Frosts - are they giving our crops and pastures the cold shoulder?
by Zita Ritchie, DPI Warnambool
Clear, calm and dry nights following cold days can lead to a radiation frost which forms white ice crystals on the ground and plants. These conditions can happen during winter and early spring where high atmospheric pressures follow a cold front, bringing cold air from the Southern ocean but settled cloudless weather.
Wind and cloud reduce the likelihood of frost by decreasing the loss of heat to the atmosphere. The extent of frost damage is determined by how quickly the temperature takes to get to zero at ground level, the length of time its stays below zero and how far below zero it gets (Note: BoM temperatures are taken at 1.2m above ground level and are usually 1-2 degrees warmer than ground temperature.)
In dairying areas frost can slow and disrupt pasture growth as well as cause damage to cereal and forage cereal crops, particularly when they are flowering.
Pastures
For ryegrass pastures, the presence of water soluble carbohydrates (WSC) in the leaves, (formed during photosynthesis) plays an important role in regrowth and the survival of plants through periods of stress such as frost and moisture stress. WSC are particularly important in sustaining plant growth when photosynthesis is unable to provide enough energy to meet the plants demands. An example of this is immediately following grazing, during cloudy weather, or at night. As a result it is important to manage pastures that maximise WSC build up to benefit plant growth and persistence.
Grazing at a shorter interval (less than the two leaf stage) should be avoided as it depletes the WSC reserves and leads to decreased plant survival, particularly during frost. Other consequences include decreased dry matter (DM) yield, decreased tillering and increased invasion of weed species.
Cereal crops
Cereal crop susceptibility to frost ranging from the most to the least susceptible is triticale, wheat, barley, cereal rye and lastly, oats. Triticale is more susceptible despite its hardy nature and adaptability to a range of soil and seasonal conditions, due to it’s longer flowering time.
Frost during the flowering period is generally the most damaging period but cereal crops can be susceptible from booting through to grain ripening. Part of the problem with frost is that it can often go unobserved, known as a “black” frost. This occurs when the temperature drops below zero but the surrounding air is dry (such as drought conditions). Ice can’t form on the plant surface and the water between cells freezes quickly forming large crystals, causing permanent damage. Once thawed, plant parts affected immediately look floppy, spongy and discoloured.
Figure 1: This graph shows that cereals are most susceptible after booting to the flowering stage. From GRDC 2009
In frost-prone regions, conditions of the crops and pastures should be monitored regularly for signs of frost and detailed records of varieties, frost dates and the area of the paddock affected should be maintained for future frost risk management.
Are you in for a frost?
This page provides frost prediction two days out which is updated each evening at the Bureau or Meteorology (BoM) Water and the Land site. http://www.bom.gov.au/jsp/watl/weather/frost.jsp
Figure 2: Above is an example of a map showing the frost potential for Victoria
Was it a frost last night? Check for temperatures from your local BoM automatic weather station. Click on the town to get the half hourly data. http://www.bom.gov.au/products/IDV60800.shtml
