Features Agronomy Weeds
Canada fleabane a minor problem in most crops

Concerns over Canada fleabane have surfaced in recent years, especially in
the corn and soybean growing regions of the US. Fortunately, Canadian prairie
farmers have researchers investigating effective tools to control this weed.

Denise Maurice, technical development manager for Agricore United, explains
that Canada fleabane, called horseweed or mare's tail in the US, is ranked 69
in the latest Prairie Weed Survey, so it is not widely distributed in
western Canada. This weed species is typically a late winter annual, although
it is occasionally a spring annual. The plant starts to bolt in late May, producing
one or more flowering stems. The stems can range from about four to 70 inches
tall and have many narrow leaves. The small daisy-like flowers bloom in mid
July and produce seed mainly in early August.

Canada fleabane is found mostly around field edges and grain bin areas. Photos Courtesy Of Rick Holm, University Of Saskatchewan.

The seeds are not dormant when they fall to the ground. As a result, most of
the seeds germinate in the same season, especially if rainfall occurs in mid
to late August. The seeds easily germinate on the soil surface, with seedlings
emerging in late summer or early fall. They grow to form small rosettes with
dark green leaves which hug the ground for overwintering. A few of the seeds
germinate in the following spring.

Maurice says, "Canada fleabane seed is very much like a dandelion seed
in that it has a pappus (parachute) attached to it, so it is carried readily
in the wind and can spread from field to field." She adds, "A healthy
plant that has not had any competition can produce about 230,000 seeds. If the
plant is under a little more competition in-crop, it is not as aggressive and
doesn't produce as much seed."

Concerns about Canada fleabane are in the news media these days because the
weed has developed resistance to glyphosate in some American states. Initially
identified in Delaware in 2000, glyphosate resistant biotypes of Canada fleabane
had been found in 14 states by 2005 and just recently, Nebraska confirmed a
resistant biotype. No glyphosate resistant biotypes have been identified in
Canada to-date.

Maurice says resistance to glyphosate likely developed in the eastern US because
farmers there often follow a two year soybean-corn rotation using both Roundup
Ready soybean and Roundup Ready corn. She explains the rapid spread of the resistant
biotype in the US is probably the result of several factors.

"One factor is the very few in-crop options to control the weed in these
crop rotations in the US. Also, the biology of the species means that seeds
that fall to the ground can germinate in that season, so you've got that quick
cycling going on," says Maurice. "On top of that, the seed is able
to spread easily with the wind. And with large plants being able to produce
so many seeds, if one plant escapes control, you can get a buildup of a population
fairly quickly."

Fortunately, farmers in western Canada have access to several control options.
In cropping systems that use tillage, the weed can be controlled by spring or
fall tillage. In no-till systems, the herbicide options that farmers used to
control many other weeds in cereal crops are also effective on Canada fleabane.
Maurice notes, "There are not a lot of herbicide labels that list Canada
fleabane as a weed that is controlled." However, recent research has shown
that the weed is effectively controlled with either a pre-harvest glyphosate
application or a post-harvest or pre-seeding application of phenoxys.

A fall herbicide application with glyphosate or 2,4-D is usually the best option.
Maurice says, "The mid August to October window dictates whether or not
you're going to have a lot of winter annuals in the next year. It's more cost-effective
to control winter annuals like downy brome or Canada fleabane or narrow-leaved
hawk's beard in the fall."

However, if a fall application is not an option, a spring application will
work for Canada fleabane. "Because Canada fleabane tends to be a late flowerer,
you have a wider window to control it. But no matter what, earlier is always
better," she explains.

Small daisy-like flowers bloom in mid July and produce seed mainly in early August.

With the variety of effective control options, Canada fleabane remains a minor
weed problem on the prairies. "At present, Canada fleabane is found mainly
around the bins and along road edges," notes Maurice. "We do crop
scouting across western Canada and we covered about two million acres in 2006.
We found some Canada fleabane in farmers' fields but generally scattered to
slight populations."

Maurice concludes, "At this time, I'm not seeing that Canada fleabane
is going to be a significant threat in the short-term, because of the fact that
phenoxys readily control it and because, with the crop and herbicide rotations,
we are at a lower risk for development of glyphosate resistant Canada fleabane
than in the US. Perhaps in southern Manitoba if you are growing specialty crops
like field beans or sunflowers with fewer herbicide options, it may be a greater
concern. But in wheat, barley and canola, our main crops, it shouldn't be much
of an issue.

"Herbicide tolerant crops, including those based on glyphosate herbicide,
can remain useful components of crop production systems when properly managed.
Proper use of this technology is an important component of an integrated weed
management program to preserve the long-term benefits and manage the threat
of resistance."