Use Caution with Weather Prediction Tools and Dicamba Recordkeeping

With a promising extended forecast, we may finally get a lot of field work done around the state. With planting and spray season upon us, we decided it was a good time to test some of the weather prediction tools that have been advertised throughout the winter. On the afternoon of April 19th and the morning of April 20th, we drove out to our research plots at Throckmorton Purdue Agriculture Center (TPAC) to measure wind speeds with our handheld Kestrel 3000 held at 3 feet above the ground. We used the National Weather Service guidelines of measuring wind for 2 continuous minutes. We then compared the wind speeds from our handheld device to several apps and websites that pinpointed our location on a smartphone. The different results we found on April 19th are in Table 1, and the results from April 20th are in Table 2. Remember that according to the labels for Xtendimax, Engenia, and FeXapan, wind speed and direction must be measured at boom height at the start and stop of each application. Some state regulatory agencies are allowing applicators to use some of the available apps for weather data recording.  Most of these apps will use currently reported weather data and models to try to pinpoint a forecast to your location.  However, we wanted to see how much the weather model data compared to our handheld device.

Table 1. Wind data collected from multiple devices at 3 PM on April 19th. N/A means the device or app does not measure or report data for that column.

Source Wind Direction Wind Speed Wind Gusts
Kestrel 3000 (Handheld) N 6.9 MPH 10.8 MPH
RRXtend Spray (App) N 7 MPH N/A
Engenia Spray Tool (Website) N 12 MPH 24 MPH
DTN Ag Weather (App) NNE 5 MPH N/A
Pocket Spray Smart (App) N 15.8 MPH N/A
National Weather Service (Website) N 13 MPH 20 MPH
The Weather Channel (App) N 14 MPH N/A


Table 2. Wind data collected from multiple devices at 10 AM on April 20th. N/A means the device or app does not measure or report data for that column.

Source Wind Direction Wind Speed Wind Gusts
Kestrel 3000 (Handheld) NE 3.1 MPH 4.7 MPH
RRXtend Spray (App) NNE 0 MPH N/A
Engenia Spray Tool (Website) NNE 2 MPH 9 MPH
DTN Ag Weather (App) ENE 5 MPH N/A
Pocket Spray Smart (App) N 3.6 MPH N/A
National Weather Service (Website) N/A Calm N/A
The Weather Channel (App) NE 6 MPH N/A

On April 19th, our wind speeds were approaching the high end of the legal limit to apply Xtendimax, Engenia, and FeXapan. Our handheld unit measured winds gusting slightly over 10 MPH, which means we could not spray until the wind gusts were 10 mph or less. The RRXtend Spray App and DTN Ag Weather App reported wind speeds of 7 and 5 MPH, respectively. However, neither of these two apps report wind gusts. The herbicide manufacturers and Office of the Indiana State Chemist do count wind gusts over 10 MPH as off-label, so additional information would need to be gathered if using these apps since wind gusts are not reported, only the average wind speed. The rest of the apps we used reported wind speeds over 10 MPH, indicating a no-spray situation.

On April 20th, we were on the opposite end of the wind speed limit for the new dicamba formulations. When wind speed was measured in the field with the handheld device, we had favorable wind speeds and wind gusts which would allow an applicator to spray if the wind is not blowing towards a sensitive crop. Pocket Spray Smart, DTN Ag Weather, and The Weather Channel indicated favorable wind speeds, while the rest of the apps reported wind speeds below the 3 MPH threshold.

Xtendimax, Engenia, and FeXapan have the tightest wind restrictions of herbicides we will be applying in the corn-belt, but the differences in the data provided by various sources should be a cause of concern when planning any application. A handheld device used in the field will be most accurate and meet the legal requirements for the three new dicamba products, but we understand the need to look at weather conditions before mixing and heading to the field. Given the variability that we found amongst the 7 tools we tested, we recommend using and recording from at least two different sources when evaluating if wind conditions are right to make any herbicide application. Using two sources will do two things for the applicator.  One, it will provide a second set of data to provide more information on whether or not we should spray that day. Second, in the event of an offsite movement event and subsequent inspection, this will let the inspector know that you the applicator did your diligence in attempting to abide by the label requirements for spraying the Xtend soybean dicamba herbicides.

We will continue to evaluate these and other available tools over the next several weeks to check the consistency of these apps compared to in-field wind measurements.


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