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Early weed control in canola imperative

May 1, 2013, Winnipeg, MB - Canola growers will want to get the drills moving this spring as soon as the snow has cleared and the fields can support equipment, says the Canola Council of Canada. But even though seeding has started later than usual, early weed control remains a high priority.

Fields with a large population of weeds, especially advancing winter annuals, should get a pre-seed burnoff, says says Kristen Phillips, Canola Council of Canada (CCC) agronomy specialist. "There is evidence that a pre-seed burnoff will result in higher yields versus no pre-seed burnoff, so you're often better to spray before seeding rather than trying to control the weeds in crop."

After the snow has cleared, start walking the fields to see what weeds are present and how fast they're advancing. Seeding can begin fairly soon after a burnoff. For annuals and winter annuals, glyphosate needs only 24 hours to get to the growing point. After a day, the crop can be seeded. For perennial weeds, three days should be enough in sunny and warm conditions but five days is recommended before seeding if weather is cloudy or cool.

Herbicides in general tend to work best in warm sunny conditions when weeds are actively growing and cycling nutrients into their growing points. Applications made when cool cloudy days follow cool nights will result in lower herbicide efficacy than applications made in warm sunny days.

Applications made in cold conditions often result in poor weed control. For example, cloudy days with highs of 10 degrees C after a night near 0 degrees C will tend to result in herbicide performance at the low end. Biological activity would have stopped during the night, and would not start up again until the plant warmed to at least 5 degrees C — and even then it would be very slow. No biological activity means no herbicide activity.

The question many growers will have this spring is whether lower efficacy from an early spray in cold conditions is better than delaying a week for better conditions if it means the weeds are that much bigger. "The better decision, from a herbicide efficacy point of view, is to wait the week," Phillips says. "Ideally, a grower wants a day or two of warm sunny days and night time lows of 4 degrees C or higher before spraying."

A grower looking at the calendar may balance the risks and choose to seed before spraying. "In that situation, those weeds will have a whole extra week or more before the crop emerges, and can advance very quickly in good conditions," Phillips says. "Growers who make this decision may want to apply in the narrow post-seeding pre-emergence window to keep weed competition in check before the crop comes up rather than wait for post emergence."


May 1, 2013  By Canola Council of Canada


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