Pre-seed burnoff: Tips for best results

Early weed control is important for canola profitability. Pre-seed burnoff removes perennial and winter annuals weeds that are growing quickly and taking up moisture and nutrients. Delayed applications can make these early weeds more difficult to control.

A small number of weeds (even just a few per square foot) that emerge before or with the crop can do much more damage to yield than a larger number of weeds flushing later.

The Canola Encyclopedia includes pre-seed burnoff in its section on weed management timing.

Tips for best results

Scout for weeds

Can you see perennials and winter annuals that were not controlled in the fall? They are easier to control with an early spray. Early weed control with lower efficacy is generally preferable to late control with higher efficacy – as long as weeds are not frost damaged. (See more on frost below.) Any weed flushes that emerge later can be hit with an early in-crop application.

Wait for warm, sunny daytime conditions

This is especially helpful when targeting perennials or winter annuals that may be a little larger. Ideally, you want a day or two of warm sunny days and night time lows of 4°C or higher before spraying.

Add a tank mix to glyphosate

Main reasons to tank mix are:

  • One, the tank mix reduces selection pressure for glyphosate-resistant weeds, protecting glyphosate as an effective mode of action.
  • Two, using two modes of action can improve weed control, especially for weeds like cleavers and kochia.

Note that with a tank mix, you will want to use higher water volumes than you would for straight glyphosate. While tank mixing requires good sprayer clean-out between fields, the effort helps to protect glyphosate efficacy for the long term. The Canola Encyclopedia section on Timing includes links to the provincial crop protection books. These books are updated annually with pre-seed burnoff herbicides.

Timing

Farmers can seed fairly soon after spraying. For annuals and winter annuals, glyphosate needs only 24 hours to get to the growing point and set the control process in motion. After a day, farmers can seed the crop. For perennial weeds, wait three to five days, depending on weather conditions. If sunny and warm, translocation will take place fairly quickly so 72 hours (three days) should be enough.

Even if seeding is two or three weeks away, a pre-seed burnoff now could keep these weeds from getting too big to control. See point 6 in this article.

Weeds to watch for in fields planned for canola:

  • Kochia. Kochia germinates early (80 per cent of kochia seedlings emerge before the crop), making pre-seed burnoff can be an effective management strategy. However, because kochia germinates early, plants can be sufficiently large and survive standard herbicide rates in situations where pre-seed burnoff is delayed. Kochia has widespread resistance to herbicide groups 2 and 9, and confirmed resistance to groups 4 and 14. Many kochia populations have stacked resistance to multiple groups. With pre-seed burnoff of kochia patches, (1) apply early, (2) use an effective tank mix and (3) scout after application for escapes that could be resistant. How to contain herbicide-resistant kochia
  • Winter annual cleavers. These are best sprayed in the pre-seed window with rates needed for control. Cleavers with large established roots are probably winter annuals and will need a higher rate. If roots are small and look like they germinated in the spring, a lower recommended rate may be enough. With large winter annual cleavers, it is best to wait 3-5 days before seeding to give the herbicide a chance to work.
  • Flixweed. This weed is a clubroot host. Clubroot spores become active with a soil temperature around 15°C and temperatures of 20-25°C are optimal for infection. While soil temperatures may not be at that level, flixweed growing strong will have a lot of root area – which will be optimal for early infestation and possibly large gall formation. Within three weeks, galls can be mature enough to produce spores. So waiting that long to control the weed could allow it to contribute to the clubroot spore-bank. At this stage, it is also more difficult to control with in-crop rates.
  • Volunteer canola. This weed and competes with the crop for nutrients and water and sunlight. Volunteers in a canola crop do not make a positive contribution to yield. Growers also have other reasons to get rid of them: Volunteers do not have seed treatment, so they can introduce seedling diseases and increase flea beetle pressure. Also, volunteers in non canola years provide a host for blackleg and clubroot, reducing the effectiveness of crop rotation for managing these issues. Even if a clubroot-resistant variety, the second generation volunteers may not have the same level of clubroot resistance, which means they could have big spore-producing galls.

Assess frost damage

After a frost, wait for at least one night with a low of 5°C or warmer and one day of warm, sunny conditions. If heavy frost caused tissue damage to more than 40% of weed leaf area, wait for new growth to show before spraying. If spraying was done more than 48 hours before a frost event, efficacy on living plants will be retained and the plant will continue to decline when it warms up again. This frost article includes considerations for weed management.

Post-seeding, pre-emergence

If fields miss a pre-seed burnoff, one option is to apply glyphosate (or glyphosate with an appropriate tank mix partner) in the narrow post-seeding/pre-emergence window. Shallow-seeded canola can emerge within five days under warm and generally moist soil conditions. Since growers need to give weeds a couple days to start growing again after the seeding operation, this window may only be a couple days long. However, spraying and missing a few of these weeds could be more beneficial than waiting to spray in-crop. Check for crop emergence before applying products that need to be applied pre-emergence.

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