At what temperature does frost occur in a cereal crop?Cold damage occurs when plants are exposed to temperature less than 5°C down to -2°C. If this occurs during pollen development (Z39 – 45) it can cause spikelet damage.
Why does rain make it worse?A canopy that is wet from a light shower of rain is often more prone to frost damage. This is because rain contains ice nucleators such as bacteria or dust. These ice nucleators raise the freezing point of water (Christner et al 2008). As a result, a slightly wet canopy may get freezing damage warmer temperatures compared to a dry canopy. This might happen at home when you put a beer in the freezer and forget to take it out. Initially you might see that it isn’t frozen but as soon as you open it, it freezes immediately as the super cooled liquid freezes. Super cooling is when a liquid, free from ice nucleators remains as a liquid below its freezing point. Opening the beer has a similar effect, promoting rapid ice crystal formation. A similar reaction can happen within plant tissues out in the paddock, particularly with wet conditions.
When is the crop most susceptible?Cereal crops are most susceptible to frost damage during and after flowering but are also susceptible from stem elongation through grain filling. Pulses and canola are particularly susceptible during pod filling where affected pods have absent, mushy or shrivelled and distorted seeds.
What does frost damage look like?The nature of frost damage depends upon the plant development stage at which the frost occurs. Grain: Frosted grain at the milk stage is white eventually turning brown with a crimped appearance. It is usually spongy and when squeezed it doesn’t release a milky dough substance. Frosted grain at the dough stage is shrivelled and creased along the long axis, like a pair of pliers has crimped the grain in the middle. Flowering: Frosted anthers are white turning a dull brown colour, ovaries turn a dull brown and are spongy when squeezed. They begin to shrivel as no grain is developed. Also the head will be underdeveloped and/or have bleached florets. Stem: Pale green to white ring on the peduncle (the stem below the head), or between the internodes which can lead to a crimped, cracked/blistered appearance with a rough texture. The damaged area may turn white/brown and the head/stem may bend over.
How do I check for frost damage?When: Inspect crops when they are between ear-emergence and grain-fill, after the temperature drops below 2°C (screen temperature). Damage is usually most evident 7−10 days after a suspected frost event. Where: Examine the crop in more susceptible lower parts of the landscape first and if the crop is damaged proceed to higher ground. How: Walk through the crop and examine a whole plant every 10−20 paces. If the head has not emerged from the boot, check that the developing head has not been damaged. You will need to carefully dissect the plant from the top down to find the head of the plant inside the leaf sheaths. If the crop has flowered, open the florets to check if the grain is developing. After a frost event, tag a few heads with tape and note the stage of development. Return a week later to determine if head/grain development and grain filling is continuing. To check for stem frost, remove the leaf sheath from around the stem from the flag leaf to the roots checking for a pale to white ring, shrunken or wrinkled/blistered appearance.
Is stem frost damage as bad as flowering frost damage?Stem damage may not be as bad as flowering frost damage, provided there are viable grains and mild weather conditions during grain filling. The stem structure is similar to a bunch of straws where water and nutrients travel to the head/grains. Not all straws may be affected, allowing water and nutrients to still reach the head/grains. Dye can be used as an indicator of how much damage has occurred. To demonstrate this, cut a plant at the base and place outside in a mixture of water and food dye for 24 hours. The ease of which the dye travels up the stem will give you an indication of damage. Normally it’s the xylem (capillary tubes which transport water) that are the issue, (the phloem which transports the sugars can rebuild), if it is hot or water stressed, the heads can lose ability to maintain evaporative cooling because of restricted water flow and can over heat. This can result in white bleached heads after a hot day. Lodging of cereals can also be an issue if windy conditions occur during late grain filling stages. Stem frost damage late in grain filling can be confused with root diseases such as, crown rot, rhizoctonia or septoria. To determine the cause of damage, remove the leaf sheath from around the stem from crown to peduncle to check for a pale to white ring, shrunken or wrinkled appearance.
Why do crops with high yield potential get frosted more?Position in the landscape influences temperature variations, frost damage and yield more than management practices. High production areas in a paddock are often lower in the landscape with the increased moisture and better soil types often promoting higher yields. Current frost research indicates that crops sown with high seeding rates, high nitrogen and higher yield potentials may be more susceptible to frost. It is thought that high inputs creates denser canopies which shades the soil, minimising soil heat retention and the ability of the soil heat bank to buffer the frosts. There is also more sychronisation of the canopy development so a greater portion of the canopy may be exposed. As a result these crops can experience greater frost severity, duration and damage compared to crops grown with more conservative approaches. Despite this, conservative nitrogen strategies are only recommended for the more severe frost prone parts of the landscape as the opportunity cost of these strategies often outways the direct cost from frost damage.
What is the soil heat bank?The soil heat bank refers to the amount of heat absorbed and retained by the soil during the day. This heat is then radiated back into the crop canopy overnight to warm flowering heads, minimising frost damage. The amount of heat stored depends on a number of factors such as row spacing which affects canopy closure, soil colour, stubble loads and soil moisture. A moist soil profile will store more heat than a dry soil. The soil heat bank captures heat during the day and radiates heat into the canopy overnight to warm flowering heats and minimise frost damage.
What management options do I have to minimise frost damage?
- Identify frost prone paddocks – with topographic electromagnetic, yield maps and paddock history
- Consider enterprise in a zone – cropping/sheep balance
- Review nutrient management – targeted nitrogen, potassium, copper inputs
- Modify soil heat bank – stubble levels, crop canopy
- Select appropriate crops – oats, barley, wheat, canola
- Manipulate flowering times – stage sowing time, mix long and short season varieties
- Fine tune cultivar selection – wheat, barley susceptibility during flowering
- How do I manage frost?
- Can trace elements improve frost tolerance?
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