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.
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 Pioneer sales representative or agronomist can provide a specific field recommendation.
Figure 2: Grassy weed pressure in herbicide tolerant canola near the end of the CWFP.
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.
*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.
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)
- Wild buckwheat (annual)
- Wild oats (annual)
- Green foxtail (annual)
- Volunteer wheat (annual)
- Cleavers (annual)
- Chickweed (annual)
- Volunteer canola (annual)
- Spiny annual sow thistle (annual)
- Lamb’s quarters (annual)
- 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.
References
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. https://canoladigest.ca/march-2019/agronomy-insights-mar2019/.