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What’s Changed for Soybean Dicamba Use in 2018
Dicamba use is a hot topic for soybean growers. Purdue University Extension weed scientists Bill Johnson and Joe Ikley, University of Illinois Extension weed scientist Aaron Hager, and Mark Loux, an Extension weed scientist with Ohio State University, have compiled the following guidelines for dicamba use in soybeans for 2018.
1. Approved dicamba products. As of early November 2017, there were only three dicamba-containing products approved for preplant, preemergence, or postemergence use in Roundup Ready Xtend soybeans. These products are Xtendimax (Monsanto), FeXapan (same thing as Xtendimax but sold by DuPont), and Engenia (BASF). It is a violation of federal and state law to use anything but approved formulations of dicamba on Roundup Ready Xtend soybeans.
Other dicamba products can be used at least 14 days preplant, if the appropriate waiting interval is followed per the label for non-Xtend soybeans.
2. Wind direction. The labels state that a buffer is required if wind is blowing toward a sensitive area. The label adds that dicamba should not be applied if the wind is blowing toward a sensitive crop.
In 2017, it appeared that many applicators did not follow this restriction, perhaps because a specific distance to the sensitive area was not specified and sensitive areas and crops were not well defined. Realistically, if the sensitive crop is within ½ mile or less of the target field, common sense would suggest it might not be a good idea to apply to that field.
If wind is blowing toward extremely sensitive vegetation, such as non-Xtend soybean varieties, don’t spray until the wind is blowing away from the sensitive crop on the day of application and also for the next two to three days after application.
3. Wind speed. The labels allow spray applications when wind speeds are between 3 and 10 mph. These wind speeds are to be measured at the boom height. This is more restrictive than in 2017 when applications could be made when wind speeds were up to 15 mph, depending on the product used.
In 2017, many applicators overlooked wind gust speed. Many applicators may have focused more attention on average wind speed, rather than wind gust speed. As a result, many spray applications were made during days when average wind speeds were less than 15 mph, but in many instances wind gusts were in excess of 15 mph.
It is strongly recommend to not apply on days when wind gusts exceed 10 mph, even if sustained wind speeds are less than 10 mph. It is not always easy to find a window with these lower wind speeds. The reality is that in some years, it can be challenging to make applications of dicamba products that have very strict label precautions with regard to wind.
4. Time of day. The labels now allow applications to be made only between sunrise and sunset. This is to restrict applications to when temperature inversions are less likely to occur. If the time-of-day restriction was in place in 2017, there would have been substantially fewer hours in June where applications could be made. Accounting for conditions that allowed equipment traffic, west-central Indiana would have had only 48 hours in June with wind speeds between 3 and 10 mph between the hours of sunrise and sunset.
5. Temperature inversion. During a temperature inversion, very small spray droplets remain suspended in the air and do not settle on plants or the soil surface. These droplets will move when wind speed increases later in the day. It is strongly recommend that you use an app like Spray Smart or something similar to determine whether or not a temperature inversion exists. If there is a temperature inversion, do not spray until the inversion has lifted.
6. Buffers. Another frequent violation of the label in 2017 was failure to implement buffers near sensitive areas. Many applicators took the approach that if the wind was blowing away from the sensitive crop, dicamba could be applied right up next to the sensitive crop. University research in 2017 demonstrated that even the new formulations of dicamba can volatilize and move on dust particles for up to three days following application. Wind directions can change on day two or day three and move volatilized dicamba or dicamba dust to sensitive vegetation. So, the establishment of buffers is extremely important if you are near a sensitive area.
7. Nozzles. Consult the websites for the respective herbicides to find the list of approved nozzles and spray pressures to apply the approved dicamba products to Xtend soybeans.
8. Spray additives and tank-mix partners. The list of approved spray additives changes frequently, so it is important to regularly check the websites.
All approved dicamba products require the use of a drift-control agent from the list of approved drift-control agents on their respective website. Adding any other products (including foliar fertilizers, insecticides, herbicides, or fungicides) that are not listed on the website for the respective herbicide constitutes a label violation.
Finally, do not add ammonium sulfate or anything containing ammonium sulfate, as this produces more of the volatile form of dicamba. There are approved nonammonium sulfate-based water conditioners to reduce hard water antagonizing glyphosate that is tank-mixed with an approved dicamba formulation.
Keep in mind that you can do everything per the label but still have offsite movement. This happens because of the following.
- The new dicamba formulations have the capability of volatilizing and moving on dust particles.
- Fine spray particles can remain suspended in inversions.
- Dicamba can move with runoff water after heavy rainfall events.
To reduce the probability of both primary and secondary dicamba movement events, consider the following recommendations.
- Do not spray when the forecast indicates that wind gusts will exceed 10 mph. It is impossible to predict when a gust of this magnitude will happen or how long it will last. Gusts that reach 30 mph can move spray particles and vapor for great distances.
- Reduce boom heights to the extent practically possible, in order to get close to the 24-inch boom height limit specified on the label. Simply reducing the boom height from 48 to 24 inches has been shown to reduce the distance traveled by drift particles by 50%. One of the most effective ways to safely lower the boom height without running the boom into the ground is to reduce sprayer travel speed. Also remember that any travel speed over 15 mph is off-label. The labels also now recommend that travel speeds be reduced to 5 mph when making applications on the field edges.
- Avoid application when temperature exceeds 80°F. Assuming that these dicamba products have some potential for volatility, the risk of this occurring increases with temperature.
- Consider applying dicamba only preplant, preemergence, or very early postemergence. Over 90% of the offsite movement complaints resulted from postemergence applications. Our assumption is that applications earlier in spring will have less likelihood to cause problems even where dicamba moves, due to the absence in many cases of any developed vegetation to injure. Temperatures are also likely to be lower when applied preplant/preemergence vs. postemergence, possibly reducing the risk of movement via volatility.
- Have conversations with neighbors to know what crops and technologies are being planted around Xtend soybean fields. Many offsite movement cases in 2017 occurred where neighbors planted Xtend and non-Xtend soybean adjacent to each other. Knowing what sensitive crops are in the vicinity of your Xtend fields will enable better decision making about use of dicamba in a given field.
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