Early weeds have large impact on canola yield potential. Research has shown a yield advantage of 3 bu./ac. when weeds are removed at the 1- to 2-leaf stage of canola instead of waiting until the 3- to 4-leaf stage. That advantage rises to 7 bu./ac. when comparing weed control at the 1- to 2-leaf stage versus the 6- to 7-leaf stage.
This earlier timing, when possible due to logistics and weather, can make a big difference to overall crop profit with no added cost.
Weather influences on efficacy. Wind and soft, moist fields are common weather delays to timely spraying. Cool conditions are also a factor to consider.
After 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 cover. No biological activity = no herbicide activity.
Ideally, you want a day or two of warm and sun with nighttime lows of 5°C or higher before spraying. If applied more than 48 hours before the frost event, efficacy on living plants will be retained and the plant will continue to decline when it warms up again.