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Early Fall Frost Effects on Canola

The amount of frost damage is dependent on various factors including crop stage, degree and length of frost, relative humidity, and presence of rain or dew.

To determine when to swath after a frost, check the extent of damage 2-3 days after the frost. Frost damage will be evident on the outside of the pods, which will often show a white, speckled appearance (caused by ruptured cell walls leaking fluid). Immature and watery seeds will start to leak their inner fluid, and begin to shrivel. The outside of the seeds will begin to turn dark green, instead of a light-medium green. If the majority

of the seeds remain turgid, delay swathing to allow for further seed maturity. If the pods are severely damaged and are beginning to desiccate, swath during periods of dew or high humidity to reduce the amount of pod shelling and pod drop.

Severe frost after flowering can result in significant yield reductions and grade loss. The amount of fall frost damage to canola depends on its stage of maturity. Dry, mature seeds can be frozen with little or no effect on seed quality and viability. However, seeds with moisture content of 20% or more will suffer frost injury. Generally, the higher the seed moisture content, the greater the chance of frost injury. A frost of -3EC is enough to kill immature seeds containing 50 to 60% moisture while those with less than 20% moisture will normally escape damage. Frost does not damage all seeds to the same degree since the canola plant flowers progressively from the lower to the upper parts of stems.

Frost that results in green seed is the major cause of downgrading in canola. Green seed occurs due to a failure of the seed to complete the normal chemical processes involved in "degreening." Even a light frost can fix the green color, preventing additional chlorophyll clearing, regardless of how favorable the following weather conditions. Sub-lethal frosts from 0 to 1EC can disrupt the biological enzyme system that breaks down chlorophyll in seed nearing maturity. Frost damaged seed dries down or desiccates very rapidly. Once seed moisture content is down to about 20%, the biochemical activities within the seed have slowed to a very low level so that little or no further enzyme chlorophyll clearing can take place.

Growers considering swathing early to avoid damage from future frosts need to remember that swathing should be done at least 2-3 days (depending on temperature) before the anticipated frost. This will allow the seeds to dry down and raise their solute level sufficiently, and be less likely to be damaged.

Source: Canola Watch Report - Special Frost Bulletin - August 20, 2004

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