Mindfully.org
Home | Air | Energy | Farm | Food | Genetic Engineering | Health | Industry | Nuclear | Pesticides | Plastic
Political | Sustainability | Technology | Water



Outcrossing Between Canola Varieties

A Volunteer Canola Control Issue

Phil Thomas, Provincial Oilseed Specialist / Alberta Agriculture Food and Rural Development 24mar00

Outcrossing 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. This outcrossing 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.

Now however, with six different canola systems being grown, outcrossing is much more of an issue. The six different systems are:

Pollen from plants of any of the above canola systems can outcross to any nearby canola plants whether or not they are the same or of a different system. In other words the pollen of novel-trait herbicide tolerant canola plants can outcross to nearby non-herbicide tolerant canola or canola with other herbicide tolerances. The Brassica napus species (Argentine varieties) is mainly self pollinating with outcrossing 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 metres 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 metres are normally sufficient for purity of pedigreed seed, although regulations for production of hybrid seed specify isolation distances of at least 800 metres to ensure that no stray pollen fertilizes the female plants.

The recent introduction to Canadian canola growers of specialty fatty acid varieties and herbicide tolerant varieties, developed by selection, induced mutation and/or gene transfer, has increased the need for clear strategies to reduce the risk of gene flow and minor problems with volunteer canola "weeds". Professionals working with canola, including the regulatory agencies involved with granting approval of new novel trait varieties, were fully aware of the outcrossing potential in B. napus varieties. This included the concern of herbicide tolerant trait transfers creating novel volunteer canola. Their conclusion was that this problem was minor and manageable. Research studies at Agriculture & AgriFood Canada in the 1970's showed that outcrossing 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 outcrossing rates between large commercial fields ranging from 0.1 to 1.5% at 20 metres to 0.1 to 0.4% at 100 metres. Despite the low probability of outcrossing recorded in 1998 the large number of canola flowers and small seed produced ensures a substantial number of outcrossed 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% outcrossing 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 metre. 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.

In 1998, outcrossing 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. As the tolerance was not anticipated by the producer, the same herbicide was applied to control the volunteers, and it was ineffective. Multiple herbicide tolerant canola volunteers were identified, as would be expected through outcrossing. Canola volunteer plants tolerant to Liberty, Roundup and ALS inhibitors and combinations of these 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 of volunteers should include multiple strategies:

For pre-seeding or chem-fallow weed control on canola stubble, add a phenoxy like 2,4-D to glyphosate (Group 9 products like Roundup) or use a Group 22 herbicide like Gramoxone PDQ. Control of volunteers on or adjacent to Roundup Ready stubble will not be effective with glyphosate alone.

Use herbicide mixtures when using Groups 2, 6, or 9 products. All canola volunteers regardless of system are controlled by inexpensive Group 4 herbicides such as 2,4-D and MCPA.

Rotate herbicides (not in groups 2, 6, 9 or 10). Rotation of herbicides is important for weed resistance management and for the control of herbicide resistant volunteer canola.

After harvest leave canola seeds on or near the soil surface as long as possible. A high percentage of seeds left on the soil surface will germinate in the fall and be killed by frosts while remaining seeds will germinate in the early spring and can be controlled either through tillage or by herbicides in the succeeding crop. Seeds which are incorporated into the soil develop induced secondary dormancy and can persist for up to 4 years in the soil.

Use tillage immediately prior to seeding to control volunteer canola. Tillage can play an important role in weed control in conventional systems, and you can maximize weed control benefits by shortening the interval between tillage and seeding operations.

Include silaging and green manuring to control volunteers.

Isolate fields with different herbicide systems. Do not grow them side-to-side in the same field or across the fence line. To minimize the amount of field-to-field crossing, the current research data suggests a minimum of 175 m isolation between fields.

Rotate canola with cereal, pea and forage crops. Control of multiple herbicide tolerant canola within a canola crop may not be possible. Diversifying a rotation permits use of a wider selection of herbicides. Lengthening the rotation depletes the volunteers from the soil weed seed bank.

Scout fields for volunteer canola which are not controlled by herbicide application. Early detection allows time for control before seed set.

Grow competitive crops. Boost seeding rates, choose competitive varieties, seed early, and place a balanced fertilizer close to the seed.

Reduce the loss of seed during canola harvest. Swath at between 20 and 35% seed color change on the main stem reduces shatter loss. Properly adjust the combine so that seed losses are minimized. Higher combining speeds increase seed loss.

Use certified seed. Pedigreed seed use reduces the probability of multiple herbicide tolerant canola volunteers due to outcrossing.

source: http://www.agric.gov.ab.ca/crops/canola/outcrossing.html 26mar01

If you have come to this page from an outside location click here to get back to mindfully.org