Features Agronomy Weeds
Weed resistance management

Weed resistance to herbicides is a real and complex issue for growers today
and resistance to many herbicide groups can be found in all parts of western
Canada. While weed resistance has been known in western Canada for several years,
it is sometimes forgotten when growers sit down to map out their planting options
and herbicide rotations. Rob Neyedley, technology manager for Monsanto Canada,
would like to see this change, urging growers to think carefully about their
herbicide rotations and make resistance management a regular part of their crop
management plans.

"With more glyphosate options available to growers and a number of herbicide
tolerant cropping systems available in the marketplace, it is no surprise there
has been an increased level of talk about glyphosate resistance in the past
couple of years," says Neyedley. "Growers should take weed resistance
seriously, but they also need to look at the data and focus their weed resistance
management strategies at the right targets."

Globally, weed resistance to glyphosate is low, with only 11 species confirmed
as resistant to glyphosate. Historically, glyphosate use in North America has
resulted in a low incidence of glyphosate resistant weeds and after more than
30 years of use, there are no glyphosate resistant weed species in Canada.

The incidence of resistance to glyphosate (Group 9, glycines) is low when compared
to other products in Group 1 – ACCase (Centurion, Select, Fusion, Assure
II, Poast, Horizon, Achieve, Puma, Venture) and Group 2 – ALS (Refine,
Express, Frontline, Muster, Odyssey and Pursuit) herbicides (see Figure 1).
Glyphosate has shown a pattern of resistance occurrence, similar to the Synthetic
Auxins (like 2,4-D).

"The research shows that the use of Roundup brand herbicides in Roundup
Ready canola, grown in crop rotation, does not significantly change the risk
of glyphosate resistant weeds occurring in western Canada," says Neyedley.
"In reality, glyphosate is actually a low risk herbicide for developing
weed resistance. Plus, in western Canada, Roundup brand herbicides used in Roundup
Ready canola are rotated with other herbicide groups in different crops, which
could also lower the risk of weed resistance developing."

Another way Neyedley suggests growers might consider looking at weed resistance
is to consider how glyphosate provides an effective alternative to some other
herbicides they might use, many of which have a higher risk of resistance occurrence.
For instance, an average of one in six fields in western Canada has wild oats
resistance problems.

"Herbicides commonly used by growers are similar in that they control
weeds but they differ in the level of risk for development of weed resistance,"
cautions Neyedley. "It is important growers don't lose sight of this fact."

Research conducted at the University of Alberta and Agriculture and Agri-Food
Canada (AAFC) seems to support this assertion. Researchers have classified the
various herbicide groups based on incidence of occurrence and field use history
(see Figure 2). For example, Group 1 herbicides are classified as high risk
herbicides and resistance has developed after 10 or fewer applications.

Managing resistance to Group 1 herbicides, therefore, can be difficult for
growers using systems such as the Liberty Link system, where the use of Group
1 herbicides is often recommended to improve grass weed control and when growers
also rely upon Group 1 herbicides to control grasses in rotational crops such
as cereals and pulses. This is not a problem in a typical Roundup Ready crop
rotation.

"Glyphosate can be applied at multiple times through the year in pre-seed,
in-crop and pre-harvest situations, but it is usually applied in various fields,"
explains Neyedley. "These applications target different weed species and
populations which minimizes overall selection pressure for glyphosate resistance."
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Also see The time to prevent glyphosate
resistance is now on page 20.