More on Offsite Movement of Dicamba Products
By now, I am fairly certain that most members of the Midsouth agricultural community are aware of the controversy surrounding the use of dicamba herbicide products on RR2 Xtend soybeans. This stems from the off-target deposition events that were associated with the spraying of these products on tolerant crops in 2017. I am also fairly certain that most interested parties have read or heard about all of the activity this issue has stimulated in recent months.
Drs. Bill Johnson and Joe Ikley (Purdue Univ. Extension), Aaron Hager (Univ. of Illinois Extension), and Mark Loux (Ohio State Univ.) recently (Nov. 2017) published a concise summary of the points that producers/applicators should consider when including dicamba herbicide products in their weed control programs for RR2 Xtend soybeans. Although most of the below points from that article have been published/disseminated previously, they are presented here to further iterate the importance of proper stewardship when applying a dicamba herbicide product to tolerant crops.
• Remember that selection for dicamba resistance in weeds will occur each time dicamba is applied; therefore, over-reliance on this new technology will lead to the development of dicamba-resistant weed populations. Hopefully, the selection for glyphosate resistance in weed populations that resulted from over-dependence/use of glyphosate will not soon be forgotten.
• Federal labels for the dicamba herbicide products contain very detailed/specific instructions that are designed to reduce the risk of their offsite movement when applied to dicamba-tolerant crops. It is likely that state-specific labels will be developed to supplement the federal label. For example, click here for Missouri restrictions and requirements for 2018 dicamba use (especially significant is the “Dicamba Notice of Application Form” which must be completed online prior to each day’s application of a dicamba product). Click here for the Tenn. Dept. of Agriculture site that lists new rules and regulations pertaining to dicamba applications in 2018.
• All of the dicamba herbicide products (Xtendimax, FeXapan, and Engenia) are now restricted use pesticides. Thus, they can only be applied by licensed/certified applicators who have completed required annual training. The above-listed three products are the only ones approved for preplant, preemeregence, and postemergence use in/application to RR2 Xtend soybeans.
• Realistically, a 0.5-mile buffer should be the minimum to ensure enough distance between a dicamba-tolerant crop that receives a dicamba application and a dicamba-sensitive crop. This is to ensure that changes in wind direction during the 2-3 days following application will not likely move volatilized material to an area with sensitive plants.
• Applications of dicamba herbicide products may only be made between sunrise and sunset to avoid times when temperature inversions are more likely to occur.
• Each product’s website will be frequently updated with a list of approved spray additives, tankmix partners, and drift control agents (these are required). Thus, these websites should be checked often to ensure current awareness of the approved products in each category. The websites for this information appear below.
www.xtendimaxapplicationrequirements.com
www.fexapanapplicationrequirements.dupont.com
The authors provide the following additional suggestions to reduce offsite movement of applied dicamba products.
• Offsite movement can occur even when all label requirements are met. This is because 1) all of the products can still volatilize and move on dust particles, 2) fine spray particles can remain suspended in inversions, and 3) dicamba can move away from the application site with runoff water after heavy rainfall.
• Dicamba products should not be applied during the allowed daily window if there are forecasts for wind gusts that exceed the 3-10 mph wind speed (measured at boom height) allowed for application. Wind gusts that exceed the allowed wind speed for application can move spray particles and vapor far offsite. Thus, more attention should be paid to wind gust forecasts than to average wind speed forecasts.
• The allowed spray boom height is critical for the prevention of offsite movement since heights greater than the allowed 24 in. will significantly increase the potential for offsite movement.
• Avoid application of dicamba herbicide products when air temperature is >80°F since the risk of volatility increases with increasing temperature.
• Preplant and preemergence applications of the dicamba products are less likely to cause offsite problems since there will be less likelihood for the presence of sensitive vegetation and a greater likelihood of lower temperatures. However, this approach does not allow for the intended postemergence application of these products to control weeds that have emerged in tolerant crops. A compromise approach is to apply these products very early postemergence, and avoid later postemergence applications.
Several of the above points may be moot for many Midsouth soybean producers since Arkansas has proposed that no dicamba applications be made between April 16 and October 31 and Missouri prohibits applications after June 1 in the Bootheel counties. Other Midsouth states likely will impose similar restrictions on the application of dicamba herbicides based on date. Such calendar date restrictions could effectively exclude some or most postemergence applications to soybeans in those states, and thus limit the utility of this new tool for controlling weeds that emerge in a growing soybean crop and that are resistant to other classes of herbicides.
Regardless of the policies/regulations designed to resolve present dicamba-related issues, two important facts remain.
• Use of dicamba herbicide to control herbicide-resistant weeds in dicamba-tolerant soybeans can and hopefully will be an important component of soybean weed management systems.
• To protect this vital technology for future use in soybean weed control systems, companies and producers must come together to explore all possible avenues to ensure dicamba’s safe, long-term availability for its intended purpose.
Composed by Larry G. Heatherly, Dec. 2017, larryheatherly@bellsouth.net