What is herbicide resistance?

Herbicide resistance is a weed's ability to survive and reproduce after it’s exposed to an effective herbicide. “Tried and true” methods aren’t working like they have in the past. And it’s only going to get worse – unless we Mix It Up.

Types of herbicide resistance

Can weeds be resistant to more than one herbicide or herbicide Group?

Resistant weeds may be resistant to only one herbicide Group or to two or more herbicide Groups. And two terms commonly used to describe how weeds resist herbicide activity include "target-site resistance" and "metabolic resistance":

Herbicides work like a lock and key mechanism: by de-activating important enzymes in weeds, they essentially "lock" them out. However, if the enzyme changes its shape, the herbicide may not be able to bind and deactivate it. And that’s when the herbicide no longer works anymore on that weed.

Here are some examples, (from least difficult to most difficult):

  • Resistance Single herbicide resistance Single Herbicide Resistance
    A biotype that is resistant to only one active ingredient. (Example: weed is resistant to active ingredient X which is a Group A, but is still susceptible to active ingredient Y, which is also a Group A).
  • Cross-resistance Herbicide resistance cross Cross Herbicide Resistance
    A biotype with resistance to two or more active ingredients belonging to the same herbicide Group. (Example: weed is resistant to multiple or all active ingredients in Group A, but is still susceptible to other herbicide modes of action).
  • Multiple resistance Multiple herbicide resistance Multiple Herbicide Resistance
    A biotype with resistance to two or more herbicide Groups. This may be the result of two or more different resistance mechanisms. (Example: weed is resistant to Group A and Group B herbicides).

Metabolic resistance prevents herbicides from reaching their target sites (usually enzymes) by:

  • Reducing herbicide absorption or translocation,
  • Detoxifying a herbicide's active ingredient,
  • "Storing" the herbicide in a cellular site that is not vulnerable to the active ingredient, or
  • Repairing damaged tissue.

Do herbicide-resistant weeds still show symptoms of herbicide damage, or would they appear untouched?
Unfortunately, it can be both. Typically,you might not identify resistant populations until there have been multiple failures with the same product or products in the same herbicide Group, identified by patches of “weed escapes.” For example, if you have observed that a herbicide has become less effective over the past three years, you’ve selected for resistant biotypes.


The evolution of herbicide resistance

In fighting herbicide resistance today, it’s important to know where it came from and how the problem has developed over the past several decades. As you can see, resistant weeds are on the increase in Canada.

Resistance is increasing. Significantly.

Canada is still in a good position to manage the onset of resistance, but it’s critical that all growers take action now. We’re in this together, and together, we can find simple ways to control resistance and keep farms productive, sustainable and profitable. Another concern is the fact that there aren’t any new herbicide patent applications on the horizon.

Don’t put all your eggs in one basket.

There has been no new herbicide patent application since 2005 and it can take up to 14 years to develop a new herbicide Group. The bottom line is, we can’t wait around for the development of the next herbicide Group; but we can still take action. It’s time we diversify current systems, tools and approaches to ensure they retain their effectiveness over the long-term.

Decline of patent applications

Resistant weeds in Canada

The following are resistant weeds that are known to exist across the country.

Weed Group(s)
Ball mustard Group 2
Chickweed Group 2
Cleavers Group 2, Group 4
Cow cockle Group 2
Green foxtail Group 1, Group 3
Hemp-nettle Group 2, Group 4
Kochia Group 2, Group 9
Narrow-leaved hawk's beard Group 2
Shepherd's-purse Group 2
Sow thistle Group 2
Stinkweed Group 2
Wild buckwheat Group 2
Wild oats Group 1, Group 2, Group 8

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Weed Group(s)
Chickweed Group 2
Cleavers Group 2
Green foxtail Group 1, Group 3
Kochia Group 2, Group 9
Lamb’s quarters Group 2
Persian darnel Group 1
Redroot pigweed Group 2
Russian thistle Group 2
Shepherd’s-purse Group 2
Stinkweed Group 2
Wild mustard Group 2
Wild oat Group 1, Group 2, Group 8

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Weed Group(s)
Chickweed Group 2
Cleavers Group 2
Green foxtail Group 1, Group 2, Group 3
Hemp-nettle Group 2
Kochia Group 2, Group 9
Pale smartweed Group 2
Powell amaranth Group 2
Redroot pigweed Group 2
Stinkweed Group 2
Wild mustard Group 2, Group 4, Group 5
Wild oat Group 1, Group 2, Group 8

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Weed Group(s)
Wild carrot Group 4
Common lamb's quarters Group 5
Green pigweed (also known as Powell's amaranth), red root pigweed, common ragweed Group 5
Barnyard grass, yellow foxtail, old witch grass Group 5
Late flowering goosefoot, wild mustard, common groundsel Group 5
Canada fleabane (also known as mare's-tail) Group 22
Red root pigweed, green pigweed (also known as Powell's amaranth), Eastern black nightshade, ragweed Group 2
Green pigweed (also known as Powell's amaranth) Group 7

Understanding modes of action

In 1997, herbicides were organized into different Groups based on their mode of action, or the way they kill weeds. Today, according to the Weed Science Society of America, there are 29 different Groups.

"Tried and true"
methods aren't working like they have in the past.

Herbicide groups often contain several products or brands. Products within the same Group have similar or even identical modes of action. Some groups have many products, others only a few, and some may contain only one herbicide because it has a unique mode of action. A product may contain more than one Group if it contains more than one active ingredient.


How resistant weed populations increase

The weeds in your field may look the same on the outside, but they may not have the same processes happening on the inside. These weeds are called “biotypes” and some of these biotypes may be resistant to certain herbicide Groups. If the same herbicide Groups are used repeatedly on the same field every year, then the biotypes which are resistant will remain in the field.

Squeezing crop rotations can favour herbicide resistance

Growing the same crop too often could mean repeating the same herbicide program too often. That means using the same herbicide, from the same Group, repeatedly, rather than rotating to herbicides with different modes of action. Since the same herbicide Groups can be used on commonly-grown crops in your area, the same population of weeds will be repeatedly exposed to the same herbicide Group, presenting greater opportunity for resistance to develop.

Overuse of modes of action

Herbicides impart a very high level of selection pressure on a resistant weed population – for example, an effective wild oat herbicide will control every wild oat in a field except the few plants that are resistant. Initially, it appears that a herbicide has worked so well that it's tempting to continue to use it. Over several years, though, repeated use of the same herbicide or group gives resistant weeds the opportunity to set seed, multiply and establish themselves in a field. Once established, moving across a field with a combine, a cultivator or even a planter/drill can spread those resistant seeds.

Don't let your
weeds go
to seed

Weed seed deposition

Controlling weeds before they produce seed will help reduce the spread of resistant plants. Simply put, don’t let your weeds go to seed.

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