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Frost and Green Seed in Canola

Immature canola seed naturally contains a high level of chlorophyll, the green pigment that allows plants to photosynthesize, or turn the sun's energy into biomass.  

Fully and properly matured canola will have no chlorophyll, which we typically measure in Western Canada as per cent green seed. In high green seed samples, processing removes the chlorophyll with the oil component imparting a dark colour to the oil which is difficult and expensive to remove. In addition, high levels of chlorophyll are associated with increased oxidative rancidity and difficulties in hydrogenation.

The Canadian Grain Commission's (CGC) Official Grain Grading Guide accounts for distinctly green seed, but the measure is subjective. The Official Grain Grading Guide also has a reference to colour and as such has provisions for pale green and slightly immature seeds that are not distinctly green. The Guide notes that, "Whole seeds that are green may be as a result of thin seed coats of certain canola varieties. Whole green seeds of these varieties are not indicators of elevated chlorophyll levels and therefore are not considered distinctly green or assessed as part of colour evaluation. Only seeds which are distinctly green throughout when crushed are assessed as distinctly green". However, the grader determines per cent green subjectively and has leeway within the regulations. 

The correlation between per cent green seed, as it relates to grades, and chlorophyll measured in parts per million (ppm) is not very tight. A general rule of thumb would suggest the following: 

Table 1: Relationship between grade, per cent green seed and chlorophyll in parts per million (ppm)

CGC Grade 
Per Cent Green Seed  Chlorophyll
No. 1 Canada 
 < 2 < 25
No. 2 Canada 
2 - 6 26 - 45
No. 3 Canada 
6 - 20 46 - 100
Sample > 20 > 100

At some point in seed formation, all canola will contain a high amount of chlorophyll. If maturity progresses normally, the chlorophyll will clear out through natural metabolic activity. Enzymes, proteins that facilitate biological processes, are responsible for the removal of chlorophyll and, in canola, they are only active at temperatures above 5ºC and at seed moisture contents above 20 per cent. 

Environmental Factors

Several different environmental factors in combination with agronomic practices can affect the ability of seed to rid itself of chlorophyll. The two most common are frost and extreme hot dry weather at or near swathing. As mentioned previously, enzymes are responsible for clearing of chlorophyll.  Frost can denature or destroy these enzymes and render the plant unable to rid itself of chlorophyll regardless of the temperature and moisture conditions following a frost. On the other hand, extreme heat immediately post swathing can dramatically reduce the moisture content of the seed in a very short amount of time, leaving the seed with too little moisture to support enzymatic activity. As well, variety selection, disease and variable plant stand can and will affect green seed count, but are not of concern this late in the growing season. 

Temperature and Moisture

We know that frost during maturity will affect green seed and, if the temperature is low enough to destroy the enzymes responsible for clearing the green seed, no amount of time, heat or moisture will help, nor will the green seed cure out once the crop has been harvested and binned. If canola is not mature enough to swath, unfortunately, producers do not have many options to reduce the effects of an early fall frost.  Seed must be below 20 per cent moisture to avoid the effects of frost.  This is because much of the damage from frost comes from the freezing of water and the formation of ice crystals. If the moisture content is low enough, this will not happen to any great degree. 

Swathing early is often not an effective management strategy to mitigate the risk of frost and can result in severe yield loss due to shrivelling of immature seeds. In addition, swathed canola, if above 20 per cent moisture, is susceptible to frost damage just like standing canola. Swathing to avoid frost damage is only effective if the canola is swathed at least 72 hours before a frost so that the moisture content can fall below 20 per cent. 

Symptoms

A noticeable symptom of a damaging frost on canola near maturity is that the pods "sweat" and release plant juices. This happens because ice crystals have destroyed the integrity of the cell walls. Other symptoms of a damaging frost will be white specking of the pods, shrivelled seeds that resemble raisins, and wilting of the plants.  However, most of these symptoms may not be visible for two to four days following the frost.  

We have no hard and fast rules as to how much frost canola can take at this or any stage, which for most of the province is two to three weeks away from physiological maturity. The amount of frost a crop can withstand depends on several factors including acclimatization, dew and ultimately crop stage. Acclimatization is a plant hardening off by several days of cold weather before a frost. Plants that have gone through this react better to frost than plants that have not. However, canola seedlings react better to hardening off before early spring frosts than do plants nearing maturity. Dew will also affect a canola plant's ability to withstand frost.  Heavy dew on the plant will act as an insulator and decrease the effects of freezing temperatures. At the end of the day, no one can predict with any great certainty how a particular canola crop will fare at a particular temperature, it is a wait and see situation. 

Markets

Green seed is a major discounting factor for canola and historically the price drops by $10 to $15 per tonne, per grade. Buyers have historically discounted the price for sample grade canola, that which has a green seed count greater than 20 per cent, by one half compared to No. 1 canola. The discounted price for off grade and sample canola has not been the only problem in the past, finding a market and moving it off the farm has been a challenge as well. Recently, markets have been emerging in Saskatchewan for sample and off grade canola for use in the feed industry and the production of industrial products like biodiesel fuel conditioners.

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