Three recent events trigger this article. First, a call from a landowner with significant concerns after watching a herbicide application with winds alleged at 20 mph blowing towards their berry crop. Second, yesterday’s wind that was gusting well over 20 mph and preventing herbicide applications at a time when we need to spray our trials just like many custom applicators and growers who need to spray corn fields. Third, a call asking about the risk of dew affecting glyphosate performance if spraying in early mornings to avoid the wind. 

 

All together, these items highlight two of the major risks with postemergence herbicides – drift and the pressure to spray. The risk of drift is certainly real with numerous rural residential properties, vegetable, organic, or other sensitive field crops, and other sensitive sites. Last week’s rains, wet fields, and winds have placed many applicators under pressure to catch up. Be careful and be smart out there. Use drift reducing technology such as drift reduction agents, spray nozzles that increase droplet size, higher spray volumes, and lower boom heights. Remember that these steps are drift reducing technologies, not prevention strategies. The best combination of technologies will not prevent drift in a high wind.   

 

If you have a highly sensitive site (also known as a claim that would be expensive to settle), consider if you can delay the application until the wind direction shifts away from the site or delay the application until early morning or late evening when wind speeds may be lower. An application with a consistent 3-5 mph wind blowing away from a highly sensitive site is ideal as herbicides never drift upwind (studies have proven this just in case there is any doubt).

 

As for the question of spraying glyphosate in early morning or late evening to avoid the wind, is this a good idea with the potential of dew at these times? The clear advantage of spraying at this times of day is being able to continue spraying, hopefully when weeds are smaller and easier to control. The two concerns are if dew will affect performance and if glyphosate performance will be reduced with the early or late time of day. Based on the limited amount of research on dew’s effect, I would not be overly concerned about dew unless the weeds are literally dripping wet such that spray drops cause the dew to run off the leaves. While the dew may dilute the concentration of the glyphosate, which could lessen performance, this is likely compensated for by the greater hydration of the leaf surface (i.e. slower drying and better absorption). I am more concerned about the time of day effect on glyphosate’s performance. Research has shown glyphosate applications before 6 a.m. or after 7 to 8 p.m. can have reduced weed control. This effect will be seen more on broadleaf weeds, on larger weeds, and with lower glyphosate rates. While this potential exists, I would still recommend spraying at these hours to avoid wind near sensitive sites and use full glyphosate rates for the weed size being sprayed and spray while the weeds are still smaller. This should minimize the overall risk of reduced glyphosate performance. 

 

Fortunately, the weather has been calm today and I hope everyone has a great week making effective herbicide applications without risks of drift.