By Aaron Hager, Weed Scientist

The long-awaited label allowing dicamba use in dicamba-resistant soybean was granted Nov. 9, 2016, by the EPA, although only one commercial product received that label. 

Many Illinois farmers anticipate this technology will provide a much-needed solution to challenges caused by weed populations resistant to herbicides from multiple site-of-action groups and other difficult-to-control species. Without question, there are instances and scenarios in which dicamba will improve control of certain weed species, but dicamba will not bring back the “good ole days” of POST-only weed control programs in soybean. Current expectations of what this technology can accomplish tend to be a bit more optimistic than what the technology actually will be able to deliver.

Where It Fits

Horseweed control. Many farmers experienced significant challenges attempting to control marestail before planting no-till soybean in 2016. The “traditional” burndown tank mix of glyphosate and 2,4-D was not as effective on marestail as many have come to expect. Anecdotally, we suspect resistance to glyphosate is widespread in Illinois marestail populations, and that this likely contributed to many control failures with burndown herbicides. Marestail control with spring-applied 2,4-D can be quite variable, and poor control is common when a glyphosate plus 2,4-D tank mix is applied to a glyphosate-resistant marestail population.  

Dicamba can provide some remedies because the recently approved dicamba label allows up to 1 pound dicamba acid equivalent to be applied prior to planting dicamba-resistant soybean. This can provide better and more consistent control of glyphosate-resistant marestail compared with 0.5 pound acid equivalent 2,4-D. Note: Although the new label allows soybean to be planted immediately after dicamba application, it’s still advisable to wait a few days following application before injuring the weeds with the planting operation.

Annual morningglory species are a well-known bane of glyphosate. Control of species such as tall and ivyleaf morningglory with glyphosate is rarely adequate, but dicamba is much more effective on these species. Add to this list common and giant ragweed, two other species that have evolved resistance to glyphosate and ALS-inhibiting herbicides.  Dicamba certainly can provide better control of herbicide-resistant ragweeds than can glyphosate or ALS inhibitors.

Where It Maybe Fits

Waterhemp. Before the widespread evolution of glyphosate-resistance in waterhemp, glyphosate was considered by many weed scientists to be an excellent herbicide against waterhemp. That same adjective is not ascribed to dicamba. Most university weed control guides list dicamba as good or very good on waterhemp, but not excellent. Dicamba can improve control of pigweed species, but it will never be as effective as glyphosate once was.  

Illinois farmers have made great strides toward utilizing more diverse herbicide programs for waterhemp control than they were using a decade ago. We suggest that dicamba should be used in a way that does not reduce this diversity. It is imperative to maintain a diverse weed management approach to prolong the effective utility of dicamba. Illinois waterhemp populations have evolved resistance to herbicides from six site-of-action groups, and resistance to dicamba is not a question of “if,” but “when.”

Some Realities

What are some limitations of utilizing dicamba for weed control in soybean? The current label contains several mandates related to the actual spray application procedure that are somewhat unique. For instance, there are limitations on boom height, sprayer speed and nozzle type that applicators must follow. An infield, downwind buffer of 110 or 220 feet (depending on application rate) must be maintained. One of the most significant limitations is the inability to tank mix with other herbicides. There is an avenue by which other herbicides can be approved for application with dicamba, but if the current label remains unchanged during the 2017 growing season, applicators will be required to apply dicamba alone. In other words, farmers will make a separate application of dicamba and another application of other needed herbicides.

Many claims have been made about yield potential of dicamba-resistant soybean varieties and the lower volatility potential of the recently labeled dicamba formulation.  We believe that our clientele should be aware that the University of Illinois weed science program has not had the opportunity to evaluate yield or volatility potential. We do not have data that dispute these claims, but neither do we have data to support them. University variety testing programs are now releasing results from yield trials that include dicamba-resistant varieties. Shawn Conley, soybean and wheat extension specialist at the University of Wisconsin, recently published an article summarizing his research with dicamba-resistant soybean varieties in 2016.

Of particular concern is the apparent confusion about particle drift and volatilization.  While the newly labeled formulation is reportedly less likely to volatilize after application, there is absolutely nothing unique about the formulation that will reduce physical drift during application. Off-target movement of dicamba is of particular concern due to the number of sensitive dicot species grown in Illinois. 

The new formulation of dicamba is no more likely to drift than any other herbicide formulation, but the symptoms that drift did occur can be induced at extremely low concentrations of dicamba. Several years ago, we were able to induce soybean leaves to “cup” with as little as 1/10,000 pint of dicamba. Many media reports suggest that use of older, more volatile dicamba formulations was largely responsible for the widespread off-target injury that occurred in areas of the mid-south during 2016, but volatility is generally a minor component of off-target movement when compared with actual physical drift during application.

One last item to consider is that this new registration for dicamba use in dicamba-resistant soybean will expire on Nov. 9, 2018. EPA documents indicate the registration will automatically expire “…unless the EPA determines before that date that off-site incidents are not occurring at unacceptable frequencies or levels.” In other words, the continued availability to utilize dicamba in dicamba-resistant soybean is very much dependent on those who use it.

Much more undoubtedly will be shared about dicamba use in dicamba-resistant soybean from a variety of sources. We do believe dicamba can provide a solution to unique weed management challenges, but we also believe not all weed management challenges can be met with dicamba. Other herbicide-resistant crop technologies, such as LibertyLink and Enlist (once all import approvals are received), also can provide solutions and remain viable options for soybean producers. Proper stewardship of all technologies only helps prolong their effective utilization.