Use Seed Colour Change To Determine Swath Timing – for Jul. 23, 2010

Even in a good year, canola maturity varies between varieties, fields and conditions. In a rough year like this one, a tough seeding (and reseeding) season and excessive rain in many areas has added to the variability of fields. Staging a patchy field for swathing or harvest timing is especially difficult.

To best gauge swath timing, Jim Bessel, senior agronomy specialist with the Canola Council of Canada, says you’ve got to get out into the field and look inside the pods — it’s the seed colour change that really matters, and pod colour can be misleading.

“The problem with using pod colour change as a maturity indicator is that weather conditions can give false indications. Sun scald can make pods look ready but the seeds might still be green,” he says. It is important to distinguish between sun scald and pod colour change because sun scald can make the plant appear riper than it actually is. Sun scald occurs when plants are ripening during periods of stress, such as heat. “The main symptom is purpling on the stems and pods,” Bessel says. “The purpling is an abiotic stress response caused by periods of stress when the plants are ripening. The purpling is likely due to higher levels of anthocyanin pigment and a lack of chlorophyll in the naturally senescing tissue and often shows up more in some varieties than others.” Confirm sun scald by checking the underside of the pods or branches (areas not exposed to the sun) for normal pod colour.

The other factor in gauging canola maturity is variety. Bessel says that using the variety’s posted “days to maturity” isn’t a bad starting point, but the reality is it’s only a guideline as to when to start really looking at the seeds. Heat, wind, drought and length of the flowering period can all affect the rate of maturity.

“Some varieties mature at different rates independent of pod colour changes,” he says. “Seeds can be turning colour and pods not be and vice versa.” The only real way to know if the crop is ready or not is to get out into the crop canopy, open up some pods and roll the seed between your thumb and forefinger to determine if the seeds will squish or not.


When you’re out in the field, sample the crop in the same way you might when doing soil samples or scouting for insects. You need to get into the field, into low and high spots and away from the headlands to get a decent handle on maturity. Bessel says to start looking at the bottom third of the main stem, where most of the yield comes from.

You’re getting very close to swathing when: “The bottom third of the main stem will have turned, the middle third will likely be spotty and changing colour but may have some green, firm seeds that are not translucent and the top third may still have translucent seeds that you can squish between your thumb and forefinger,” he says. When 60 per cent of the lower third of the main stem is turned and 40 per cent of the middle third have turned, it’s likely time to pull the pin and cut the crop.

Bessel adds that thin stands of canola are more likely to branch out, making maturity less uniform. While branches typically carry less of the yield, in a tough year like this one, they may be a life saver. The main stem is still the key driver in determining swath timing, but take into account the volume and maturity of the seeds on the branches as well.

Dry weather, wind and heat can also cause seeds to mature very quickly; even just one or two days of good weather can significantly speed maturity. “It’s not an exact science; determining swath timing is a judgement call, but things can change quickly, so once the crop is getting close you should be out there every two or three days.”


Bessel says that while straight cutting canola is gaining in popularity, it’s not a one-size-fits-all practice. “If you’ve got an even crop with a thick, interconnected canopy, it might be a good candidate for straight combining,” he says. Swathing isn’t without its pitfalls — light swaths can blow away and cutting of the plants can cause some shattering losses. But Bessel says that leaving a canola plant standing longer can also lead to pod drop, incurring significant losses. Also, leaving a crop standing longer than necessary once the seed is mature can result in increased shattering losses.

And do new varieties developed with straight cutting in mind really make a difference? Bessel says it’s too soon to tell, and that growing conditions and the actual plant stand seem to matter more. He says that the decision to swath or straight cut really does need to be made on a field-by-field basis, regardless of the variety grown.

What is interesting, Bessel says, is that swathing does not seem to speed actual maturity of the canola seeds. “Swathing does help dry down the stems faster, which can help at harvest, but actual seed maturity is about the same between both practices. Straight cutting canola can sometimes be like putting silage through the combine because of the tough green stems,” he says.

“It takes time and fuel to swath, yes, but straight cutting can use up more horsepower and fuel, as well as take longer (than threshing a swath) if the stems are tough,” Bessel says. “It comes down to a time management decision, as well. Not everyone has time to swath every field.”


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