Adapted with permission from "Out-crossing Between Canola Varieties - A Volunteer Canola Control Issue" by Phil Thomas - AAFRD Provincial Oilseed Specialist.
Out-crossing between different varieties of canola has not been much of a concern to canola producers for the past few years - unless they were seed growers. Now however, with a number of different canola systems being used, out-crossing is much more of an issue. Pollen from plants of any of the canola systems can outcross to any nearby canola plants whether they are the same or from a different system.
The recent introduction to Canadian canola growers of identity-preserved varieties (e.g. Natreon*-quality Nexera* varieties) and herbicide tolerant varieties has increased the need for clear strategies to reduce the risk of gene flow and problems with volunteer canola "weeds".
This out-crossing or gene flow between varieties was a concern for canola growers in the past. This was especially important when Canada switched production from rapeseed to canola in the 1970's, and later with the production of high erucic acid/low glucosinolate varieties grown in the same vicinity as conventional canola varieties.
Understanding Canola Pollination
The brassica napus species (Argentine varieties) are mainly self-pollinating, with out-crossing rates of 20 to 30%.
- The vast majority of canola pollen, which is heavy and sticky, falls to plant surfaces and the ground within a few meters of its source.
- A small percentage (5-10%) becomes airborne and floats like dust particles on the wind.
The majority of the pollination between plants is through pollen movement by insects such as bees. Research studies by Agriculture & AgriFood Canada have shown that isolation distances of 100 meters are normally sufficient to prevent cross-pollination that could affect the purity of pedigreed seed. Regulations for production of hybrid seed, however, specify isolation distances of at least 800 meters to ensure that no stray pollen fertilizes the female plants.
Professionals working with canola, including the regulatory agencies involved with granting approval of new novel trait varieties, were fully aware of the out-crossing potential in B. napus varieties. This included the concern of herbicide tolerant trait transfers creating novel volunteer canola.
A Minor and Manageable Situation
The conclusion was that this problem was minor and manageable†. Research studies at Agriculture & AgriFood Canada in the 1970's showed that out-crossing levels or pollen flow from large commercial fields to small plots tended to be at low levels (46m = 2.1%, 137m = 1.1% and 366m = 0.6%).
More recent research in 1998 found substantially lower out-crossing rates between large commercial fields ranging from 0.1 to 1.5% at 20 meters to 0.1 to 0.4% at 100 meters. Despite the low probability of out-crossing recorded in 1998, the large number of canola flowers and small seed produced ensures a substantial number of out-crossed seed could still be produced per acre.
Some seed may shatter onto the ground before or at harvest and germinate the following season with the succeeding crop. At a 0.2% out-crossing rate in a field yielding 25 bushels per acre, a shatter and harvest loss of 3% would result in about 10,000 seeds per acre, or about 4 seeds per square meter. Although nearly all the plants originating from such seed would normally be killed by frost, herbicide treatment and/or tillage, some could survive to compete with the succeeding crop and warrant further chemical or mechanical control.
A Recent Case
In 1998, out-crossing between canola varieties was documented in a producer's chem-fallow field in northern Alberta. Pollen flow from an adjacent field in 1997 conferred herbicide tolerance to volunteers. Tolerance was not anticipated by the producer, so the same herbicide was applied to control the volunteers, and it was ineffective. Canola volunteers tolerant to multiple herbicides were identified, as would be expected through out-crossing.
Control Strategies
Canola volunteer plants tolerant to Liberty™, Roundup®, Pursuit™, Odyssey™, Freedom™ (and soon other herbicides) are likely present in many fields across western Canada. The volunteer plants most likely to be noticed by growers will be Roundup tolerant because Roundup is used for chem-fallow and pre-seeding burn-off whereas Liberty and ALS inhibitors like Pursuit are not. Similarly, ALS-inhibitor tolerant volunteers may remain in cereal or pea fields which used only Group 2 herbicides.
Control of volunteer canola, whether herbicide tolerant or not, can be achieved by a variety of methods currently used by producers. Effective control should include multiple strategies. Among them would be:
- proper control of volunteer canola in summerfallow;
- using tillage or chemicals, control volunteers prior to seeding;
- keep good records and thoughtfully rotate crops and herbicides.
Phenoxy herbicides still play an important role in Dow AgroSciences chemistry
In cereal crops, no varieties have yet been shown to have resistance to phenoxy herbicides (2,4 D and MCPA). This makes certain Dow AgroSciences products valuable tools for combating all volunteer canola. Curtail* M, Attain*, Prevail* and Prestige* all have a phenoxy component. Use these herbicides in a balanced herbicide rotation plan as an effective way to manage herbicide tolerant canola on your farm.
New chemistry for volunteer HTC control in cereals
Florasulam is Dow AgroSciences' new Group 2 active ingredient for a wide spectrum of broadleaf weeds. It is part of two new products being evaluated for registration: Frontline* and Spectrum*. Along with control of key problem weeds such as chickweed, cleavers and wild buckwheat, a key attribute of florasulam is that it controls ALL varieties of volunteer canola, including Roundup Ready varieties and other herbicide tolerant canola (HTCs).
Florasulam has characteristics that differ from other Group 2 herbicides such as the "IMIs" (e.g. Odyssey™, Pursuit™) or "SUs" (e.g. Freedom™, Refine™).
Example of the difference: Frontline delivers 60% to 80% control of volunteer SMART canola, yet this canola is tolerant to Odyssey, Pursuit and Freedom.
What else do you need?
Dow AgroSciences has a number of products available to help in dealing with herbicide tolerant volunteer canola. For more information on crop protection solutions, call our Solutions center at 1-800-667-3852.
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