C.O.R.N. Newsletter: 2017-08

Even if you are planting RR Xtend soybeans, the answer to this could be – maybe nothing.  A number of growers have told us that even though they are planting RR Xtend soybeans, they plan on “letting the dust” settle this first year and stay with their regular herbicide program.  And then of course there are also some solid reasons to use dicamba in a preplant or postemergence treatment, depending upon what has been done in the field already and whether previous practices have been ineffective for control of certain herbicide-resistant weeds.  A few other things to consider before we cover some of the dicamba use options:

• Be sure to know the dicamba labels and stewardship guidelines well.  It might be worth assessing fields now to determine whether some just should not be treated with dicamba based on sensitive surroundings, and whether for others the wind direction the day of intended application will be a major consideration. 
• While we use the term “dicamba” throughout this article for the sake of brevity, only three dicamba products are labeled for use in Xtend soybean herbicide programs – XtendiMax, FeXapan, and Engenia.  Use of other dicamba products is not legal.
• For those planting both nonXtend and Xtend soybeans where dicamba use is planned, some type of system to ensure that dicamba is applied to the right fields might be in order.
• Check the websites for the dicamba products for approved mixtures and nozzles while in the planning stages, and be sure not to deviate from this list.  We have these links in the Jan 23 dicamba post on our blog – u.osu.edu/osuweeds.
• If relying on a custom applicator for dicamba applications, be aware that some are still declining to apply dicamba on soybeans this year, and other are charging higher fees for dicamba.  This is good information to have now.

There are several possible approaches to use of dicamba in Xtend soybeans.  We tend to look at this with regard to three weeds, marestail, giant ragweed, and waterhemp (and common ragweed in areas), since current herbicide programs control everything else well usually, and these are our major herbicide-resistant weeds in the state.  Our assumption is that where it is used, dicamba will be one component of the comprehensive two-pass approach of burndown and residual herbicides followed by postemergence herbicides that gets used on most of our soybeans.  So our thoughts here are really along the lines of where dicamba can be plugged into this program to replace or supplement the other herbicides. 

Where current herbicide programs for RR soybeans are still effective for control of these more problematic weeds, there’s not necessarily a need for dicamba.  One reason for use could be to reduce the selection pressure for resistance to certain herbicides due to their continuous use or overuse.  An example of this is the reliance on site 14 herbicides (Valor and other flumioxazin herbicide, Flexstar and other fomesafen products, Cobra/Phoenix) in preemergence and postemergence applications for control of common ragweed that is resistant to glyphosate and ALS inhibitors.  Continued use of this practice in soybeans is pushing the ragweed to become resistant to site 14 herbicides as well, and swapping in dicamba for fomesafen or Cobra in the postemergence treatment can mitigate this.  The same thing can be said about waterhemp, based on the importance of POST-applied site 14 herbicides for control of glyphosate/ALS resistant populations, and the tendency for this weed to fairly rapidly develop resistance to anything used postemergence even as often as every other year. 

There can be issues of cost/effectiveness where dicamba is a better choice.  For example, postemergence combinations of glyphosate + dicamba are likely to be more effective on glyphosate/ALS-resistant giant ragweed than combinations of glyphosate + fomesafen (Flexstar etc) for about the same money.  Dicamba can have a good fit in preplant burndown programs for control of marestail, to replace or supplement 2,4-D.  The advantage to dicamba on marestail occurs primarily in fields not treated with herbicide last fall, since those are the fields where the variability in 2,4-D effectiveness has been observed for the most part.  Dicamba will be more expensive than 2,4-D, but also more consistently effective in that specific situation.

There are instances where dicamba will just be more effective than current herbicides, especially in weed populations for which herbicides have been so badly mismanaged for years that multiple resistance occurs and the population continues to increase.  For growers who have multiple resistance already in ragweeds and waterhemp, the ability to use dicamba in postemergence applications may be the only good option left.  The ability to use dicamba postemergence can certainly be helpful for marestail where the combination of preplant herbicides and suppression by soybeans fails to result in adequate control.  Keep in mind when making seed choices that use of glufosinate in LibertyLink soybeans accomplishes most of what has been mentioned here also.  And that failure to use any of these new technologies judiciously can result in selection for resistance to them as well and possibly reduce their utility fairly rapidly.  Our assumption is that growers who have mismanaged current technology can just as easily mismanage and break any new tools as well.  I would say “you know who you are” here but I would guess these growers are not avid CORN newsletter readers.

Finally, using dicamba instead of 2,4-D in the preplant burndown treatment does away with having to wait 7 days to plant soybeans, which certainly allows for more flexibility in the spraying/planting operation.  Dicamba and glyphosate can be applied anytime before or after planting (and even after soybean emergence), but keep in mind the residual herbicides included in mixtures cannot.  Almost all of the residual products have to be applied before soybean emergence.  Any residual product that contains flumioxazin (Valor) should be applied within 3 days after planting per product labels.  This restriction is in place to minimize the risk of injury to soybeans when application occurs too close to crop emergence.  In our experience, risk of soybean injury with any product containing flumioxazoin or sulfentrazone (Authority) increases when applied at or after planting, compared with application a week or more prior to planting.  Our suggestion for these products in the “Weed Control Guide for Ohio, Indiana, and Illinois” is that they be applied at least a week ahead of planting, even in dicamba systems where a wait between application and planting is not required. 

And a reminder also that weather can wreak havoc with the best plans.  Planning to plant a lot of soybean acres first, with the assumption that the dicamba/glyphosate/residual mixture can be applied to all of these acres prior to soybean emergence (or within 3 days of planting where flumioxazin is used) can look good in theory.  Insert a big rain somewhere in there and this can become more difficult to achieve.  From a time and weather management perspective, the better approach would be to start applying dicamba/residual mixtures over the next several weeks as fields dry out, instead of trying to do it all at or after planting.  And just a final note – while dicamba is more effective on marestail than 2,4-D is, waiting too long in spring to apply it can result in some problems controlling larger plants.  In one 2016 OSU study, control of overwintered marestail plants, where stem elongation had started, was reduced when mixtures of dicamba and residual herbicides were applied in early May.