Seed first or spray weeds?

Early weed control is an important step in profitable canola production. Saskatchewan research on preseed weed control in wheat showed that early weed control was more important to yield than early seeding.

Pre-seed weed control will manage weeds that emerge ahead of seeding, reducing crop competition for light, moisture and nutrients.

Growers waiting to seed may find that fields too wet for the seeding unit may support the sprayer — although deep ruts are not great for the seedbed. High flotation tires on the sprayer will reduce rutting.

Cool, wet conditions that are holding up seeding can also reduce herbicide efficacy, but 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 present and not frost damaged.

Clark Brenzil, provincial specialist, weed control, with the Saskatchewan Ministry of Agriculture, explains how cool, cloudy conditions affect herbicide efficacy:

In the case of herbicide applications following a nighttime frost or near-frost event, the herbicide activity on a cloudy cool day would be next to zero. Biological activity would have stopped during the night, and would not start up again until the plant warmed to at least 5°C — and even then it would be very slow. A few hours between 5°C and a daytime peak of 10°C would not be enough warmth to get plant metabolism going to a point where herbicide was all that effective, especially with the cloud. No biological activity = no herbicide activity. Ideally, you want a day or two of warm sunny days and night time lows of 4°C or higher before spraying. If applied more than 48 hour before the frost event, efficacy on living plants will be retained and the plant will continue to decline when it warms up again.

If faced with a decision to spray some fields and not others, be sure to spray Clearfield fields before seeding. Otherwise you have to wait until the two-leaf stage of the crop to spray.

Further reading:
Weed out volunteers, perennials and winter annuals
The Canola Encyclopedia section on weed control timing