Top Crop Manager

Swath timing critical after a frost

Frost changes everything.

November 26, 2007  By Bruce Barker

The killer frosts that rolled through many parts of Manitoba, Saskatchewan
and Alberta in mid August 2004 left many farmers scrambling to decide when to
swath their damaged canola. John Mayko, agronomic research and extension manager
with the Canola Council of Canada at Mundare, Alberta, says time of swathing
canola became critical in getting the best canola quality in those frosted conditions.

These pods show premature browning of damaged seed.

Mayko says that three main scenarios developed in 2004, depending on the severity
of the frost and its impact on maturity. Hopefully, that early frost will not
occur in 2006, but if it does, the lessons learned provide guidance when it
comes to swathing frozen canola.

In normal years, the optimum stage to swath for both yield and quality is when
all seeds contain about 30 to 35 percent moisture – when approximately
50 percent of the seeds in the pods on only the main stem will have changed
or started to change colour. However, the early frost of 2004 and abnormally
cool weather had agronomists reconsidering the decision-making process.


Scenario 1 – light frost
With light frosts, there is little evidence of frost damage. The pods remain
mostly green or tan-green, with few, if any, white spots on the outside of the
pods. The seeds inside are mostly intact.

Mayko recommends growers check the colour of the inside of the seeds as well.
If the outside of the seed is starting to turn, the inside should be yellow
or light-lime green. If the seed is still green on the outside and the inside
of the seed is still watery, the outside seed coat should remain intact and
turgid. In this case, leave the crop to continue to mature until further seed
and or pod colour change occurs.

If the seeds start to shrivel up, they are likely frost damaged and little
will be gained from delaying swathing. Careful monitoring of the crop should
be done until the crop is deemed ready to swath as a normal crop.

Scenario 2 – moderate frost
When moderate frost damage occurs, it will show up as only white specks on the
outside of the pods, or only a few seeds in immature pods are affected and are
not intact, or only a few seeds in the upper plant parts are prematurely browning.

Premature browning occurs when the outside of the seed has been damaged by
the frost and the inside of the seed remains dark green. In these cases, the
fields should be left and monitored on a daily basis to evaluate further crop
changes. If no further damage is evident, the remaining crop may continue to
fill and mature. If the seeds continue to deteriorate and/or pod integrity declines,
Mayko recommends swathing to save as much seed as possible from shattering.

Scenario 3 – severe frost
Under severe frosts, there is considerable whitening of the outside of the pods
for a majority of the plants. In this case, the pods will begin to shrink and
desiccate rapidly. Swathing should begin immediately to reduce pod shelling
and pod drop, and to conserve any seeds in those pods.

Mayko says that careful monitoring of the canola crop can help improve quality
by swathing the crop at the optimum time, taking frost damage into consideration.

"The crops should be monitored regularly, every day if possible. In many
areas in 2004, the frost damage did not manifest itself until five or six days
later because the cool and wet weather delayed the symptoms," says Mayko.

Making the most of a bad situation
When frost is in the forecast and the crop is still standing, swathing may be
an option to help protect the crop from frost damage. Three degrees of frost
is enough to kill immature seeds containing 50 to 60 percent moisture, while
those ready to swath at about 35 percent moisture will normally escape damage.

Information from Saskatchewan Agriculture, Food and Rural Revitalization indicates
that swathing at least 24 hours prior to a frost, preferably 48 to 72 hours
prior, can reduce the green seed count even at early maturity stages, such as
zero to five percent seed colour change (about two weeks ahead of normal swathing).

At the normal swathing stage (50 to 60 percent seed colour change), temperatures
of minus three degrees C for 60 minutes will have little effect on chlorophyll

Visit the Council's frost damage awareness
page at: production/Frost/frost.html

The Bottom Line
Unfortunately in 2004 we experienced all three frost scenarios mentioned
in the article. In hindsight, we regret swathing as early as we did after the
frost, however, we also had up to 50 percent hail on a lot of our canola, so
we wanted to get a start in case it did start to warm up. We have found over
the last few years that swathing early has resulted in smaller seed and lower

In 2004 in particular, swathing early likely resulted in a higher green seed
and a higher count in damaged kernels. Shattering does not appear to be as big
an issue as it was with conventional varieties. As canola is 25 percent of our
production, our most stressful production decisions every year involve the correct
time to swath canola. What we have decided is that not all canola will be swathed
at the correct time but we need to determine when the majority will be cut close
to the optimum time.

One eye is on the six to 10 day forecast and the other on each field. As the
article states, we would prefer to see up to 50 percent of the seeds on the
main stem start to change colour. The danger with leaving it too late is that
as canola dries out it is more difficult to get through the swather opening
and has the potential to be damaged by the swath roller or not settle into the
stubble and be more prone to wind movement. Warren
Kaeding, Churchbridge, Saskatchewan


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