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Herbicides at night
Spraying herbicides at night is an option – if that’s the only time when winds are calm enough to reduce the spray-drift risk. Good lighting, GPS guidance, well-marked obstacles and alert operators are immensely important.
Sunlight and warm conditions help with herbicide translocation in the plant. Applications made at night can be effective as long as weeds were growing actively the evening before spraying.
A Farming Smarter study of night spraying of various herbicides in Alberta concluded that:
- The herbicides in pre-seed burndown and in-crop trials performed most effectively when applied in the day time (noon-2:00 p.m.). Night time (midnight – 1:00 a.m.) gave better results than the least effective dawn time (4:00-5:00 a.m.). Farming Smarter’s research showed a substantial advantage of day and night time applications over the dawn time application.
- Midnight application could be useful alternative when conditions limit day time opportunities.
- Moisture-stressed plants or a major rainfall event shortly after application could render herbicides totally ineffective.
Glyphosate vs glufosinate
The following table from a University of Minnesota study (Martinson 2005) on herbicide-tolerant soybeans shows that rate and temperature are the two key considerations in efficacy for glufosinate and glyphosate. Time of application is not that important for glyphosate, but it is fairly important for glufosinate. Even so, if spraying at night when winds are calm enough to spray means lower efficacy, it’s still a lot better than not getting the fields sprayed at all.
Spraying flea beetles at night
Night spraying is also an option for insects if days are too hot and windy for insecticide efficacy. Because nighttime temperatures are cooler, this can be an advantage for some insecticides, such as pyrethroids that high temperature restrictions.
John Gavloski, Manitoba provincial entomologist, says the practice does present some uncertainty about the flea beetle target. “Little is known about flea beetle behaviour at night, such as how much time are they spending up on the plants, where contact with them would be easier. The insecticides should have enough residual though, so I would expect good control the following day should feeding have declined over the evening.”
More…
Canola Encyclopedia chapter on weed management
Night spraying: Pesticide efficacy with night time applications