Spraying and Dew--Is There an Issue?
The southern states are fortunate to have a cadre of highly qualified and knowledgeable weed science research and extension specialists who work very hard to ensure that southern crop producers have all the facts pertaining to applying herbicides for weed management. Dr. Eric Prostko, Professor and Extension Weed Specialist at the Univ. of Georgia, is one such individual. I have cited his work and information on numerous occasions on this website.
An article titled “Does dew affect herbicide performance? The facts on the matter” by Dr. Prostko appeared in the Jan. 17, 2018 online Southeast FarmPress. In this article, Dr. Prostko presents his views and supporting documentation on this subject.
He first states that, in his 30 years of herbicide application experience, he has never had a weed control failure because of the presence of early-morning dew. He further states that his herbicide spraying activities have been dictated by wind, which means that those activities usually occur in the morning between sunrise and 9AM when there is less likelihood of a wind concern.
He next defines what dew is according to the National Oceanic Atmospheric Administration’s (NOAA) National Weather Service (NWS). That definition can be found here. NWS’s definition of dewpoint can be found here.
And finally, he cites research results from a Univ. of Minnesota study (“Time of Day of Application Effect on Glyphosate and Glufosinate Efficacy” by Martinson et al., 2005, online, Crop Management, doi:10.1094/CM-2005-0718-02-RS). The below table is taken from that publication. Of the seven factors measured in this study in conjunction with herbicide application, dew was found to have the least effect on glyphosate and glufosinate efficacy.
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Factors determined by multiple regression to influence weed control following application of glyphosate and glufosinate herbicides. |
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|
Glyphosate |
Glufosinate |
|
Variable/Factor |
Rank |
|
|
Herbicide rate |
1 |
1 |
|
Air temperature |
2 |
2 |
|
Weed height |
3 |
4 |
|
Adjuvant |
4 |
5 |
|
Relative humidity |
5 |
6 |
|
Time of day |
6 |
3 |
|
Dew |
7 |
7 |
Results from other studies where other herbicides were evaluated showed that herbicide efficacy when applied before or at sunrise was reduced even in the absence of dew. So it is quite likely that efficacy of herbicides that are applied at the most likely time of dew presence are more affected by the time of day of their application vs. the presence of dew. Click here for the latest results from experiments conducted to determine how herbicide efficacy is affected by time-of-day of their application.
Dr. Prostko offers the following points to consider if concerned about the presence of dew at time of herbicide application.
• “In some scenarios, excessive leaf moisture can lead to herbicide runoff. I could not argue with an applicator who decided to delay if at the time of application dew or water was running off a leaf like Niagara Falls.”
• “Growers have many things to worry about every year. In the grand scheme, dew should probably not be one of those.”
• “I would prefer that growers obsess more about things such as calibration, rate, timing, GPA, tractor speed, nozzles, and modes of action.”
• “If excessive dew makes you feel uncomfortable, delay the application. But your worry then will be more about the wind and potential off-target movement. As everyone knows, off-target movement is not a very popular subject these days.”