Using your Group 2’s wisely.

Strategies for managing Group 2 resistance in pulses

Written by: Bryce Geisel

Herbicide resistance is particularly challenging for pulse crops. Not only are they poor competitors to weeds, pulses are more limited in herbicide options than many other crops. As a result, they rely almost exclusively on Group 2 herbicides for in-crop broadleaf control. This is an issue when some of the most prevalent weeds in Western Canada have known populations of Group 2 resistance, including wild mustard, kochia, cleavers, lamb’s quarters and stinkweed.1

Weed Herbicide group resistance ("+" indicates multiple resistance)
Alberta Manitoba Saskatchewan
Ball mustard 2 - -
Common chickweed 2 2 2
Common hempnettle 2, 4 2 -
Common lambs-quarters - - 2
Cowcockle 2 - -
Cleavers 2+4, 2 2
Stinkweed 2 2 2
Green foxtail 1,3 1, 2, 3, 1+3 1, 3, 1+3
Kochia 2, 2+9 2, 2+9 2, 2+4, 2+9
Narrowleaf hawksbeard 2 - -
Pale smartweed 2 2 -
Persian darnel 1 - 1
Powell amaranth - 2 -
Redroot pigweed - 2 2
Russian thistle 2 - 2
Shepherd’s purse 2 - 2
Spiny sowthistle 2 - -
Wild buckwheat 2 - -
Wild mustard 2 2,4,5 2
Wild oats 1, 2, 8, 1+2+8 1, 2, 8, 1+2+25, 1+2+8+25, 1+2+8+14+15 1, 2, 8, 1+2+8

Reported populations of resistant weeds in Canada.
Source: Heap, I. The International Survey of Herbicide Resistant Weeds.1

How resistance comes about.

Herbicides work by inhibiting important enzymes or processes key to a weed’s survival. This is known as the “mode of action”. Herbicides that target the same enzymes or plant processes (e.g. photosynthesis) are given the same Group number. For instance, herbicides that fall under Group 2 all target the acetolactase synthase (ALS) enzyme responsible for producing amino acids. This is essential to plant growth and development.

Weeds resistant to herbicides contain natural, genetic variations that make the herbicide ineffective. This will occur in a weed population regardless of whether a herbicide is applied or not. The most common one is a result of one or more mutations to the herbicide’s target enzyme. It changes the enzyme, preventing the herbicide from binding and allowing the pathway to continue uninterrupted, resulting in continued plant growth. With Group 2-resistant weeds, the mutation is in the ALS enzyme.2 The structural change only prevents the herbicide from binding and doesn’t affect enzyme function itself, allowing the weed to continue to grow and develop.

Once resistance appears, continued use of the same herbicide or herbicide Group will only screen for resistant weeds and eliminate the susceptible ones. Unless another mode of action is introduced, the resistant ones will continue to grow and set seed, increasing the resistant population until they eventually dominate the field. Weeds that remain actively growing after a herbicide application amongst controlled weeds of the same species warrant further scrutiny beyond the assumption of a sprayer miss. This could be the start of herbicide resistance in that field.

Target-site resistance visualized

Source: Adapted from Beckie, H. 2009. Herbicide resistance update.

How quickly resistance spreads.

As it turns out, the risk of developing resistance depends on the herbicidal mode of action. Researchers have classified this risk based on the number of applications it takes to select for resistance. For Group 2 herbicides, the risk is incredibly high as fewer than 10 herbicide applications are enough for a weed with resistance to show up in a field.3 That’s because plants have several naturally occurring forms of the ALS enzyme,4 and all it takes is a random mutation in one gene to develop resistance. In other words, Group 2-resistant weeds occur more frequently than most other Groups in any given field. So the more Group 2 herbicides are used, the more likely you are to come across a resistant weed.

Source: Saskatchewan 2016 Guide to Crop Protection. Adapted from: Beckie, H.J. 2006.3

Certain weeds are also more likely to develop resistance to herbicides. These weeds typically produce large amounts of seed with high levels of genetic diversity. Because of their short lifecycle and high levels of seed production, annual weeds are particularly prone to resistance.

Conserving Group 2 chemistry

Group 2 chemistry is one of the most important herbicide Groups used, having products applied in cereals, canola, and soybeans in addition to pulse crops. The more Group 2 resistance continues to spread, the fewer herbicide options we have. Plus, it limits cropping decisions as pulses have few options of other chemistries. If resistance isn’t kept in check, the need for multiple chemistries will lead to higher costs.

Targeting Group 2 resistance is twofold: managing existing resistant weeds and preventing the onset of further resistance. That’s not to say farmers have to stop using Group 2 chemistry altogether – it just needs to be managed. To conserve the use of this chemistry, consider the following best practices:

Break out the Group 2 herbicide once per season

Even though different products within the same group may target different weeds, the underlying chemistry is similar. Simply rotating into a different product or brand that belongs to the same herbicide group will not help prevent herbicide resistance. Rotate into alternative modes of action within a season and avoid applying Group 2 herbicides in consecutive seasons.

Apply multiple modes of action with overlapping weed spectrums

This prevents selection pressure from one active ingredient. For instance, Group 1 herbicides do not effectively control cleavers. Used in a tank mix with a Group 2 herbicide, the application ultimately relies on Group 2 action and selects for Group 2-resistant cleavers. For MMOA to be an effective strategy, active ingredients must have overlapping weed spectrums so resistant weeds aren’t given an opportunity to set seed. If Group 2 resistance already exists in your fields, ideally two other modes of action will be used to avoid selecting for multiple resistance.

Conserve your Group 2’s

Wherever possible, use non-Group 2 herbicides in other crop rotations. This will help save Group 2 products for when there are few other options, like pulse crops.

Layer your herbicides

Consider rotating your herbicidal modes of action in your two-pass approach. Start your fields off clean with an effective burndown, and choose a product with residual activity to keep weeds small for the in-crop herbicide. The pre-seed burndown will take care of any emerged weeds, while your residual activity and in-crop herbicides can be used to target flushing or later emerging weeds.

In addition to sustaining Group 2 herbicides, be sure to take up other practices that can help manage herbicide resistance. This includes scouting before and after herbicide applications, using proper label rates and keeping detailed records of herbicides. Minimize weed seeds by planting certified seed, using clean equipment and establishing your crop early on to outcompete the weeds. Take an integrated approach to managing herbicide resistance, and help lengthen the effectiveness of all available chemistries.


  1. Heap, I. The International Survey of Herbicide Resistant Weeds. Online. Internet. November 15, 2016. Available www.weedscience.org
  2. Hall, L. Defence mechanisms. Top Crop Manager. July 2016.
  3. Beckie, H.J. 2006. Herbicide-resistant weeds: Management tactics and practices. Weed Technology 20(3): 793-814.
  4. Rainbolt, C.R., Thill, D.C., Yenish, J.P. and Ball, D.A. 2004. Herbicide-resistant grass weed development in imidazolinone-resistant wheat: weed biology and herbicide rotation. Weed Technology 18:860-868.

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