Spraying herbicides at night is an option – if that’s the only time when winds are calm enough to reduce the spray-drift risk. It goes without saying that good lighting, GPS guidance, well-marked obstacles and alert operators are immensely important, but from the perspectives of herbicide uptake, efficacy and label requirements, night spraying is OK as long as weeds are not under any stress and are growing actively going into the evening.
Sunlight and warm conditions help with herbicide translocation in the plant, but applications made at night can also be effective. 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 (12-2 p.m.). Night time (12 p.m. – 1 a.m.) gave better results than the least effective dawn time (4-5 a.m.). [Farming Smarter] saw a substantial advantage of day and night time applications over the dawn time application
- Although night time application performance was less often effective than day time application, it performed better more often than dawn time and, therefore, it could be useful as an alternate spray application timing when opportunities for day time application are limited.
- The results also suggest that moisture-stressed plants or a major rainfall event shortly after herbicide application could also reduce efficacies, potentially rendering the herbicides totally ineffective, upsetting the performance patterns most often seen in the study. Read the report.
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.