Fall frost: Swathing and straight combining considerations

September 7, 2017 - Issue 24

Part 1. Swathing

Frost on pods can stop plant development and lock in green. It can also cause pods to split. However, a light frost may have no effect at all, and the crop will be better left to mature fully.

To determine your situation after a frost:
—Check standing canola the morning after a frost.
—Before taking any action, wait at least 4-6 hours after frost to 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.
—Light to moderate frost damage may take longer to show up. 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.

This was taken 4 hours after a minus 7°C frost. Green pods are already turning white and popping open.

Responses for heavy and light frost

Heavy (“Sharp!”) 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.

For canola with high seed moisture, frost in excess of -5°C is generally lethal, resulting in non-viable seed. 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.

Light (“Gentle”) 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.

What to look for during daily monitoring:

—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.

Frost and 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.

Part 2. Crop left standing for straight combining

Pod splitting or pod drop due to tissue damage from the heavy frost usually starts within a day after the frost event. If the canola crop is still sound, growers can probably stick with original plan to straight combine.

If frost damage is fairly heavy, getting brittle plants into a swath could protect them from wind whipping, which could increase losses in standing canola. However, swathing means you handle these brittle crops twice. The act of swathing may create just as many losses as leaving the field standing.

Here are a couple thoughts to consider while making this decision:

–Would you sleep better if the crop was swathed? If yes, swathing is probably the best decision.
–What is the state of seed in the most frost damaged pods? If frost hit top immature seeds hardest, these will shrivel up and won’t contribute much to yield anyway. If frost damage was minimal to those lower pods that will contribute more to yield, you may be able to leave the crop standing — if that was your original plan — without significant losses to these pods.
–Will pod shatter tolerant varieties reduce pod shatter after a frost? This may be worth a conversation with the seed provider.
–Will crop swathed that late make a nice tight windrow? Plants at that stage of maturity are lighter and stiffer and may not pack down into a window that will protect them from wind.

If you’re still torn, swath part of the field, leave the rest, and do a comparison. That experience might help the next time you encounter this situation.

Here’s a map showing lows for the past week…

Further reading:

Frost in the forecast: Swath now or leave it standing?

Canola Watch