Frost's impact on crops depends on the crop type and variety, stage of development, temperature and other weather conditions, and the capacity of the crop to compensate for the damage. This is all influenced by complex interactions between environmental and plant factors. Identifying the symptoms of damage and estimating the likely impact of the damage on crop prospects are essential to making decisions about crop salvage.
Minimum temperature can vary widely, depending on the microclimate of the crop and location, and crop development stage. Farmers are generally aware of the variation between paddocks and the susceptibility of those low-lying paddocks. In addition to minimum temperature, the length of time below critical temperatures for plant growth and the number of repeat cold events are also important. But other factors such as soil water content, soil colour and surface roughness can also influence the temperature.
The most frost-sensitive stages of grain crop plants are around flowering and early grain filling. Plants can be 'hardened' by exposure to low temperature (<7°C), which reduces the chance of damage, and 'de-hardened' by exposure to higher temperature (>15°C), which increases the chance of damage.
Rapid changes in the sensitivity of crops, such as wheat, around flowering mean that small (a few days) differences in the timing of flowering can be critical in the type and extent of damage observed. Such small differences can be caused by environmental factors such as spatial differences in nutrient and plant water availability.
The impact of the damage caused by frost also varies with the capacity of the plant to recover.
Crop symptoms vary depending on the exposure of susceptible tissues to damaging temperature. For picture and explanations see the Cereals — Frost Identification: Back Pocket Guide and the Pulse and Canola — Frost Identification: Back Pocket Guide.
How symptoms relate to grain quality, quantity
Frost damage to the head around flowering reduces the number of grains available to be filled. Often the remaining grain is well filled and of high quality. If spring conditions are particularly dry, reduction in grain number can benefit grain quality. Stem and head frosts will limit grain filling but can also lead to higher screenings and low hectolitre weight as well as a higher protein content. Starch and protein quality can also be affected by frost damage.
Great care needs to be taken in interpreting the impact of frost on grain quality and quantity because of the potential for 'unaffected' grain to 'contaminate' samples.
Program 3.7.3 Contact: Dr Doug Abrecht 08 9081 3111
National, North, South, West