Frost forecast and swath timing

If frost is forecast, should you swath canola now or leave it standing? The answer depends on (at least) two things: (1) How far advanced is the crop? (2) How cold will it get?

Nice night for swathing. Credit: Clint Jurke

Here are the four scenarios that these two questions present, and considerations for all four.

1. Crop is past 30% seed colour change (SCC) on the main stem, plants have relatively few side branches, and heavy frost is forecast.

This is not the ideal SCC for yield, but at 30% SCC, most of the seeds will be firm to roll. You may want cut this ahead of the frost to prevent locking in green on the most immature seeds. Note however that crop needs to dry down to a seed moisture level of 20% or less to be relatively safe from the damaging effects of frost. This can take three good drying days, so swathing has to occur at least three days before the frost to achieve this benefit.

2. Crop is past 30% SCC, green seeds are firm to roll, and light frost is forecast.

The degree of frost is hard to predict, so will a light frost really be light? That is a question we can’t answer here, but going on the gut feeling that frost will be light, leave this crop standing. It would have to be cut about three days before the frost to get pods dried enough to keep them from popping open.

3. Crop is at 5% SCC and heavy frost is forecast.

At 5% SCC on the main stem, this crop is really green and a heavy frost will bring high yield loss and lock in high amounts of green for those seeds that do make it to the combine hopper. The best option is probably to leave it standing and take your chances that the frost is light and the crop will have a few more days to mature. Each additional day standing will benefit this crop. Research conducted by the Canola Council of Canada over approximately 28 site-years indicates that significant yield increases could be achieved by swathing at 60-70% seed colour change (SCC). Swathing at 60% SCC resulted in 8% more yield (which was equivalent to about 3 bu./ac.) than at 30-40% SCC, 12% more yield than at 10-20% SCC, and 19% more yield than swathing before 10% SCC. Swathing at 5% SCC in anticipation of the frost could reduce yields by 20% or more.

4. Crop is at 5% SCC and light frost is forecast

This crop will benefit from all the standing time it can get. Wait out the frost, but monitor afterwards to assess the degree of pod damage and increased risk of shelling if left standing after a harder frost than expected.

Further considerations:

Many nights of frost. If frost is forecast for several nights in a row, canola with a high percentage of green immature seed might not have much chance to mature further. Growers could cut the crop and accept that any yield potential from immature seed is likely lost.

Logistics and timing. If growers still have a lot of canola to swath, they may need all the swathing days they can get. If today is a good day to swath, then go swathing but know that any yield potential from immature seed will likely be lost.

Crop standing for straight combining. Frost can actually be a benefit as it provides some extra dry-down of green stems. Note, however, that frost at the point when pods are brittle and quite dried down will likely have little-no impact, and pod-shatter tolerance can also help maintain pod integrity if the crop wasn’t quite that dry yet.