Sorting Out the Soybean Herbicide Resistance Traits

By:  Mark Loux, Ohio State University

The world of soybean herbicide resistance traits has gotten more complex over the past several years.  The good news is that we have new options for control of herbicide-resistant weeds, although it can be a little difficult to sort out which one is best for a given situation and whether the possible downsides of certain traits are tolerable.  The following is a quick rundown of what’s available and some things to consider when selecting seed.  This is not meant to be an extensive evaluation/description of these systems because including all the possible configurations of herbicide use and the stewardship stuff would probably kill the possibility that anyone reads the rest of the article.  We also do not attempt to include all of the possible seed trade names.  For ratings of herbicide effectiveness on certain weeds, check the tables in the “Weed Control Guide for Ohio, Indiana, and Illinois”.

Roundup Ready (RR1, RR 2 Yield, etc.) – the original herbicide resistance trait.  Resistant to glyphosate which can be applied anytime up through R2

LibertyLink – resistant to glufosinate (Liberty, Interline, etc.) which can be applied anytime up to R1.

LL-GT27 (Freedom Plus, etc.) – resistant to glyphosate, glufosinate, and isoxaflutole (Balance), although there is currently no isoxaflutole product approved for use in these soybeans.

Enlist – resistant to glyphosate, glufosinate, and 2,4-D.  Enlist One (2,4-D choline) and Enlist Duo (2,4-D choline + glyphosate) are the only 2,4-D products approved for preemergence and postemergence use on this soybean, outside of the typical use of 2,4-D ester 7 or more days ahead of planting that works on any soybean.  These products can be used any time before or after planting Enlist soybeans without a waiting period as well as postemergence through R2

Roundup Ready Xtend – resistant to glyphosate and dicamba.  XtendiMax, FeXapan, and Engenia are the dicamba products approved for preemergence and postemergence use on this soybean.  These products can be applied any time before or after Xtend soybean planting without a waiting period, as well as postemergence (prior to R1 and no later than 45 days after planting).

Note:  Dicamba and 2,4-D are different herbicides.  Dicamba cannot be applied to Enlist soybeans and 2,4-D cannot be applied to Xtend soybeans.  Just like glyphosate cannot be applied to LibertyLink soybeans and glufosinate cannot be applied to Roundup Ready soybeans.  Seems obvious but it’s a surprisingly frequent question.

All of these soybean herbicide trait systems have utility in certain situations.  Factors determining this are the resistant weeds present and the type of tillage.  The primary resistant weed issues in Ohio, which require herbicides other than glyphosate, are marestail, giant and common ragweed, waterhemp, and Palmer amaranth.  A few things to consider here – all of which assume that some type of residual herbicides are being used, regardless of the specific weed issues:

• Any type of tillage that mixes the upper few inches of soil and uproots weeds often renders marestail more or less a non-issue but does not really greatly affect the other weeds listed here.  Marestail can often be handled well enough with a combination of fall burndown and spring burndown + residual, although the residuals are not bulletproof.  Otherwise, use of Xtend soybeans and a preemergence dicamba product provides an effective alternative for spring burndown of marestail, as well as a dicamba POST option.  The ability to use higher rates of 2,4-D at planting in the Enlist system does not usually result in effective marestail burndown without a fall treatment, although it can be followed with a POST application of glufosinate and/or 2,4-D.  The combination of PRE and POST has allowed for effective control in our research with Enlist.  LibertyLink and the LL-GT27 soybean provide for the POST use of glufosinate to control marestail, but do not change the burndown options (glufosinate can be used for burndown in any type of soybean, prior to emergence).
• Giant ragweed requires initial control through tillage or burndown herbicides, and also postemergence control.  Most populations have lost response to glyphosate and some are highly resistant.  Burndown has not usually been an issue as long as there’s a way to get some 2,4-D, dicamba, or Sharpen in the mix, so there’s not necessarily an advantage to any of these systems.  With the exception that the Enlist and Xtend systems allow more flexibility in use of 2,4-D or dicamba, respectively, since there’s no wait between application and planting.  Using any system besides the basic Roundup Ready will also provide for more effective POST control, although the edge usually goes to systemic herbicides on bigger plants.  Rate and application parameters have a substantial effect on glufosinate activity, and optimization of these parameters can make the difference between so-so and effective control.  These same considerations apply to common ragweed as well.
• Waterhemp and Palmer amaranth require essentially the same approach, with the emphasis on residual and postemergence herbicides.  Postemergence control is complicated by herbicide resistance, need for plants to be small, and emergence that extends well into the growing season.  We assume almost all of the waterhemp populations are resistant to glyphosate and site 2 herbicides, and a number of populations are also resistant to site 14 herbicides.  So, a system that allows postemergence use of 2,4-D, dicamba, or glufosinate will provide for the most consistently effective control.  All of these can become considerably more variable in effectiveness on larger plants.  And all can require the addition of a site 15 residual herbicide to provide control of later-emerging plants.  We would consider Enlist to have an advantage over the Xtend, LibertyLink, and LL-GT27 soybeans because it’s the only system where we know we can still mix two postemergence herbicides that are still effective – 2,4-D and glufosinate.  Postemergence control in Xtend depends upon dicamba, and in LibertyLink and LL-GT27 depends upon glufosinate.  Advantages to Enlist are two-fold: 1) the mix of 2,4-D and glufosinate will be more consistently effective on larger waterhemp than any of these herbicides applied singly; and 2) mixing two herbicides with different sites of action that still have activity reduces the selection for resistance.  It’s a game of chance really, with the odds of a plant having two concurrent mutations that confer resistance to both sites of action is considerably lower (but not impossible) than the odds of a single mutation conferring resistance to one site of action.  Even when the Xtend soybean has glufosinate resistance added to its genetics, it’s hard to fathom how one could mix dicamba and glufosinate while still optimizing glufosinate activity and following dicamba stewardship guidelines.  This is of course not to say that the mix of two herbicides is always more effective than a single herbicide, although this is probably one of the founding principles of weed science.  Similar principles apply to Palmer amaranth control, except that it’s found only infrequently in Ohio still and we have not observed resistance to site 14 herbicides yet.
• Some seed dealers have stopped offering the basic Roundup Ready seed, because of the lack of POST options for these weeds.  Assuming marestail has been taken care of by burndown + residual, it is possible to make this system work by adding a site 14 herbicide (fomesafen product or Cobra) to POST glyphosate treatments.  This will be a generally more variable approach than using one of the other systems, and usually results in some degree of soybean injury.  Some waterhemp and common ragweed populations are already resistant to both glyphosate and site 14 herbicides, and Roundup Ready soybeans will not work in these fields (same goes for non-GMO since site 14 herbicides are the only POST option).
• Needless to say, there are extensive stewardship guidelines for the Xtend and Enlist systems, which can make them more burdensome to use than the LibertyLink and LL-GT systems.  The large-scale experiment with dicamba on millions of acres over the past several years has led university weed scientists to conclude that there is an unpredictable component to its off-target movement that is not necessarily controlled by following stewardship guidelines.  This is not to say that it always moves off-target of course, it’s just something that should be considered when decisions about which trait system to use are made.  We hope that the same does not occur for 2,4-D use on Enlist soybeans, but the large-scale experiment with this technology starts in 2020, so it remains to be seen.
• There can be issues with applying some of these herbicides in mixtures, and these are still evolving.  Even if it does not control the resistant weeds, glyphosate still has utility on most other weeds and can be the cheapest way to control grasses.  It can be used in mixtures with the other herbicides mentioned here in all trait systems except the LibertyLink (without the “GT27”).  Previous research and field experience have shown that control of barnyardgrass and certain other weeds can be reduced with a mixture of glyphosate and glufosinate, compared with separate applications.  Mixing dicamba with postemergence grass herbicides (clethodim etc.) can result in reduced control of volunteer corn.  All of these interactions seem to be specific to certain weeds, and also weather conditions in some cases.  Bottom line is that there’s probably more to learn about how these herbicides interact in mixtures.
• For any of these traits, it’s important to take resistance management into consideration.  Use an effective residual herbicide program, try to vary herbicide site of action between corn and soybeans, don’t overuse the same postemergence herbicides in soybeans, and use mixtures of different sites of action that have activity on the same weeds.  Follow up in mid to late season to remove waterhemp and Palmer amaranth escapes and prevent seed.