Managing frost risk

Zoning - position in the landscape

The cold air mass that comes with a frost is similar to pouring liquid nitrogen on a flat table - if you tilt the table the liquid nitrogen will slide down the surface. A frost is similar because the cold air will pool in lower areas where the air has nowhere to escape. The identification of frost prone zones would help to inform crop choice and time of sowing. In addition, in the event of a frost, knowledge of susceptible areas would help inform salvage options, for example cutting for hay and/ or grazing. After a significant frost year, harvest offers the opportunity to map frost affected areas which would help inform management strategies for the following season. 

Climate resources

Frost risk maps are produced July to October each year for growers and advisors to observe the severity and occurrence of frost events for the south west land division. These maps aid in strategic monitoring of crops for frost damage and help inform salvage options.  Historic maps have been compiled annually from 1975 showing the occurrence and severity of frost events for each month (July, August, September and October). The August, September and October data has then been combined to show occurrence and severity over these sensitive months in one map.

The Extreme weather events tool uses data from DPIRD's extensive weather station network to map extreme temperatures, either below or above a specified threshold. It provides real-time information about the location, duration and severity of frost and heat stress events, to help grain growers manage accordingly to reduce their financial impact.

Crop susceptibility to frost

All winter grain and oilseed crops are susceptible to frost. Crop selection is an important factor to consider for frost-prone paddocks. Hay production harvests biomass and hence reproductive frost damage does not reduce yield. Pasture rotations are a lower risk enterprise and oats are less susceptible to frost during the reproductive stage than other cereals. Wheat is more susceptible then barley at flowering, but it is not known if barley and wheat have different frost susceptibilities during grain fill. Canola is an expensive crop to grow and can increase financial risk on frost-prone paddocks due to high input costs.

Cereals

Oats are regarded as the cereal crop least susceptible to frost damage, followed by cereal rye, barley, wheat and triticale. Oats is thought to be about 4°C more tolerant than wheat while barley is thought to be about 2°C more tolerant than wheat.

Canola and pulses

Canola is susceptible to frost, however is least susceptible to frost damage from late flowering (90%) to the clear watery stage (about 60% moisture).

Field peas are the most frost-susceptible pulse crop followed by faba beans and lupins.

Wheat variety response to frost

Under very severe frost (for example -8°C) or multiple minor frost (several nights of -2° to -4°C) all wheat and barley varieties are equally susceptible, resulting in up to 100% sterility. No varieties are frost tolerant however wheat and barley varieties do differ in susceptibility to reproductive frost damage during booting and flowering. Its important to note that there is no point selecting less susceptible varieties for the whole cropping program if there is an opportunity cost of lower yield without frost. A new variety should be managed based on how known varieties of similar ranking are currently managed.

Ranking information for current wheat and barley varieties for susceptibility to reproductive frost is available from the National Variety Trial website via the FV-PLUS frost rankings tool. These rankings are based on each variety’s relative susceptibility to spring radiation or reproductive frost, which occurs in late winter to early spring. As part of the Australian National Frost Program (ANFP – UA00136), 72 wheat lines were assessed and a nationally consistent methodology was developed to assess field based frost damage in cereals. The frost susceptibility data is generated from ongoing research trials based at Loxton SA; Merredin, Wickepin, Brookton and Dale WA; and Narrabri, NSW over several seasons.

Time of sowing - manipulating flowering times

When wheat is sown in frost risk areas, a good tactic is to ensure the flowering window of the cropping program is widely spread.

This can be done by:

• using more than one variety and manipulating sowing date
• selecting varieties with difference phenology drivers so the crop flowers over a wide window throughout the optimal flowering window.

It should be noted that flowering later than the frost risk will result in lower yields in seasons with hot, dry finishes due to heat and moisture stress and is generally not a sound long term management strategy as the loss of yield potential with late sowing is often greater than loss due to frost.

Sowing time is a major determinant of crop yield because in partnership with the variety chosen, it determines the timing of key growth and development stages such as canopy closure, flowering and the subsequently the environmental stresses at these key stages. Crop yields are particularly sensitive to stresses in the period from about 3-4 weeks prior to flowering through to the start of grain fill as this is when grain number is determined and stem reserves are accumulated.

Staging sowing dates over a 3-6 week period is recommended. If sowing just one variety, this would provide a wide flowering window. If sowing more than one variety: sow winter wheat first; then a long season spring wheat or a day length sensitive wheat; then an early maturing wheat last. This equates to the whole wheat program flowering over a wider period, potentially exposing it to more frost risk but maximising the yield potential in the absence of frost. Even with this strategy in place it is possible to have more than one frost event that causes damage. Flowering over a wide window will probably mean that some crop will be frosted but the aim is to reduce extensive loss.

Sowing time remains a major driver of yield in all crops with the primary objective to achieve a balance between crops flowering after the risk of frost has passed but before the onset of heat stress. Crops sown early within the sowing window will establish faster and have the potential to maximise water use efficiency. However early sowing increases the chance of frost damage and can limit weed control options and increase disease pressure. The loss of yield from sowing late to avoid frost risk is often outweighed by the gains from sowing on time to reduce heat and moisture stress in spring.

To minimise frost risk there needs to be a mix of:

• sowing dates
• crop types
• maturity types.

In years of severe frost, regardless of which strategy is adopted it may be difficult to prevent damage. Flower Power, an online tool, will help inform time of sowing decisions further as it predicts wheat flowering times of up to three different varieties at the same time gives the subsequent risk of frost or heat stress for these scenarios. Ben Biddulph (DPIRD) and Garren Knell (ConsultAg) explain the risks and how to minimise the downside of a frost event without delaying planting in this Weedsmart webinar.

Managing inputs

To minimise financial risk in frost-prone paddocks, growers can apply conservative rates of fertilisers and reduce the sowing rate.

Advantages of reduced inputs

• Less financial loss if the crop is badly frosted.
• Lower input crops, though potentially lower yielding during favourable seasons, are less like to suffer severe frost damage than higher input crops with a denser canopy in frost-prone areas of the landscape.
• Input costs saved on the higher frost-risk paddocks may be invested in other areas where frost risk is lower.
• Lower sowing rates may result in a less dense canopy potentially allowing more heat to reach the ground during the day, transferring it to the canopy at night. Its important to note that there is no conclusive evidence that lower sowing rates will reduce frost damage but it is an area of current research.

Disadvantages of reduced inputs

• In the absence of frost, lower grain yield and/or protein (which is a particular disadvantage in barley and wheat delivery grades) may be the result during favourable seasons, contributing to the hidden cost of frost.
• Less vigorous crops can result in high weed populations/seed banks and subsequent high herbicide use to control these populations may reduce the sustainability of lower sowing or fertiliser rates. Herbicide resistance can develop in weed populations if various herbicide groups are not alternately applied.

Fertiliser management

Target fertiliser (nitrogen, phosphorus, potassium) on high risk paddocks and seed rates to achieve realistic yield targets should minimise financial exposure, reduce frost damage and increase whole paddock profitability over time. These nutrients could be reallocated to lower risk areas of the farm.

The relationship of crop nitrogen (N) status and frost severity and duration is complex and is the focus of ongoing research. To date there is no strong evidence that N softens wheat to frost. It is suspected that high N rates can promote increased synchronisation of canopy development, head emergence and flowering. If a greater proportion of the canopy is flowering all at the same time, this will potentially increase the frost risk, as the whole crop will be vulnerable at the same time. The 2016 trials concluded that managing wheat varieties has a greater impact on frost risk than varying nitrogen and seed rates.

Adequate potassium (K) fertiliser application is important for reducing the effects of crop stress on grain yield. Potassium plays a role in maintaining cell water content in plants, which can potentially influence tolerance to frost. Potassium deficiency results in poor water use and other nutrients, making crops more susceptible to drought, waterlogging, frost and leaf diseases. Based on 21 potassium experiments since 2011, Ma et al. (2018) concluded K fertiliser provides added protection for cereal crops against crop stress from drought and frost especially if on marginal light sands (low in K <50ppm). Luxury levels have not been shown to reduce damage economically; however work is ongoing in this field.

If a crop is deficient in copper correcting with a foliar spray at booting is economical without frost. Luxury levels have not been shown to reduce damage. The role copper play in reducing frost is not fully understood. The symptoms of copper deficiency are similar to frost often leading to misdiagnosis as frost damage. Whole-top plant test provides a rough guide if paired good/poor samples are taken, but this should be confirmed with a youngest emerged blade (YEB) test. YEB levels below 1.5 milligrams per kilogram (mg/kg) indicate copper deficiency.

Frost tolerance can not be brought by applying extra potassium or copper to a crop that is not deficient. There is no evidence that applying other micronutrients has any impact to reduce frost damage. Use soil tests to calculate conservative fertiliser rates.

The science behind the solution

In some cases more frost damage has been reported as occurring where high applications of nitrogen were applied. In other cases this has not been reported. It has been suggested, but not confirmed, that the increased biomass from high rates of nitrogen may dilute concentrations of water-soluble carbohydrates and other minerals that in turn act as natural antifreeze agents (Karow 1999). There is also some evidence to suggest wheat plants take up late applications of nitrogen at the flowering growth stage (Z31-39), as carbohydrates stored in the stems are trans located to the roots to encourage root growth. The store of carbohydrate in the stems increases osmotic potential of cells and lowers the freezing point.

Cells with high concentrations of solutes have depressed water potential. If temperature differences at crop head height are not detected, but differences in the amount of frost damage are detected, then this may indicate nitrogen has made the plants more susceptible to freezing damage at any given temperature.

Wheat trials were carried out in Borden WA during 2003, 2004 and 2005 and fertiliser and seed rates were based on soil test results and were matched to target yields of 2, 3 and 4 tonnes per hectare (t/ha) (termed low, medium and high input treatments). Across the three seasons frosts were experienced and each time the low-input treatments suffered less damage and yield loss than the high-input treatments and were the most profitable.

In a trial at Yealering WA during 2005 the Wyalkatchem wheat treatments that received a higher split application of Flexi-N at stages Z31 and Z29 had a greater amount of frost distorted grains than the lower nitrogen treatments but a greater yield. So in terms of quality the low input nitrogen treatments fared better.

Stubble management

Increasing the soil heat bank is an important factor in reducing the risk of frost. The soil heat bank refers to the residual heat captured in the top 10cm of the soil profile during the day. Overnight this heat is released into the canopy to warm flowering heads thus minimising frost damage.

The soil heat bank. Heat is captured in the soil during the day and radiates out during the night into the canopy

Manipulating stubble loads as a tool to help reduce frost risk has been a focus of DPIRD/GRDC research trials since 2013. It has been shown that stubble loads can alter spring frost duration and severity. This is driven by the stubble load restricting heat flow in and out of the soil. For example, the temperature of the air above high stubble loads is more likely to be colder at night, due to the straw and other mulches acting as an insulator.

The key findings from extensive large scale trials are:

• Reducing stubble load reduced the severity and duration of frost events and resulted in less frost damage and better yields under frost. No differences were observed between stubble height, orientation or composition. Data to date suggest it’s mainly a load issue.
• In frost prone parts of the landscape growers can minimise frost risk by reducing stubble loads back to grain yield potential, prior to seeding. This can be achieved by various approaches including cutting low, windrow burning/chaff carting, stubble mulching, raking and burning, strategic blanket burning and summer grazing.
• Without frost, once off stubble reductions did not reduce yield and may give a slight improvement in yield due to reduced disease and less nitrogen tie up, depending on site, season and variety choice.
• With multiple severe frost events, stubble reduction did not increase yield.

Although stubble retention is a very desirable farming systems practice in terms of decreasing soil erosion, increased water content and soil biological activity, a compromise must be reached to ensure frost risk is reduced. Thresholds appear to be site-specific. In terms of minimising frost risk, a good rule of thumb is that stubble loads should match grain yield. For example, in a low production environment, 2t/ha grain yield potential = 4t/ha stubble and the stubble load needs to be halved at seeding, back to 2t/ha to minimise frost risk. In a medium production environment with 3t/ha grain potential = 6t/ha stubble, which needs to be reduced to 3t/ha to minimise risk. This calculation assumes a harvest index of 0.5. See images below to get a visual of stubble loads ranging from 1-4t/ha.

Wheat stubble loads of 1, 2 and 4t/ha at York 2016

Stubble management and wind erosion

Balancing stubble management in terms of frost with minimising water and wind erosion risk will come down to individual paddock histories and future planning. Burning stubbles is one option to reducing stubbles however this method should be used strategically to ensure erosion risks are minimised. Burning in windrows is preferred over blanket burning when wind erosion is the primary concern. However, there are some situations when blanket burning is recommended.

For instance:

• Blanket burn strategically when stubble loads from previous seasons are unmanageable. For example when tyned seeding machinery is not able to handle the high levels of accumulated stubble between windrows after two or more high-yielding years.
• Blanket burn when crops have been frosted. Frosted stubble rots off at ground level and does not remain standing during seeding causing trashflow issues.

Where blanket burning will occur it is recommended to focus blanket burns on the risk areas (frost, resistant weed seeds, disease carry over and to improve herbicide efficiency) and to burn 2-3 days in front of the seeding bar to reduce wind erosion.

Soil amelioration

Strategic deep ploughing with mouldboard, one-way and square ploughs or rotary spaders can overcome topsoil water repellency and help overcome subsoil constraints resulting in greater plant establishment and vigour. Amelioration of repellent sands may reduce frost damage by improving the capacity of the soil to absorb and then release heat. Increased heat absorption by the soil can occur as a result of improved wetting and changes to the colour of the soil surface.

In 2014, a trial at Brookton was assessed to determine if alleviating the water repellent surface soil will be able to accumulate a greater thermal mass to radiate through the crop canopy and reduce the level of crop damage in a frost event. Yield differences were observed between the ameliorated and untreated plots in trials in 2014 and 2015, however the measured yield increases were partly a result of improved crop establishment and growth from overcoming the water repellency and other constraints by soil inversion . There were no consistent reductions in frost severity or duration, in the two years of trial work, hence the higher yield of the ameliorated soil was largely due to improved yield potential from overcoming soil constraints, rather than a reduction in frost damage.

In 2016, as in previous seasons, there was a lot of anecdotal evidence around the impact of soil amelioration during a frost event. Some growers reported reduced frost damage in paddocks that have been mouldboarded spaded delved or clayed. However there are many confounding factors occurring as a result of the soil amelioration making it difficult to narrow down impacts to a reduction in frost damage alone.

Amelioration impacts on:

• nutrient availability, particularly K
• reductions in water repellency
• change in soil colour
• soil water
• crusting
• reduction in root diseases
• improvements in establishment
• biomass
• delayed flowering
• yield potential.

Soil amelioration options can give longer term production benefits but are slow and expensive to implement. Therefore they need to provide significant long-lasting productivity benefits year-in and year-out in order to provide a good return on investment. The decision to undertake soil amelioration should be made primarily in regard to weed control, soil water repellency, subsoil constraints and profitability. Potential reductions in frost damage are an additional potential benefit in years when frost is an issue and soil amelioration is unlikely to be economically feasible practice for frost damage alone. Research is continuing to further understand the possible impacts of soil amelioration on frost severity and the impact of soil types and amelioration methods.

Mouldboard ploughing of water repellent yellow deep sand at Badgingarra, 2013, using an in-furrow 5-furrow mouldboard plough

Other management practices to lower risk

The following management practices may lower frost risk but have had varying levels of success:

• Sow wider row spacings on heavy soils - may allow more airflow through crops, thus allowing soil heat to rise up to canopy height during frosts. Previous trial work has shown that wide-row sowing has debatable effectiveness on both sandy and heavy-texture soil. There is also a 1% yield loss per inch increase in row spacing and compromised weed control.
• Roll sandy and loamy clay soil after seeding - this practice consolidates moist soil providing a reduced surface areas. This enables more radiant heat to be trapped and stored during the day compared with dry, loose soil. Moist and firm soil is a better conductor of heat and will cool slowly because heat removed at the surface by radiation is replaced in part by heat conducted upwards from the warmer soil below. A roller is usually towed behind the seeder machinery or it can be done post-emergent. Rolling is not usually recommended due to the extra expense, extra time required, debatable effectiveness, inter-row weed germination and increased wind erosion risk on susceptible soil types.
• Cross-sowing - crops are sown twice with half the seed sown in each run producing an even plant density and generating a complete crop canopy that still allows air flow. Two trials across two seasons have been carried out in WA. In one trial a conventionally sown wheat crop was compared with a cross-sown wheat crop at varying sowing rates. The conventionally sown crop experienced more frost damage and lower yields at all sowing rates compared with cross-sown crops. Cross-sowing is not usually recommended due to the additional cost and time taken to sow the paddock twice and the minimum trial work that has been done to look at the impact of cross-sowing.

Frost is difficult to manage and in some seasons damage will be unavoidable. An integrated approach is required to minimise the risk of frost whilst setting your paddocks up to reach their yield potential. Where you will see greater gains are in paddock choice/zoning, crop and variety choice and manipulating the canopy.

References

• Anderson WK, Garlinge JR (Eds) (2000) The wheat book: principles and practice. Agriculture Western Australia and the Grains Research and Development Corporation.
• Rebbeck MA, Knell G (2007) 'Managing frost risk: a guide for southern Australian grains', (Ed. Reuter D) (Grains Research and Development Corporation, South Australian Research and Development Institute: Adelaide).
• White C (2000) Pulse and Canola—Frost Identification: The Back Pocket Guide.  Department of Agriculture and Food, Western Australia.  Bulletin 4401.
• March T, Laws M, Eckermann P, McGowan P, Diffey S, Cullis B, Maccallum R, Leske B, Biddulph B and Eglinton J (2016) Ranking cereal varieties for frost susceptility using frost values Northern Grains Research and Development Corporation Updates Paper.
• Ma Q, Bell R and Biddulph B (2018) Potassium application alleviates grain sterility and increases yield of wheat (Triticum aestivum) in frost-prone Mediterranean-type climate. Plant and Soil, 434 (1-2). pp. 203-216.