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Managing volunteer canola in RR soybeans

Volunteer Roundup Ready canola caused a lot of trouble for Roundup Ready soybeans
in Manitoba during 2006. Part of the problem was due to larger, overwintered
volunteer canola plants that escaped a pre-seed burnoff application in mid May.
Another part was due to a large amount of volunteer canola emergence by the
first week of May.

Controlling volunteer canola is best done in cereal rotations. Photos Courtesy Of Bruce Barker.

"In 2006, we found that volunteer canola was very large at the time of
a typical pre-seed burnoff application before soybeans; these canola volunteers
proved to be a challenge to control," explains Arvel Lawson, oilseed business
development specialist with Manitoba Agriculture, Food and Rural Initiatives
(MAFRI) at Carman.

Of course, compounding the problem is that glyphosate alone does not control
Roundup Ready canola volunteers, either in a pre-seed burnoff or in-crop in
Roundup Ready soybeans. Part of the solution, according to Lawson, is to implement
strategies to minimize the problem in the first place.

Limit volunteering
Lawson says that minimizing the amount of canola seed that hits the ground is
the first step in management of volunteer canola. She cites a Saskatchewan study
that found the average harvest loss in canola is six percent. On an average
crop, that can result in 3600 seeds per square metre hitting the ground.

"That's 20 times the normal seeding rate for canola, which can result
in a lot of volunteer canola to manage in future years," she says. "While
Mother Nature takes care of some of it, you can help by managing the amount
of seed that hits the ground at harvest."

Lawson recommends growers do everything they can to prevent harvest losses
of canola, including swathing at the correct time, properly setting the combine,
and even swathing or combining at night or in the early morning to help prevent
shattering losses.

Another important approach is to minimize secondary dormancy. She says canola
can be induced into secondary dormancy, which can allow canola seeds to persist
in the soil longer. Secondary dormancy can happen when canola seed is exposed
to dry and dark conditions. Not burying seeds that fall to the ground at harvest
can help limit the induction of secondary seed dormancy in canola.

Avoid creating secondary seed dormancy by eliminating tillage.

Ideally, if the weather co-operates, volunteer canola will germinate in the
fall after harvest and under normal western Canadian conditions, die off with
freezing temperatures. If moisture is good in the fall, heavy harrowing after
harvest, when the soil temperature is still suitable for canola seed germination,
can be part of a volunteer canola management strategy.

Those seeds that do enter secondary dormancy become the persistent volunteer
canola seedbank. Volunteer canola populations have been shown to persist in
Europe for up to 10 years after production and in western Canada for up to four
years. Anecdotal accounts of longer time frames do exist in western Canada.

Lawson says that controlling Roundup Ready canola volunteers is generally easier
in the first year after the crop because the rotation is often to a cereal crop
where pre-seed and in-crop herbicide control is easier. In the spring prior
to a cereal, several registered herbicide options exist for volunteer Roundup
Ready canola control. Simply adding a phenoxy to glyphosate will take care of
the volunteers. Volunteer Roundup Ready canola control is much more challenging
in crops such as soybeans or other pulses because of limited options for pre-seed
and in-crop weed control.

"The first year after Roundup Ready canola is the year you want to control
volunteer canola. It becomes tougher after that," she says. "Wheat
is a good choice of a rotational crop.

"There are no herbicides registered for pre-seed burnoff control of volunteer
Roundup Ready canola prior to soybeans," explains Lawson. "In-crop,
you could apply Roundup and then Pursuit in a separate operation to control
volunteer Roundup Ready canola, but that option becomes expensive."

Pursuit, however, is only registered as a stand alone, foliar application on
soybeans in Manitoba and is not recommended for the Brown and Dark Brown soil
zones, and it comes with a soil residual that must be managed. Pinnacle herbicide,
while registered on soybeans, does not have volunteer canola on its label.

As a result of the restricted choices for herbicidal control of Roundup Ready
canola in Roundup Ready soybeans, Lawson urges growers to utilize a holistic
approach when growing these crops in rotation. She summarizes by saying that
the important guidelines to follow are to limit losses of canola at harvest,
avoid creating conditions favourable to secondary seed dormancy induction, which
leads to seedbank persistence, and to plan rotations carefully to make volunteer
canola control easier.