The third blog on stewardship of XtendiMax and Engenia on Xtend crops will focus on application as affected by wind speed. The supplemental labels for XtendiMax and Engenia use in Xtend crops read the same with respect to what wind speed these herbicides can be applied. The labels indicate that the optimum wind speed window to apply either herbicide is between 3 and 10 mph.
Applicators should try every way possible to apply these herbicides in that 3 to 10 mph wind window. However, the herbicide labels for both also state that they the can be applied outside of that 3 to 10 mph wind window if certain requirements are met. In the case of wind speed less than 3 mph, applications can be made (with Engenia only not Xtendimax) if one checks to see if a temperature inversion is NOT occurring. In the case of wind speed between 10 to 15 mph, applications are also permissible provided no sensitive vegetation is down wind.
The reason for the restriction of applications for winds less than 3 mph is the fear of spraying into a temperature inversion. What is an inversion? A temperature inversion occurs when air temperatures increase with height from the ground surface, which is the opposite of what normally happens (i.e. the temperature profile is “inverted”). This results in a layer of cool, still air being trapped near the ground surface below warmer air higher up.
If a pesticide is sprayed during an inversion, fine droplets of the chemical can be concentrated in the cool layer near the ground and are isolated from the surrounding weather conditions. The direction and distance which the droplets will then move becomes unpredictable due to the light variable winds common during inversions and the chemical may be transported away from the target area.
Under clear to partly cloudy skies and light winds, a surface inversion can form as the sun sets. Under these conditions, a surface inversion will continue into the morning until the sun begins to heat the ground. Inversions can be indicated by ground fog, smoke not rising, dust hanging over a road, or the presence of dew or frost.
Some applicators often use simple 4th of July smoke bombs to help detect inversions. Smoke that layers and moves laterally under low wind conditions indicates an inversion while smoke that moves upward and rapidly dissipates indicates good vertical air mixing. Inversions typically dissipate with increased winds (>3 mph) or when surface air begins to warm (3° F from morning low).
The restriction for not applying XtendiMax and Engenia above 10 mph is obviously critical to good stewardship. Some states like Mississippi and Arkansas have stressed this maximum allowable wind speed for applications by capping it at 10 mph. The Tennessee label is no different than the federal label and does allow for applications between 10 and 15 mph if no sensitive vegetation is down wind.
That means the applicator needs to physically ride down to the downwind part of the field and double if not triple check that no sensitive crop or landscape is nearby. Moreover, the applicator must be very cognizant of wind direction changes. This is clearly hard to do in the best of circumstances which is why it is stressed that applications not be made if the wind speed exceeds 10 mph.