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Don’t Ignore Herbicide Resistant Weeds

Herbicide resistant weeds in Western Canada

This list is compiled from number in brackets is the herbicide group to which resistance has been confirmed. Wild buckwheat (2) Wild oats (1) (2) (8) (multiple groups) Common hempnettle (2) (4) False cleavers (2) (4) (multiple groups) Kochia (2) Ball mustard (2) Green foxtail (1) (2) (multiple groups) Wild mustard (2) Spiny sowthistle (2) Common chickweed (2) Field pennycress (2) Redroot pigweed (2) Wild mustard (2) (4) Shepherd’s purse (2) Persian darnell (1) Russian thistle (2)

Eight questions about herbicide resistance in weeds were posed to a variety of agricultural professionals — from weed scientists to herbicide reps, from farmers to extension specialists. The message? Herbicide resistance is here and we’re not doing enough about it. Ignoring the problem won’t make it go away. Read on to find out what’s out there and what to do about it.

1. Are farmers concerned about herbicide resistant weeds?

“In focus groups, 60 per cent to 70 per cent of growers tell us herbicide resistance is an important issue,” says Jon Gough, product manager with DuPont.

Unfortunately, what growers say and what they do, seem to be two different things.

A comprehensive 2005 Purdue survey of midwestern U. S. growers found only 48 per cent of growers scout their fields to determine weed problems. Only 60 per cent of growers believe repeated use of same herbicide mode of action contributes to weed resistance. Only about half of the surveyed farmers believe rotating crops or herbicides minimize glyphosate resistance. Less than half of growers believe tank-mixing glyphosate with another active ingredient minimizes resistance. Less than a third of growers thought tillage would reduce glyphosate resistance. And only 30 per cent of farmers believe glyphosate resistance is a serious issue.

Hugh Beckie, research scientist specializing in herbicide resistant plant study with Agriculture and Agri-Food Canada says, “Growers are aware of the issue but not of resistance on their farm. They normally only change their practices once resistance has occurred and is becoming a problem.”

Chris Vander Kant, BASF western herbicides market manager agrees, reporting that most growers do not realize they have a herbicide resistance problem on their own farms until 30 per cent of a weed species is not being controlled with a herbicide.

2. Is the problem of herbicide resistance increasing?

Vander Kant says “the incidence of weed resistance to herbicides is on the rise. The number of acres where herbicide resistant weeds are present is increasing each year.”

In a presentation to the first Bayer CropScience weed resistance conference which was held in Miami, Florida in January of this year, Beckie described field surveys done from 2001-03 which found herbicide resistant weed species on 20 per cent of the cultivated land in western Canada, however random field sampling done in Alberta in 2007 found group 1 resistant wild oats had increased to 39 per cent of the fields surveyed.

“I estimate 40 per cent of annual cropped land is now infested with a herbicide-resistant weed biotype based on the latest random field surveys,” Beckie says.

Even more troubling was the response from Mike Cowbough, weed management field crops program lead, Ontario Ministry of Agriculture, Food and Rural Affairs, who reports the University of Guelph is receiving an increasing number of weed samples submitted for testing which are resistant to more than one mode of action. “Weeds that are resistant to both Group 1 and Group 2 herbicides are becoming more common.”

3. Which weeds should farmers be concerned about?

The resistant weed list is already long and getting longer. The website www.weedscience.orgidentified 70 species of weeds in the U. S. in 2008 resistant to one or more herbicides. The same website identifies 32 herbicide resistant weed biotypes in Canada. Beckie told the Bayer conference that in Canada Group 2 resistance has already been identified in 19 weed species.

Herbicide resistant wild oats, green foxtail, kochia, chickweed, cleavers, wild mustard and spiny annual sow thistle are currently of the most concern in Western Canada.

Ed Thiessen, technical crop manager with Syngenta, says in some fields 95 per cent of kochia is now group 2 resistant. “The incidence of group 2 resistant kochia is so common growers have to assume all kochia is resistant to group 2 herbicides.”

Weeds growers should also be paying close attention to, but that are not yet major concerns in western Canada, are red root pigweed, stink-weed, wild buckwheat and Russian thistle.

4. Is rotation of herbicides enough to prevent herbicide resistant weeds?

The short answer? No!

All respondents were emphatic in stating growers have to do more than simply rotate chemistries to reduce the development of herbicide resistance in weeds.

5. What should be done to reduce the development of herbicide resistance?

Use diverse crop rotations. Grow a rotation of cereals, pulses, oilseeds and specialty crops. Diversify the crop rotation lifecycle by growing fall seeded crops if possible. Including at least one year in six of fallow or a perennial forage crop in a crop rotation is one of the best ways of reducing the development of herbicide resistance.

Use a pre-seed burn off to ensure a clean seed bed.

Plant clean, pure seed

Scout fields before a herbicide application to know what weeds are present and to determine which is the best herbicide(s)to use to control that spectrum of weeds

Apply in-crop tank mixes of herbicides that have different modes of action

Use full rates of herbicides and apply them as per label directions when weeds are actively growing and before weeds set seeds.

Avoid using the same herbicide or herbicides with the same mode of action multiple times a year or year after year.

Scout fields after spraying and control any weed patches what survive a herbicide application.

6. Is weed resistance to glyphosate a problem?

There are now 17 weed species world wide that have confirmed resistance to glyphosate according to

In the U. S., 10 weed species are glyphosate resistant. In the state of Georgia, 100,000 acres are now considered severely infested with glyphosate resistant pigweed. Because of the increasing incidents of glyphosate resistance in weeds, cultivator sales in the US are actually increasing significantly.

In Canada, it is suspected there is glyphosate resistant giant ragweed in southern Ontario. Testing is ongoing to confirm this resistance.

While respondents agreed glyphosate resistance in weeds in Western Canada is not a problem at this time, it certainly has the potential to become a problem unless growers practice good stewardship to protect this important herbicide.

7. What is cost of herbicide resistance?

Trying to put a dollar figure on what herbicide resistant weeds cost a grower is very difficult.

Of course there is the cost of not being able to use a particular herbicide, and a cost of possibly having to apply more expensive herbicides or herbicide tank mixtures in order to control the resistant weeds.

But there is also a real cost arising from reduced yields due to uncontrolled herbicide resistant weeds competing with the crop. Cowbough put an actual number to the yield loss due to resistant weeds in an article he wrote a few years ago. Using yield loss data developed by the University of North Carolina, Cowbough calculated that in a soybean crop with an average of just two herbicide resistant redroot pigweed plants per square meter, the expected yield loss would amount to 15.56 per cent of the potential yield. In other words, if a clean field yields 45 bushels per acre, then just two uncontrolled, resistant pigweeds per square meter would reduce yields by 7.5 bushels per acre. At a selling price of $7 per bushel this would result in a cost of $56.16 per acre.

Cowbough added that Ontario has now adapted this yield loss calculator for Ontario crops and weeds. It is intended to assist Ontario growers in herbicide selection so growers are selecting herbicides which will control the most economically important weeds in their fields. While no comparable yield loss tool is available to western growers, prairie farmers interested in seeing the economic impact of weeds on Ontario crops can view the selector at

8. Are there any solutions to weed resistance in the works?

In the April 14, 2009 Purdue University news release “Farmers relying on Roundup lose some of its benefit,” Bill Johnson, associate professor of weed science, Purdue, says, “Farmers do not think resistance is a problem until they actually have it. And they think the chemical companies can turn on the spigots and produce a new herbicide whenever they want. The problem is, since Roundup is so effective, there’s not been any money for new herbicide discovery.”

Growers hoping that a magic bullet to address herbicide resistance will appear in the near future are likely to be disappointed. That is not to say there haven’t been advances in the last few years. Just a few years ago Bayer introduced the herbicide Infinity with an entirely new mode of action, Group 27. This gives growers another chemical rotation option. This spring BASF is introducing an entirely new active ingredient in their new burn off product, Heat.

A number of herbicide manufacturers are also introducing new herbicide tolerant traits into plants. Bayer is close to introducing a dual herbicide tolerant trait (double stack) in cotton. These plants will be tolerant of both glufosinate-ammonium and glyphosate allowing an application of both group 10 Liberty and group 9 glyphosate which will give growers two modes of action on weeds in this cotton crop variety. Bayer is also working on a triple stack combination for soybeans giving this soybean variety tolerance not only to Liberty and glyphosate but a third class of chemicals as well. Bayer hopes to have this to market around 2015.

Other chemical companies are also involved in double and triple stacking traits. Dow is working on a “fop” + 2,4-D + glyphosate resistant corn, and a 2,4-D + glyphosate resistant soybean. Monsanto is working on a dicamba + RR soybean. DuPont/ Pioneer is working on the Optimum GAT corn which has glyphosate + ALS stacks and with Liberty stacks in soybean.


Weed resistance isn’t the result of poor crop managers, necessarily. The 2001-03 resistant weed survey revealed farms larger than 1,000 acres had a higher risk of weed resistance than smaller farms. Highest risk was found in low disturbance, zero tillage systems, a slightly less risk in reduced tillage systems and a reduced risk when perennial forage or fallow included at least one year in a six-year rotation.

Gerald Pilger farms at Ohaton, Alta.

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