala and Mildura and the 52 years at Ararat we found no relationship. The r values that measure correlation are all low and indicate that this process is best understood as random.

As we showed in the last newsletter, there is not a strong signal for frost from a negative or positive Indian Ocean Dipole.

In the late 1990s, the climate scientist Neville Nicholls used to include a quote on his email signature from the Dr Who movie “I love humans, they can find patterns in anything”.

All of us, researchers, agronomists and farmers are good at recognising patterns, that is one of our superpowers as humans. The problem is that we look for pattens that are not there in tea leaves, palms of hands and weather records.

Weather records are extremely useful to learn from the past, to put seasons in context and compare sites. These records help us say something meaningful about frost risk and, with records of rainfall, evaporation, radiation and hot days, to calculate optimal flowering windows.

Because we have the weather records we can use statistics to challenge our intuition that a frosty start to a year means something about the finish.

Unfortunately, frost remains a real risk for the coming spring and it is best understood as random.

Projects such as this that keep you in touch with grower groups and the latest science on frost are valuable.

Following developments such as the new GRDC project led by Dr Kenton Porker on manipulating crops within the season may suggest some opportunities for novel in-season approaches that may help growers avoid frost damage.

Because we don’t know what will happen in spring it is wise to consider ways to prepare for a ‘softer fail’ rather than a ‘hard fail.’ This will differ on each farm. In some cases, working out hay and grazing options and being prepared to make an assessment of damage after a cold night.

Frost on barley in August

Article contributed by Peter Hayman and Dane Thomas, SARDI Climate Applications