Weed control in pulses is all in the planning.

Taking an integrated approach to resistance management in pulses

Written by: Paula Halabicki

When it comes to weed control, pulse crops can be high maintenance – but done right, they can be worth every penny. As growers are aware, pulse crops are non-competitive in nature. That’s why herbicides have been essential in preventing losses to yield and quality. But pulse crops are also sensitive to many modes of action, relying mainly on Group 2 chemistry for in-crop weed management. Widespread use of Group 2 herbicides in pulses and many other crops has led to a sharp rise in herbicide resistance. Today, 19 out of 20 resistant weed species in the Prairies have confirmed resistance to Group 2 chemistry, including wild mustard, kochia and cleavers.1

Source: Heap, I. 2016. WeedScience.org.1

Top 10 weeds by province.

Rank Alberta
1 Wild buckwheat Green foxtail Green foxtail
2 Wild oats Wild oats Wild buckwheat
3 Cleavers Wild buckwheat Barnyard grass
4 Canadian thistle Volunteer canola Wild oats
5 Dandelion Canada thistle Volunteer canola
6 Volunteer canola Spiny annual sow thistle Yellow foxtail
7 Chickweed Cleavers Dandelion
8 Lamb's quarters Lamb's quarters Redroot pigweed
9 Stinkweed Narrow-leaved hawk's beard Volunteer spring wheat
10 Hempnettle Dandelion Rount-leaved mallow

Bold = Known populations of Group 2 resistance.

Sources: Adapted from various sources.2,3,4

This has already led to serious consequences for our farming operations. If left unchecked, we could lose the use of Group 2 chemistry altogether. This limits our options for weed control, especially for pulse crops, and can raise the cost of production. Fortunately, we can manage current resistance problems and mitigate future issues by taking an integrated approach – one that includes both herbicidal and non-herbicidal strategies.

The key is planning ahead.

Rather than focusing on the pulse season alone, step back and look at your production plans over a period of several years. Are you selecting more competitive rotational crops? Are you rotating your herbicidal modes of action within and between seasons? Are you controlling weeds post-harvest or pre-seed/pre-emergence? Are you using multiple modes of effective action? These are all strategies that can help you conserve Group 2 use for the future.

Pre-season decisions.

Up the competition with rotational crops.
Growing a more competitive rotational crop in the previous season(s) will help control weeds ahead of time and reduce weed pressure. Choose a crop whose volunteers can be managed relatively easily in the pulse year. Good options include cereals and (non-Group 2) herbicide-tolerant canola.

Save your Group 2s for pulses.
Use Group 2 herbicides no more than once per season and avoid applying in consecutive seasons. Use alternative modes of action for rotational crops wherever possible. Depending on the pulse crop grown and the soil type, Group 2 carryover and stacking can be an issue. Take note of this before applying another Group 2 herbicide, as crop injury can occur with built-up residues.

Set yourself up for success.
Fall is the best time to control winter annual, biennial and perennial weed species. In the year prior to growing pulses, consider using a multiple-mode-of-action tank mix of glyphosate and a non-Group 2 post-harvest herbicide. This makes for a cleaner start the following spring.

In-season decisions.

Anticipate weeds. And which ones.
Keep records of weed species and patches. Choose herbicidal modes of action that will be effective on anticipated weeds. Avoid planting pulses in fields known to harbour perennials, biennials or Group 2-resistant biotypes.

Variety matters.
Choose more competitive pulses and varieties. Dr. Chris Willenborg at the University of Saskatchewan is looking at the competitiveness of different pea varieties and how this information can impact variety choice.5 Consider the possibility of planting mixtures of highly competitive varieties with less competitive ones.

Seed early to increase competition.
Aim to seed late April or early May to give your crops a head start. Many pulse crops are more tolerant to early spring frosts than other crops. The growing points remain underground, so new shoots can still emerge from under the soil after a damaging frost event. Seeding at a higher rate and use of a seed treatment are recommended as germination and emergence can be affected by cool soil conditions.5 Also consider narrower row spacing. Crop canopies in narrower rows are able to close faster and shade out weeds, but this will also reduce air flow and potentially impact disease development.

Critical periods for weed control.
The critical weed-free period for lentils is from the 5th to 10th node stages, while for peas it is from emergence to the 3rd node stage. Weed competition at this time will have the greatest impact on crop yield, stealing the nutrients, water and sunlight that your crop needs.

Layer your herbicides.
A pre-seed/pre-emergent burndown will provide early season control to start your fields off clean and can also address weeds that cannot be controlled later on in-crop. Tank mix products with contact and residual activity to burndown weeds and help take the pressure off your second pass. Even if residual products don’t provide complete control, they can reduce the size and number of weeds, making them easier to deal with in-crop. Ensure that your in-crop herbicide uses a mode of action that differs from your burndown – this can control a wider spectrum of weeds and help delay herbicide resistance. Also be sure to apply at correct label rates when weeds are small and actively growing to get the best efficacy. Research has shown that you can control up to 95% of weeds just by using a two-pass system.5

Use a harvest aid for weed control.
Late-season weeds can also affect yields, interfering with harvest and contributing to dockage, staining and moisture in storage – this is where a harvest aid comes in handy. Use it to control weed escapes and clean up perennials. Plus, it helps dry down your crops and weeds for an easier harvest.

Year-round considerations.

Use multiple modes of action.
Research has shown that using tank mixes or products containing multiple modes of effective action has a greater impact on delaying herbicide resistance than rotating single modes of action.5 That’s because it’s unlikely for a weed to develop resistance to more than one mode of action at once. If it escapes control from one mode, it will likely be controlled by the other, thus preventing it from reproducing and adding to the resistant population. If resistant weeds already exist, be sure to incorporate two or more modes of action that each provide control. Remember to always check that mixtures have overlapping weed spectrums, otherwise you might be depending on only one active ingredient and inadvertently screening for resistant weeds.

Keep an eye out for resistance.
Scout regularly and take note of weed populations both before and after any herbicide application. This will help identify problematic weeds and possible cases of resistance early, while you can still make an impact. Send any suspected resistant weeds for testing, and use roguing, mowing or weed wicking to prevent resistant weeds from setting seed.

Herbicide resistance management in pulses doesn’t mean giving up your Group 2 herbicides. Plan your herbicide use in advance and take an integrated approach using both chemical and non-chemical strategies. That’s the key to good herbicide stewardship and delaying the onset of herbicide resistance on your farm.


  1. Heap, I. The International Survey of Herbicide Resistant Weeds. Online. Internet. Thursday, September 29, 2016. Available www.weedscience.org
  2. Beckie, H.J. Herbicide resistance update. Presentation. Agriculture and Agri-Food Canada.
  3. Beckie, H.J. (2016) State of weed resistance in Western Canada and future outlook. SaskWheat Development Commission. Accessed http://www.saskwheatcommission.com/newspost/state-of-weed-resistance-in-western-canada-and-future-outlook/
  4. Gaultier, J. (2016). 2016 Manitoba weeds update. Crop Protection Industry Meeting. Guenther, L. (2016) Managing Group-2 resistant weeds in pulse crops across Western Canada. Grainews. Accessed http://www.grainews.ca/2016/01/19/managing-group-2-resistant-weeds-in-pulses-across-western-canada/

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