Source: Ohio State University Extension 

Last week the Environmental Protection Agency issued approval for Enlist Duo, the glyphosate/2,4-D premix for use in the Enlist corn and soybean system, in six states: Illinois, Indiana, Iowa, Ohio, South Dakota and Wisconsin. The approval came with a number of conditions that set a new precedent, and we will cover these in more detail later this fall.  

Dow informed us that they would provide more information in the near future about intentions for the scope of the 2015 launch of Enlist. They are still working on export clearances for some countries, and as with most new things, availability will be limited initially anyway. We obviously have problems with herbicide resistance, which have reduced the utility of some herbicide sites of action. So new weed management technology is needed and beneficial, and this includes the Enlist system along with the traits that provide resistance in soybeans to dicamba and group 27 herbicides (HPPD inhibitors — mesotrione and isoxaflutole).  

All of these have a fit somewhere to help manage current herbicide resistance problems, and they all have the potential to cause new herbicide resistance problems if not properly managed. There is already resistance to 2,4-D and dicamba in annual weed species, and it’s pretty much a no-brainer to say that we expect to have weed populations resistant to these herbicides at some point based on the coming potential for intensive use in oversimplified herbicide programs. Dow and Monsanto have primarily mentioned the need for residual herbicides in these systems to help prevent resistance, which gets us about 50% there (maybe). 

There is a bigger issue of how technologies are rotated over a period of several years, and whether we end up treating the current herbicide-resistant weeds too often with 2,4-D or dicamba within and over the years because at first it’s easy and effective. 

Case in point — management of marestail in soybeans with 2,4-D. One of the trends in marestail control programs has been inadequate control of marestail with a spring glyphosate/2,4-D burndown, especially when applied too late in spring and/or in the absence of a fall herbicide treatment. Lack of fall treatment results in overwintered marestail that are large, old and more difficult to control, whereas the fall treatment results in a spring burndown situation consisting mainly of a low density of young, small spring-emerging plants.  

Including other burndown herbicides with the glyphosate/2,4-D may or may not solve this problem. Experience with the Enlist system indicates that even without a fall herbicide treatment, multiple in-season applications of 2,4-D (burndown followed by POST) seem to control marestail well. Doing so will probably result in the development of resistance to 2,4-D in marestail, though, since this is the type of approach that led to glyphosate resistance — multiple applications of the same herbicide for control of the same weed.  

In this case, including residual herbicides with the pre-plant burndown herbicides certainly helps control the marestail that can emerge after planting, but doesn’t help with control of the plants that were already present when the burndown was applied. The 2,4-D has to pull the load completely on these, so the risk of resistance is not mitigated unless a more comprehensive approach is used.  

Bottom line — fall herbicide treatments remain a key component of marestail management programs, even where Enlist soybeans will be planted next spring.

The majority of our crop areas are still largely weed-free at the end of the season, with another 10% to 20% that has substantial problems with one or more weeds. It’s obvious that many growers have managed current herbicide-resistant crop technology and herbicides well, and made changes as needed to stop further infestations of resistant weeds. 

A similar approach will be necessary with new technology, because it remains to be seen whether companies have the incentive or ability to adequately steward their own products. Recent history doesn’t leave us with a lot of optimism about this, and growers should expect that the approach they ultimately take would likely have to be more aggressive and smarter than the one promoted by providers of new technology.