Agronomy •  2022-07-05

Critical Period of Weed Control in Canola

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Why Control Weeds Early?

  • Early season weed control helps protect crop yield potential, especially during the canola seedling stage when the crop is a poor competitor.
  • Weeds and canola compete for the same resources (water, sunlight and nutrients).
  • Small weeds are easier to control and can absorb and translocate herbicide better.
  • Herbicides can be less effective during times of heat and drought stress, which often occurs with later applications. Additionally, there is greater risk of crop injury with later/out of stage applications.
  • Generally, a combination of both pre-seed and in-crop herbicide applications have the greatest potential to protect canola crop yield.
  • New glyphosate herbicide tolerant canola traits allow for a wider in-crop application window; however, it is important to keep in mind the critical weed free period (CWFP) for canola and to maximize yield potential by controlling weeds early.

Critical Weed Free Period (CWFP)

  • Defined as the stages in a crop’s life cycle during which weeds must be controlled to prevent yield loss from weed competition in crop.
  • Studies from Western Canada have found that the CWFP in canola is from emergence to the 4-leaf stage of the plant. (Martin et al., 2001; Harker et al., 2008).
  • In one study in Western Canada examining the timing of weed removal in canola, it was found that delaying weed control until the 6- to 7-leaf stage of canola resulted in a 20% yield loss (Harker et al. 2008; Figure 1).
  • Another benefit of early season weed removal is the prevention of weed seed production.

Influence of time of weed control on canola yield

Pre-Seed Application

  • Applying recommended, labeled herbicides prior to seeding reduces overall risk of yield loss due to weed competition, especially if an in-crop herbicide application is delayed.
  • A pre-seed herbicide application is highly recommended, especially for fields with an abundance of winter annual and perennial weeds.
  • If a pre-seed application is not an option, consider applying control measures immediately after seeding prior to crop emergence. Keep in mind that this can be a very narrow application window.

Post Emergent Applications

  • The best time to apply in-crop herbicide applications to canola is from the 1- to 4-leaf stage.
  • After the 4-leaf stage, canola plants are much more competitive and emerging weeds have less effect on yield.
  • As the crop canopy closes, late emerging weeds have a reduced effect on yield. Second in-crop applications may produce a smaller ROI but can help manage weed escapes from the first herbicide pass or crops with low plant populations.
  • In some cases, a pre-harvest or post-harvest herbicide application is more effective at controlling weed escapes than a second in-crop herbicide application – especially in the case of perennial weeds.
  • Tank mix options are available for both pre-seed and in-crop applications to enhance weed control. Always read and follow label directions.
  • However, all situations are unique and need to be evaluated on a field-by-field basis. Your local Brevant® seeds sales representative or agronomist can provide a specific field recommendation.

Grassy weed pressure in herbicide tolerant canola near the end of the CWFP.
Figure 2: Grassy weed pressure in herbicide tolerant canola near the end of the CWFP.

Kochia competition in canola. July 2019. Saltcoats, SK.
Figure 3: Kochia competition in canola. July 2019. Saltcoats, SK.

Herbicide Tolerant Canola Systems from Corteva Agriscience

Table 1. List of herbicide tolerant canola system options available from Corteva Agriscience.

List of herbicide tolerant canola system options available from Corteva Agriscience.

*Refer to individual product labels for complete instructions on rates, tank mix partners, staging, application timing, rainfastness, etc.
**REL = Roundup Equivalent Litre. 
***Requires Surjet Surfactant.
tAvailability subject to regulatory approval.

Herbicide timing of herbicide tolerant systems available from Corteva Agriscience.

Weeds of Concern

Weed surveys are conducted in the Prairie provinces on a recurring basis. The latest prairie weed survey (2014-2017) listed the following as the top 10 weeds in canola (Canola Digest, 2019)

  1. Wild buckwheat (annual)
  2. Wild oats (annual)
  3. Green foxtail (annual)
  4. Volunteer wheat (annual)
  5. Cleavers (annual)
  6. Chickweed (annual)
  7. Volunteer canola (annual)
  8. Spiny annual sow thistle (annual)
  9. Lamb’s quarters (annual)
  10. Canada thistle (perennial)

Future Research

  • The majority of research regarding the critical period of weed control in canola was conducted over 15 years ago.
  • There is ongoing research in Western Canada, specifically at the University of Manitoba, investigating the CWFP in canola given the myriad new herbicide technologies (pre-emerge and in-crop), improved hybrid competitiveness, and changes to recommended seeding rates in canola.  


Harker, K. N., O’Donovan, J. T., Clayton, G. W., & Mayko, J. (2008). Field-Scale Time of Weed Removal in Canola. Weed Technology, 22(4), 747–749.

Martin, S. J., Van Acker, R. C., & Friesen, L. F. (2001). Critical Period of Weed Control in Spring Canola.  Weed Science, 49 (3), 326-333.

Canola Digest. Top 10 Weeds in Canola. March 5, 2019.