Data from BCG weather stations have confirmed what many already suspected; August 2014 was abnormally cold.
Last August Lubeck shivered through 17 frost events while Hopetoun suffered 20 frosts, according to data from frost weather stations positioned at BCG’s Hopetoun and Lubeck research sites.
A frost is deemed to have occurred when the temperature falls below 2 degrees Celsius (°C) at Stevenson screen height (1.2m above ground level), but its severity and impact is also dependent on its duration.
While Hopetoun and Lubeck did experience a high number of frosts last year, in some instances the temperature dipped below 2°C for only 15 minutes. Nevertheless, some impressive recordings were logged; on August 4 the mercury fell to minus 5.3°C at Hopetoun and minus 3°C at Lubeck.
These recordings were gathered for a GRDC-funded national frost initiative. As part of this project, BCG is assessing the ways in which stubble load (slashed/mulched and burnt/removed) and architecture (standing and horizontal), as well as crop architecture (such as skip row seeding), influence frost severity and duration.
Frost is estimated to cost the Victorian grains industry $30 million dollars each year from direct and indirect losses.
There are four types of frost that affect grain crops: vegetative, stem, head/flowering and grain fill frosts.
Vegetative damage, which affected many crops following five substantial frosts last July, usually results in burning of the leaf tip. Fortunately, crops typically grow out of any damage that occurs during vegetative stages (before first node) unless the frost occurs at establishment (particularly canola).
Crops are vulnerable to stem frost during stem elongation (GS31 to GS45). In 2014 it was stem frost which effected many crops in the Mallee and parts of the Wimmera. Stem frost typically occurs when the moisture (from dews or small rainfall events) freezes where it accumulates inside the leaf sheath at the nodes, and above the leaf auricle. The inter-nodal stem tissue or developing heads will freeze, resulting in physical damage to tissue and, in severe cases, cell death. Stem frost can also restrict the transport of water and nutrients to the developing head.
Head or flowering frost, which occurs if a prolonged frost hits after the head has begun to form, can be especially devastating. Intermittent sterility is commonly observed with rat tail, or malformed heads, which become obvious during grain filling. Entire heads can also be frosted. Head/flowering frost can cause a significant detrimental impact on grain yield, but often has little effect on grain quality and delivery grade of grain.
Frost during early grain filling (from about GS75-85) results from the freezing of the grain and surrounding chaff and is often characterised by physical bleaching of the head. This can impact grain screenings and lead to downgrading. It also affects grain counts and produces low falling numbers in some situations. Modern headers, however, are usually able to provide a deliverable sample if adjusted correctly.
One outcome from the GRDC frost initiative will be the release of frost ratings for most commercially grown cereal varieties this year. This information will assist growers to make varietal decisions that could reduce the risk of frost to their crops in the future.
It will build on the current advice which is to reduce exposure to possible frost damage by taking into account crop/variety maturity times and planned sowing dates.
FROST FACTS
- The most susceptible parts of the paddock are the low lying areas and the south west sides of hills where the morning sun takes longer to reach.
- Grain crops are generally at highest risk from late booting (GS45) until mid-grain fill stage (GS85), with flowering usually being the time when crops are most susceptible and have the greatest potential for damage. However, in 2014, severe frost events in August, September and into early October, combined with crops that may have been a little more advanced than usual due to the warm start, meant noticeable damage occurred as early as first node (GS31).
- Frost damage is a cumulative process; several minor frosts (-1ºC to-2ºC) can cause as much damage as one singular but colder event of -5ºC.
- The full impact of a frost event, and its economic effect, can take more than a week to become apparent, although some symptoms of frost damage, such as blistering of pea pods, will appear within the first 24 hours.
- A crop affected by frost at flowering can still be cut for hay and some income can be recouped. However, the cost of this option first needs to be assessed, taking into consideration potential grain yield, seasonal outlook and access to hay markets.
- Crops with adequate supply of moisture and nutrition can compensate for frost damage by producing secondary tillers (if frost occurs early as was the case in 2014). The later the frost, the harder it is for the plant to compensate.
- Frost damage can be assessed by tagging crops after a frost. Plants should be growing by about one centimetre per day (without water stress); after two days it should be easy to spot if a plant has grown by two centimetres or not. Impacts on developing grain (GS70-80) should be apparent after three to four days, as grain should be growing at roughly 0.5 to 0.8 mm per day.
- The time of sowing is critical for frost and heat stress; if the crop matures too early it may be frosted, too late and it may experience heat shock. A balance of sowing times will provide a broader range of maturity and spread the risk. Models such as Yield Prophet® can help identify optimal times.
