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Ohio State University Extension


The difficulty in legally applying dicamba – weather factors

Labels for the dicamba products approved for use on Xtend soybeans, Engenia, FeXapan, and XtendiMax, were recently revised, and are now generally more restrictive in an effort to prevent some of the problems with off target injury that occurred in 2017.  Whether these additional restrictions do much to prevent volatility is doubtful, but this aside, one of the problems with labels this restrictive is the difficulty in even finding enough time to make legal applications.  It can be an interesting exercise to review past weather conditions with the goal of determining legal windows of application, taking these restrictions into account.  Weed scientists at Purdue University conducted this type of analysis for west central Indiana for June 2017, and came up with a total of 48 hours when it would have been legal to apply (hours of dawn to dusk, wind speeds between 3 and 10 mph).    

We conducted a similar analysis, used weather information for June of 2017 at the Dayton International Airport.  Assumptions in determining the legally sprayable hours for the month:   30 days, 6 am to 7 pm, wind 3 to 10 mph.  Days with rain were considered to have no sprayable hours, along with those when rain occurred the next day, since it’s not legal to apply if rain is in the 24-hr forecast.  We did not take inversions into account since it could not be determined whether these occurred from the weather data, but we did some additional calculations just to show what happens if hours when temperature was above 85 were omitted (when spray droplet evaporation and volatility would be more likely).  We also ignored the fact that maximum wind and wind gusts exceeded the hourly weather observations on almost all days, and this could further restrict applicators, depending upon how often they measured wind speed.  The spreadsheet with all of these figures is shown below.  Here’s how it shook out:

- There was a total of 390 possible hours to apply, assuming that applicators are willing to work 13 hours a day every day of the month.  Taking into account the wind speed and rain forecast, there was a total of 77 hours when it would have been legal to apply the dicamba products.  Omitting any of these hours when temperature exceeded 85, number of sprayable hours declined to 70.  
- Disregarding the 85-degree limit, there was a total of 12 days with at least 2 legally sprayable hours, but only 8 days with more than 3 sprayable hours.   
- Days with legally sprayable hours were not evenly distributed throughout the month.  The second half of June was fairly windy apparently.  There were 9 days with at least 2 sprayable hours between June 1 and 16, but only three days between June 17 and the end of the month.  And there was a total of only 11 sprayable hours in those three days after June 17.
- We ran some some rough calculations on how much ground can be covered by an applicator running a 100 ft boom at 15 mph relative to the legally sprayable hours.   An applicator running at 15 mph with a 100 ft boom can in theory cover 181.8 acres per hour, which we rounded to 170 acres per hour factoring in some load and transport time (readers probably have a much better idea of the actual feasible number here then we do).  So an applicator could possibly apply dicamba to about 13,000 acres in those 77 hours of legal spray time.  But in the second half of June when there were only 11 hours, the applicator could cover only 1870 acres.
- The weather data can be mined for a couple of other interesting pieces of information.  First, in following the dicamba labels, it’s important to determine to which fields dicamba can be applied on a given day based on wind direction and the location of sensitive crops or areas.  The weather information from Dayton indicate that wind direction is frequently quite variable throughout the day, which makes this determination more complicated.  Looking at the 12 days with sprayable hours, the wind directon had variability of 90 degrees or less on 8 days, and more than this on the rest.  Assuming that the wind remained low enough to apply during a day, one would still have to continually assess what sensitive areas might be downwind based on changing wind direction.  
- One can also determine for those days with legally sprayable hours, whether the primary wind directions remain relatively unchanged over the next two days following application.  This can be important if the one assumes that dicamba could revolatilize over those following two days and move with prevailing winds.  In this case, for only 5 of the 12 days with sprayable hours did wind direction remain relatively the same over the following two days.  So for the other 7 days, if some revolatilization did occur subsequent to application, and the wind direction changed, the potential exists to injure crops that were not actually downwind from the treated field on the day of application.  
It’s probably possible to quibble with some of these figures depending upon what parameters for day length are used, etc, and what location is used for weather information.  But this does not negate the overall message – managing dicamba applications will be complicated.  Just some food for thought as we move into another year of weed killing.  Safe holidays everyone.

Crop Observation and Recommendation Network

C.O.R.N. Newsletter is a summary of crop observations, related information, and appropriate recommendations for Ohio crop producers and industry. C.O.R.N. Newsletter is produced by the Ohio State University Extension Agronomy Team, state specialists at The Ohio State University and the Ohio Agricultural Research and Development Center (OARDC). C.O.R.N. Newsletter questions are directed to Extension and OARDC state specialists and associates at Ohio State.