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Controlling volunteer canola in soybean

By Angie Peltier, University of Minnesota Extension and Joe Ikley, North Dakota State University Extension

Many farm fields in northwest Minnesota are able to grow canola quite well. Unfortunately most are actually attempting to produce soybean and the canola are volunteers -very competitive (and often herbicide tolerant) weeds. Drive-by scouting won’t necessarily reveal the canola plants until they begin to bolt and their bright yellow flower petals can be observed towering over the soybean canopy.
Figure. Flower canola can be easily seen above a soybean canopy (photo credit: Manitoba Agriculture).

Why is canola such a problem in soybean fields?

Although it is grown as an agronomic crop throughout the northern Red River Valley, canola has several weedy characteristics.

Shatters easily. 

When grown as an agronomic crop, pods shatter and drop from plants during mechanical harvest, with an average of 312 lbs. of seeds lost per acre (Brown et al, 1995).  When growing as volunteers, each canola plant can produce between 500 and 2,000 seeds that can also be lost during harvest activities.  This harvest loss of such a small seeded weed species (1,000 seeds = 0.009 lb) can add up to thousands of additions to the weed seed bank.

In fact, one of the best ways to manage volunteer canola in future soybean fields is to slow combine speeds when harvesting your canola crop.

Emerges early. 

Canola has a base temperature of 41 degrees, meaning that seedlings can emerge quite early in the spring (Lawson et al. 2006). Soybean has a base temperature of around 50 degrees.  The growing conditions required for both plants means that canola plants can get a significant head start on soybean growth and development.

Row spacing matters. 

Losing at least 5% of soybean yield is losing at least 2 bushels from a 40 bu/A crop. The density of volunteer canola plants required to cause a 5% yield loss is different in soybeans in different row spacings.  More volunteer canola plants per square yard were needed to cause a 5% yield loss in 7.5 inch row soybeans (avg.: 2.7 plants), while fewer volunteer canola plants were needed to cause 5% yield loss in soybeans grown in 30 inch rows (avg.: 2.1 plants; Gulden et al. 2017).  Why? Canopy closure is faster in narrower row soybeans and canopy closure helps soybean better compete with weeds.

Volunteer canola’s kryptonite. 

Despite volunteer canola’s seeming advantage due to its weedy characteristics, volunteer canola has one weakness – winter survival. Regardless of whether in soybean or canola production fields, canola seeds lost during harvest are capable of surviving only up to three winters. In a study in Saskatchewan,  researchers found that the maximum canola seed viability after one, two and three winters was 44, 1.4 and 0.2 percent, respectively; seed was less capable of surviving when exposed to fall rain events, likely due to some fall germination (Gulden et al. 2003).  This means that if we strive for zero canola seed additions, we can deplete the vast majority of the seed bank within 1 to 2 years.

Complicating management: Herbicide tolerant canola. 

Canola plants rely primarily on self-pollination to produce seed, although a significant amount of out-crossing still occurs.  Out-crossing is where one plant’s pollen fertilizes another plant’s ovaries to produce seed. The rate of out-crossing, which tends to decrease with distance, is estimated to be between 20 and 47% (Gulden et al. 2008).  Canola varieties engineered to express herbicide tolerant traits provide tolerance to glyphosate (ex. Roundup), glufosinate (ie. Liberty), or imazamox/sulfonylurea herbicides.  When canola varieties expressing different herbicide tolerance traits are grown in nearby fields, out-crossing can produce seed that is tolerant to multiple herbicide groups (Hartman, 2006).  A conversation with a seedsman in Roseau County, Minnesota revealed that this is not a problem only in Canada.  Conversations with neighboring canola producers can be an important way to reduce the chance of having to wrestle with multiple herbicide tolerant canola by maximizing the distance between fields in which canola varieties with different herbicide tolerance are grown.

Chemical control of volunteer Roundup Ready canola.  

The weed science team at NDSU has done a wonderful job summarizing what their field trials have revealed regarding success (or lack thereof) in managing Roundup Ready volunteer crop plants in the “2020 North Dakota Weed Control Guide” (Ikley, 2019). On pages 38 and 39 of this guide you will find a table titled, “Roundup Ready Soybean – Herbicide to Apply in Tank-Mix or Sequentially with Glyphosate for Control of Weeds not Controlled by Glyphosate” that has an entire column dedicated to how well each product has performed against Roundup Ready volunteer canola. Another table in the guide, on page 128, is titled, “Control of Volunteer Glyphosate Resistant (GR) Crops”. This table focuses in on how each herbicide product performs against Roundup Ready canola at different growth stages, from pre-emergence to several vegetative stages, beginning bolt and beginning flower.  It is always important to check herbicide restrictions regarding soybean growth and development at the time of application to ensure that your planned herbicide applications will be on-label.

Pre- and Post-emergence herbicide options few in certain cropping systems. 

Without a corresponding herbicide tolerance trait, there tend to be fewer herbicide options that control grasses that are tolerated by small grains or corn. Similarly there are fewer broad leaf herbicides that would both be effective against Roundup Ready canola and tolerated by soybeans.

As we in the Red River Valley have more complex cropping systems than folks in corn/soybean country further south, keep close at hand the table listing herbicide crop rotation restrictions on pages 112 through 114 of the guide. Among those herbicide products listed as providing good to excellent control of volunteer Roundup Ready canola in the previously detailed efficacy tables (pages 38-39, 128), most can only be applied before soybean emergence (pre) and many would likely be impractical if sugarbeets are in your cropping system.

On the lower end of sugarbeet rotation restrictions are Sharpen (pre-emergence only), Fierce (pre only), Boundary (pre only), Flexstar (pre/post) and Metribuzin (pre only), with a minimum of 4-6, 15, 18, 18 and 18 months between application and sugarbeet planting, respectively.  Authority Supreme (pre only) has a 24 month restriction.  As sugarbeets tend to be quite sensitive to injury or death from some herbicides a long crop rotation restriction (CRR) must be followed with a bioassay of the soil to ensure that the herbicide concentration is low enough to allow the sugarbeet crop to establish.  Examples of products that are rated as having good to excellent efficacy against Roundup Ready canola in soybean but requiring a bioassay before planting sugarbeet are Authority MTZ (pre only, 24 month CRR) Authority First/Sonic (pre only), First Rate (pre/post) and Surveil (pre only, 30 month CRR), BroadAxe XC (pre only, 36 month CRR) and Authority Assist (pre only) and Pursuit (pre/post, 40 month CRR; Ikley, 2019).

The crop rotation restriction tables on pages 112 through 114 also contain restrictions for using various herbicides in rotations that include alfalfa, barley, canola, corn, CRP grasses, dry beans, field pea, flax, oats, edible legumes, potato, safflower, soybean, sunflower, hard red spring wheat and durum wheat.

Literature Cited

  • Callihan et al. 2000. Guide to identification of canola, mustard, rapeseed and related weeds. University of Idaho, Moscow.
  • Gulden, R.H., Geddes, C.M. and Gregoire, P. Economic and biological implications of volunteer canola in soybean. Research summary to provincial funding agency.
  • Gulden, R.H. and May, W. 2015. Management of volunteer glyphosate resistant canola in glyphosate-resistant soybean crops: Dec. 2014-Nov. 2015.
  • Gulden, R.H., Shirtliffe, S.J. and Thomas, A.G. 2003. Secondary seed dormancy prolongs persistence of volunteer canola in western Canada. Weed Sci. 51:904-913.
  • Gulden, R.H., Thomas, A.G. and Shirtliffe, S.J. 2004. Secondary dormancy, temperature and burial depth regulate seedbank dynamics in canola. Weed Sci. 52:382-388.
  • Gulden, R.H., Warwick, S.I. and Thomas, A.G. 2008. The biology of Canadian weeds. 137. Brassica napus L. and B. rapa L. Canadian J. Plant Sci. 88:951-996.
  • Ikley, J. 2020. 2020 North Dakota weed control guide. NDSU Extension. W253-20. Fargo, ND.
  • Lawson, A.N., Van Acker, R.C. and Friesen, L.F. 2006. Emergence timing of volunteer canola in spring wheat fields in Manitoba. Weed Sci. 54:873-882.

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