Default to waiting. Don’t make a snap decision to swath. Before taking any harvest action, start with this sequence:
—Check standing canola the morning after a frost but wait at least 4-6 hours before making a harvest decision. This will allow the full extent of frost damage to become evident. The crop may look undamaged that morning but by the afternoon wilting, desiccation and pod splitting may begin. This crop may need to be swathed to preserve yield, but keep in mind that high green counts are likely.
—Cold temperatures do not automatically mean frost damage. If no damage is evident after the first day and you decide to leave the crop, scout again after 2 to 3 days to reassess.
—If most or all seed is mature and you planned to swath the day after a frost anyway, then don’t bother waiting 4-6 hours. Just start swathing.
Responses for heavy and light frost
Heavy frost… below -2°C: Assess the damage in early afternoon. Check pods for a white, wilted appearance. Pod shatter and pod drop could begin within a day, especially with warm sunny afternoons. If pods are desiccating rapidly, swathing right away will preserve as much yield as possible. Note that even with a heavy frost, canola fields can sometimes make it through with very little damage, but keep watching. Frost in excess of -5°C is usually lethal for seeds with high moisture (especially 40% or more). At such low temperatures, ice crystals physically disrupt structures such as membranes and enzymes. Pods of immature canola crops frozen at lethal temperatures have been observed to turn black, whereas mild frost turns pods white or white-speckled. These seeds usually shrivel up and are lost before or during harvest.
Light frost… above -2°C: Hold off swathing. Check in the afternoon for wilting to make sure frost damage was not heavier than expected. You may see some speckling on the stem and pods, but this is of little concern as long as the plant is still alive. If no wilting, leave the crop standing and check daily. If the majority of the seeds remain green and immature, 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.
Other questions and considerations
What does frost do to canola quality? A killing frost will reduce quality, but that can’t be helped — whether you swath today or wait. Immature seeds (moisture content higher than 20%) will be damaged. Seeds with less than 20% moisture will normally escape damage. Green seed is the major downgrade that results from frost.
At what seed moisture level is frost no longer a risk for green content or pod integrity? 20% or less. How this corresponds to seed colour change (SCC) will vary with plant architecture and other factors, but it approximately equates to 50 to 60 per cent SCC.
What if farmers had planned to apply a pre-harvest aid for straight combining? For glyphosate and Heat especially, they should likely wait until after a few warmer nights (lows of 5°C or higher) to get weeds and crop growing again to improve uptake and efficacy. The frost may also serve to provide a dry-down benefit similar to the spray.
Why does green seed lock in with frost? The majority of the canola green seed problems in Western Canada are usually the result of frost. Even a light frost can fix the green colour by damaging the enzymes that clear the chlorophyll in higher moisture seed, preventing additional clearing regardless of how favourable weather conditions may become following the frost. A killing frost also causes rapid dehydration of seed and plant tissue, which may be just as important for reducing the ability of these enzymes to function properly. The seed enzymes that clear chlorophyll work rapidly at high temperatures but slowly at low temperatures. Read more.
How does low plant population increase frost risk? With fewer plants per square foot, the plants are bigger with more branches. Seeds in side branches are later and less mature, which means lower plant populations are at higher risk of losing yield and quality to frost. The wide range of seed maturity within the same plants also makes decisions on swath timing more difficult because seed colour change on the main stem may not be a good indicator of overall seed maturity across the plant.
If canola is ready to swath, should I just keep swathing even if it’s the morning after a frost? Yes.
Does frost hurt already swathed canola? It can. A heavy frost will lock in high green counts unless the crop has dried down to a seed moisture of 20 percent or less. Canola deep in a swath may be somewhat protected from the lowest temperatures, but thermometer readings in the canopies of standing canola versus the middle of swaths suggest this blanketing effect is minimal.
Frost is in the forecast. Should I swath now? In most cases, waiting is probably the better option but the decision depends on (at least) two things: (1) How far advanced is the crop? (2) How cold will it get? Read this article for suggestions based on four scenarios.
Frost hits canola left standing for straight combining. Do I change plans? Frost provides some natural desiccation that may help dry weeds and green stems in fields left for straight combining. Some growers actually wait for frost before combining, using it as a tool to aid in crop dry down — but waiting for frost may not be advised if the field is ready and no frost is forecast. After a frost, do watch for pod shatter. Straight combining may be best performed shortly after a frost if pod shatter seems likely. Urgency may not be as high for varieties with pod shatter tolerance. Read more.