Pre-seed burnoff: Tips for best results

Early weed control is important for canola profitability, and pre-seed burnoff is a good way to remove perennial and winter annuals weeds that are growing quickly and taking up moisture and nutrients. Waiting can also make them that much more difficult to control. A small number of weeds (even just a few per square foot) emerging before or with the crop can be much more damaging to yield potential than a larger number of weeds flushing later.

The Canola Encyclopedia includes pre-seed burnoff in its section on weed management timing. Here are a few more things to consider…

Scout for weeds. Can you see perennials and winter annuals not controlled in the fall? These will get larger by the day, so getting them early will make them easier to control. You may also find small annuals starting to emerge. Early weed control with lower efficacy is generally preferable to no control at all or 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.

Specific weeds to watch for.

  • Winter annual cleavers 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 is a clubroot host, which adds to the incentive to spray it early. 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. Waiting that long will make it more and more difficult to control the weed in-crop.
  • Volunteer canola is a 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.

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. Two main reasons for this 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. It will also mean good sprayer clean-out is required between fields. But the extra effort is worth it to protect glyphosate efficacy for the long term. The Canola Encyclopedia has a list of pre-seed herbicide options, including glyphosate tank mixes, for use prior to seeding canola.

How long between spraying and seeding? Seeding can occur 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, the crop can be seeded. For perennial weeds, the recommended delay ranges from 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.

Seeding is two weeks away. Should you spray? 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.

Post-seeding, pre-emergence. If pre-seed burnoff is missed, 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, and since growers need to leave weeds for a couple days to start growing again after the seeding operation, you many only have a couple of days to spray within this window. The time lag between seeding and spraying is necessary to allow weeds buried by the seeding operation to re-emerge and to allow weeds uprooted to overcome the stress placed on them. However, spraying and missing a few of these weeds could be more beneficial than waiting to spray in-crop – but check for crop emergence before applying products that need to be applied pre-emergence.