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.
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.