By Larry Steckel, Extension Weed Specialist
A topic of conversation lately has been dicamba injury on soybeans. There are four ways dicamba can move off target and injure soybean: sprayer contamination, temperature inversion, spray drift and volatility.
The most infrequent way for dicamba to move off target is a temperature inversion. This is typically more often associated with aerial application. This is a unique environmental condition where a layer of cold air is sandwiched between two layers of warm air. The spray solution, particularly the smaller droplets, hang above the cold layer and take a long time to reach the earth and thus travel a great distance (possibly miles). As a result, they can end up coming down on the wrong field.
At the other end of the spectrum, sprayer contamination has been the most common way that dicamba has injured soybean. Typically, a thorough cleaning of the tank followed by running a solution of household ammonia and water through the system, then allowing it to sit for a few hours, has been sufficient in removing dicamba from hoses and fittings.
Physical drift is the second most common way dicamba can move from the target field to other areas. Good old common sense is the main cure for this problem. If the wind is getting up and blowing toward a sensitive area, stop.
Finally, herbicide volatility is a way some herbicides can move off the intended target area. Dicamba is a great example here, as in the Banvel formulation it is one of the most volatile herbicides on the market. The Clarity formulation is better but still can be quite volatile compared to most other herbicides.
What happens when herbicide moves via volatility? In these cases, after a herbicide application is made and under certain environmental conditions, it is possible for that herbicide to turn into a gaseous state and move into the air. Dicamba formulated as Banvel or Clarity can have this happen for up to 24 hours after application. This is typically only an issue with warm air temperatures.
Clarity and Banvel are designed and indeed labeled to be applied in March and April for burndown or to small corn. The air temperatures during that time of the year are almost never warm enough to be conducive for volatility. They are not designed for use in June and July in Tennessee as 80-90 F temperatures greatly increase the probability that these herbicides will move off the target and with a small breeze move on to a sensitive crop.
So what about yield loss from dicamba movement onto soybean? There is no hard and fast rule on yield loss in these cases. In general, soybeans in the vegetative stages have more time to recover and often will see little yield loss if good growing conditions occur the remainder of the season. Soybeans in the reproductive stages have a higher likelihood of seeing significant yield loss.
However, there are many factors that go into the ultimate yield loss at the end of the year. Factors such as soybean maturity, where group 3 soybeans have less time to recover than a late group 4 variety; soybean stand where soybeans with thicker final stands can handle more dicamba injury than a thin stand; and most importantly, the growing season where soybeans with dicamba injury will not recover as well if they go through a prolonged hot and dry spell.
Quite frankly, the best way to determine yield loss is with a weigh wagon or yield monitor at the end of the year if some areas of the same field are not affected by dicamba injury.