THERE has been plenty of media chatter in recent weeks, not about the forgettable 1988 thriller movie, Jack’s Back, but the potential 2017 winter-crop horror story starring the enigmatic Jack Frost.
Frost risk is an accepted part of successfully growing winter crops in Australia; every season, somewhere across the country, grain production is lost to frost. The extent of those losses will vary from year to year according to location and landscape factors as well as the climate.
So what causes frost? Clear, calm and dry nights following cold days are the precursor conditions for a radiation, or hoar, frost. These conditions are most often met during winter and spring where high pressures follow a cold front, bringing cold air from the Southern Ocean, accompanied by settled cloudless weather. When the loss of heat from the earth during the night decreases, and the temperature at ground level drops to zero, a frost occurs. Wind and cloud reduce the likelihood of frost by decreasing the loss of heat to the atmosphere. The extent of frost damage is determined by how quickly the temperature takes to get to zero, the length of time its stays below zero, and how far below zero it gets.
A frost event during flowering can cause sterilisation of the floret, as the pollen, ovary, or both are damaged by below-zero temperatures. In general, wheat is more susceptible than barley to such events due to the relative length of their flowering periods, but it can also vary according to variety.
That said, with spring less than two weeks old, very few cereal crops are at the flowering stage so it is too early to see wide-scale frost-aborted flowers.
However, it is not too early to suffer stem frost damage. Stem frost occurs when a small amount of water settles inside the leaf sheath above the penultimate node and adjacent developing tissue of the boot. This is most likely to happen if a light shower of rain occurs at dusk and then a frost follows.
Many of the reports emanating from the Riverina region of southern NSW relate to the morning of 28 August. The overnight low in some areas was reported to be less than minus six degrees, and less than zero for more than six hours. Similar reports are also surfacing in the Murray/Mallee region of South Australia on the same morning.
Identifying stem frost damage can be very difficult and it normally takes quite some time for the extent of such events to become evident to the market. Cereal losses of up to 70 per cent have been reported, but such examples are few, and isolated at this point.
The one bright light in this story is that affected plants, depending on available moisture, will send up new tillers to compensate for the lost heads, and such stories are also starting to emerge in the Riverina.
In northern New South Wales and southern Queensland, frost damage to the chickpea crop is widespread. Many paddocks have been flowering for some weeks now and the frequent frost events are taking their toll. Chickpeas have a low tolerance to frost due to the exposed nature of the flowers. However, as long as the plant can access sufficient moisture, they will simply reflower and produce new pods, minimising overall production loss accordingly.
In 2016, the rain didn’t stop until late October and the chickpea plants reflowered for weeks. This year almost the entire chickpea growing region has had less than 40pc of average rainfall since the 1 April. This simply means that the plants will not have the required moisture to replace the abandoned flowers and significant production losses will result. A combination of the lack of rainfall and the recent frost damage will see this season’s chickpea production fall to less than 50pc of last year’s record.
As growers and agronomists thoroughly inspect paddocks across the country, more horror frost stories will undoubtedly emerge. It is the extent and the geographic spread of the resultant damage that will tell the tale in terms of lost production. For most grain growers across Australia, this season had had enough challenges without Jack crashing the party.
Source: Nidera Australia