Numerous observations on off-target herbicide issues often pointed to temperature inversions as a source for the problem. This occurs in one of two ways. First when herbicide applications occur in a light fog bank that are generated by temperature inversions. These can be avoided by being vigilant and shutting down application during an inversion. The second way is when a volatile formulation of a herbicide like 2,4-D or dicamba pick up as a gas hours after application and join low lying fog which are generated by an inversion. These can be avoided by only using labeled low volatile formulations of 2,4-D as there is no label for dicamba in our row crops. This means that dicamba cannot be legally sprayed in row crops.
How often do temperature inversions occur in west Tennessee? Research we conducted in 2021 funded by the United Soybean Board designed to study low level inversions shed some light on this question. We set up two weather stations that measure temperature at 18”, 66” and 120” above the soil. One of these stations was placed on the crest of a hill and the other was set up about a mile away in a creek bottom between a field of corn and a field of soybean on the UT Martin farm. The weather station in the bottom was 40’ less in elevation than the one located on the hill.
In the month of June, there were 18 temperature inversions measured on the hill and 22 inversions documented in the creek bottom. The temperature inversions for both locations typically, though not always, started between 5 to 6 o’clock in the evening. What was interesting is that the temperature inversion measured on the hill often dissipated early in the morning (4 to 6 a.m.) while the inversions in the bottom would often linger until 8 a.m. or later.
I know when it comes to temperature inversions most have moved from being happily ignorant they even exist to really trying to watch for them. The problem this year has been the fact that there have been too few hours fit to spray. In order to be most efficient with time, based on these data, good fields to start with in the morning would be ones that have some elevation and then move to the bottoms later during mid-day hours.
