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Roundup Ready canola as a resistant weed

Roundup Ready canola may be a new California crop at some point in time, but is currently one of California’s newest weeds. Roundup Ready canola is a weed because of its ability to produce a significant percentage of secondary dormant seed when harvested under a Mediterranean climate. This secondary seed dormancy in combination with its glyphosate resistance makes canola a new difficult California weed.

Roundup Ready canola may be a new California crop at some point in time, but is currently one of California’s newest weeds.

Roundup Ready canola is a weed because of its ability to produce a significant percentage of secondary dormant seed when harvested under a Mediterranean climate (Lutman et al. 1998). This secondary seed dormancy in combination with its glyphosate resistance makes canola a new difficult California weed.

Most annual crops only produce volunteer plants during the year following production; however canola is known for shattering large amounts of seed before and during harvest (Mallory-Smith & Zappiola 2008) and when buried, some seed enters a “secondary dormancy.” Even if all volunteer canola is controlled before it produces seed in the first year following canola, seedlings will continue to emerge for many years from dormant seed. Most of this dormant seed emerges in the first four years four years (Lutman 2003), but some can emerge up to 10 years after burial in the soil (D’Hertefeldt et al. 2008 and Lutman 2005.

Canola is a higher quality oil developed out of oilseed rape. Canola varieties are selections from several mustard species, but most varieties produced in the United States originated from Brassica napus, commonly called rapeseed mustard.

Rapeseed mustard and the other mustard species, to which canola is related, are present in California as wild weeds.

California situation

In trial and commercial field locations since 2006, California canola has shown similar shattering losses and seed dormancy to those in Canada and England (Lutman 2003). In the fall of 2006, the University of California Cooperative Extension conducted four small plot canola variety trials in Butte, Yolo, Fresno, and Imperial Counties. In addition, growers in Colusa and Yolo Counties planted several commercial fields of canola. With high energy prices, the California interest in canola was primarily for biodiesel and for planting seed production, but this interest was short lived. Canola for oil/biodiesel is not an economically viable crop in California at the present time.

The Butte County trial was planted on one tenth of an acre, but one acre of land around it has been left fallow and no volunteering canola has been allowed to produce seed. Even though no additional seed has been produced since the summer 2007 harvest, in the fall of 2009 (3rd year of “weedy” volunteer) over 500 canola seedlings emerged from this small one tenth of an acre area planted to canola in the fall of 2006. Similar “weedy” volunteer canola emerged through 2009 at two of the monitored Yolo county locations.

The transportation of farm equipment to and from these 2007 trials and production fields scattered some canola along county roads and state highways. This scattering of seed along the roadside is typical of field crops production. However, unexpected reproducing roadside populations of canola were located during the winter of 2009. Roundup Ready corn and cotton have been widely planted over the past 10 years in California, but have not become established along roadsides as reproducing weeds.

Aggressive control with effective herbicides and hand pulling of escapes along some of the county roads has resulted in very effective control, if not eradication. Some state highways with very limited control efforts have expanding canola populations along those roadsides. Disturbance of roadside soil promotes secondary dormant seed through shallow burial.

Importance of glyphosate resistance

Wild types of rapeseed mustard are not common weeds in California agricultural fields and at present are easily controlled by glyphosate. The seed dormancy of canola makes it a difficult weed to control, persisting in a field where it was once planted for years, if not indefinitely. Canola’s glyphosate resistance in combination with canola’s seed dormancy makes it a challenging weed for roadsides, orchards, vineyards, fallow fields, and Roundup Ready crop fields, anywhere where glyphosate is an important herbicide.

Glyphosate is one of the most common and valuable herbicides in California agriculture. Stephen Powles of the University of Western Australia has described glyphosate as “a once-in-a-century herbicide” (Powles et al. 2006). Glyphosate is effective on many broadleaf and grassy weeds, both annual and perennial weeds, with extensively proven animal and environmental safety. If glyphosate is a “once in a century herbicide” a replacement herbicide for glyphosate is likely decades into the future.

Each time another weed, for example ryegrass (Powles et al. 2006), develops resistance to glyphosate it makes weed control more complicated, more expensive, and decreases the value of the herbicide. If Roundup Ready canola spreads along roadsides and into orchards and fields it will make glyphosate less valuable in those situations.

Canada and the northern United States

Canola is the most important oilseed crop in Canada (Harker et al. 2000), planted on millions of acres of farmland and has recently become an important crop in the northern United States. In these areas, canola is commonly grown in rotation with wheat where several phenoxy herbicides can be widely used to control volunteer canola. In California with cotton, grapes, and other phenoxy sensitive crops, all phenoxy herbicides are very restricted. This limits the available herbicides for controlling Roundup Ready canola. California’s diverse agriculture and restricted phenoxy herbicides makes “weedy” Roundup Ready canola control much more difficult than in Canada and the northern cereal growing areas of the United States.


As long as canola for oil is not an economically viable crop in California agriculture, avoiding establishing it as widespread glyphosate resistant weed is a reasonable goal. Industry, regulators, and Cooperative Extension can work together to prevent Roundup Ready canola in California from becoming a widespread weed. “Weedy” Roundup Ready canola would diminish the value of glyphosate wherever it became established on any of the millions of acres in California where glyphosate is an important herbicide.


D’Hertefeldt, T., R. B. Jorgensen & L. B. Pettersson. 2008 Long-term persistence of GM oilseed rape in the seedbank. Biol. Lett.

4: 314-317. doi:10.1098/rsbl.2008.0123.

Harker, K.N., R.E. Blackshaw, K.J. Kirkland, D.A. Derksen & D. Wall. 2000 Herbicide-tolerant canola: Weed control and yield

comparisons in western Canada. Can. J. Plant Sci. 80:647-654.

Lutman, P.J.W., K. Berry, R.W. Payne, E. Simpson, J.B. Sweet, G.T. Champion, M.J. May, P. Wightman, K. Walker & M. Lainsbury.

2005 Persistence of seeds from crops of conventional and herbicide tolerant oilseed rape (Brassica napus). Proc. R. Soc. B

272:1909-1915. doi:10.1098/rspb.2005.3166.

Lutman, P.J.W., S.E. Freeman & C. Pekrun. 2003 The long-term persistence of seeds of oilseed rape (Brassica napus) in arable

fields. F. Agric. Sci. 141:231-240. doi:10.1017/S0021859603003575.

Lutman, P.J.W. & F. Lopez-Granados. 1998 The persistence of seeds of volunteer oilseed rape (Brassica napus). Weed Seedbanks:

Determination, Dynamics and Manipulation : 23-24 March 1998, St. Catherine's College, Oxford. Aspects of Applied Biology


Mallory-Smith, C.A. & M.L. Zapiola. 2008 Gene flow from glyphosate-resistant crops. Pest Manag Sci 64:428-440. doi:


Powles, S.B. & C. Preston. 2006 Evolved glyphosate resistance in plants: biochemical and genetic basis of resistance. Weed Tech


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