Frost damage concern for south-eastern Australia
Growers and advisers are urged to get out in paddocks and check plants for frost damage after unusually low temperatures and widespread reports of impacted crops in the past fortnight.
While crops are commonly thought to not be susceptible to frost until flowering, with temperatures in some agricultural areas reaching -4°C (Figure 1) many crops in earlier growth stages have been affected.
Damage has been observed in wheat, lupins and barley, but wheat has been most severely affected. Crops have been damaged at any growth stage from first node (GS31) onwards.
GRDC panel member and western New South Wales agronomist John Minogue has seen very severe crop damage in the West Wyalong region and north and west of the area.
“Reports are coming in from agronomists and farmers of widespread stem frost in cereals. Advanced lupins are also quite badly affected and the extent of damage to canola is unknown. The damage appears to have emerged after two frost events of around -4 degrees in the past month.
“Large areas of crops were at the second node stage (GS32) and even as early as the head only a few millimetres from the boot being affected. There have also been reports of damage east and south of West Wyalong, but not to the same extent. There appears to be pockets of damage that were more advanced than most, generally those that were sown earlier.”
Growers inspecting crops for damage should look out for the following symptoms (Table 1).
Where frost is observed, there is no simple answer that experts can provide on the chances of the crop recovering.
Table 1: Symptoms of frost during early growth stages.
Source: Adapted from Managing Frost Risk: A Guide for Southern Australian Grains
|Crop growth stage||Inspection Details||Frost symptoms in wheat||Example|
| Vegetative (before stem extension)
|| Examine leaves
|| Leaves are limp and appear brown and scorched.
| Elongation (before and after head emergence)
|| Pull back leaf sheath or split stem to inspect damage
|| The stem has a pale green to white ring that usually appears sunken, rough to touch and soft to squeeze. The stem or nodes can also be cracked or blistered. Stems can be damaged on the peduncle (stem below head) or lower in the plant. If the head had emerged at the time of the frost then it is likely that the flowering parts or developing grain has also sustained damage. If the head is in the boot then ongoing monitoring is required to assess the level of damage.
| Flowering and post-flowering
|| Peel back the lemma (husk), inspect the condition of the florets (floral organs) in the head
|| Flowering is the most vulnerable stage, because exposed florets cannot tolerate low temperatures and are sterilised. Grain will not form in frosted florets, but some surviving florets may not be affected. Pollen sacs (anthers) are normally bright yellow but become dry, banana-shaped and turn pale yellow or white.
University of Adelaide frost researcher Tim March says it is difficult to predict how crops may respond.
“If the majority of the heads are damaged in the majority of a crop it’s likely that the crop is not going to be worth harvesting. But where about 50 percent of the heads damaged, there’s no way to say what will happen. It depends on what’s happened within the vascular tissues of the plant, as well as the conditions for the plant between now and harvest, such as spring rainfall.”
GRDC Southern Panel member and YP agronomist Bill Long says growers need to consider their circumstances and decide quickly. Growers have a number of management options to consider (Table 2).“Growers should inspect their crops to determine how severe the damage is so that they can make a decision on the way forward. If the conclusion is that harvesting won’t be economical, there are a number of options, and the best choice depends on a number of factors that the grower needs to assess with their agronomist,” he said.
“If the choice is to cut hay, then this is best done soon as hay quality is affected by significant rainfall events.” In doing this, it may be necessary to consider the current market situation. The GRDC has initiated a number of emergency measures to deal with the frost event (see What is your GRDC doing?).
Table 2: Management options for frost damaged crop, each with advantages and disadvantages.
Source: Managing Frost Risk: A Guide for Southern Australian Grains
|Options||Potential Advantages||Potential Disadvantages
|Harvest||Salvage remaining grain
More time for stubble to break down before sowing
|may be greater than return
Need to control weeds
Removal of organic matter
|Hay / Silage||Stubble removed
Additional weed control
|Costs $35-$50/t to make hay
Quality may be poor
|Chain / Rake||Retains some stubble (Reduces erosion risk)
Allows better stubble handling
|Costs $5/ha raking
||Inadequate stock to use feed
Remaining grain may cause acidosis
Stubble may be difficult to sow into
|Spray||Stops weeds seeding
Preserves feed quality for grazing
Gives time for final decisions
Retains organic matter
|Difficulty getting chemicals onto all of the weeds with a thick crop
May not be as effective as burning
Boom height limitation
Expense $5/ha plus cost of herbicide
Some grain still in crop
|Plough (Cultivate)||Recycles nutrients and retains organic matterbr/> Stop weed seed set
Green manure effect
|Requires offset disc to cut straw
Soil moisture needed for breakdown and incorporation of stubble
|Swath||Stops weed seed set
Windrow can be baled
Regrowth can be grazed
Weed regrowth can be sprayed
|Relocation of nutrients to windrow
Low market value for straw
Poor weed control under swath
Expense - swathing ($20/ha)
Spraying ($5/ha per herbicide)
|Burn||Recycles some nutrients
Controls serface weed seeds
Pemrits re-cropping with disease control
Can be done after rain
|Potential soil and nutrient losses
Organic matter loss
GRDC Project Code UA00136, CSP00143
Region South, North