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Commentary on Auxin Herbicides and Applying Them to Auxin-Resistant (AR) Crops

We are all aware of the ongoing debate about the use of dicamba growth-regulating herbicide on dicamba-tolerant soybean. The debate does not center on its utility as a herbicide that can be used to combat herbicide-resistant (HR) weeds, but rather on its safe use.

I am not qualified to discuss the technical aspects of the issues that have arisen around this new technology. However, several of the weed scientists/specialists in the US soybean-growing region have written about this or have recorded the major points associated with the dilemma created by this new technology.

Therefore, in this blog I will provide links to many of those resources, as well as a brief summary of the subject matter contained in each one. This is being done to provide you with an up-to-date, one-stop source for much of the expert commentary/advice/counsel that these individuals have provided on this subject.

First, I urge you to read a complete and thorough review of the potential issues surrounding the use of auxin herbicides 2,4-D and dicamba on tolerant crops that appeared in a 2012 article published by Purdue University Extension. Especially pay attention to the section “Factors Affecting Off-Site Movement” which includes segments on drift, which is the physical movement of spray particles by wind away from the target, and volatility, which is the movement of the gaseous form of the herbicide after its deposition on the intended target. These are the two contributing components to off-site movement of herbicides. Click here for a detailed discussion of these topics.

The Purdue article discusses the background leading to the development and evolution of genetically engineered (GE) 2,4-D-resistant technology (Dow Agrosciences Enlist Weed Control System) and dicamba-resistant technology (Monsanto’s Roundup Ready Plus Xtend System) in crops, a description of the two technologies, and concerns about, factors affecting, and methods of minimizing off-site movement of the two herbicides when they are used on tolerant crops.

The Enlist Weed Control System combines traits for resistance to 2,4-D, glyphosate, and glufosinate into soybean varieties. Dow Agrosciences developed Enlist Duo herbicide that contains both glyphosate and 2,4-D to be used in this system.

The Roundup Ready Plus Xtend System combines traits for resistance to both dicamba and glyphosate into soybean varieties. Monsanto developed Roundup Xtend herbicide (awaiting regulatory approval) that contains both dicamba and glyphosate to be used in this system.

Monsanto’s ExtendiMax and BASF’s Engenia (pending state registrations) herbicides that contain only dicamba have approved labels for use on soybeans. Click here and here for the supplemental labels for soybeans.

Since the Purdue article was written, dicamba-resistant soybean has become available for use by producers, and this has resulted in dicamba being used where previously it was not. This has resulted in a veritable plethora of issues associated with its use and misuse. And that has led to the articles that have and are addressing these issues. The remainder of this narrative will link to those articles (and one podcast) by the indicated authors.

In the 38-2016 issue of the C.O.R.N Newsletter published by the Agronomic Crops Network of Ohio State University Extension (title: The ExtendiMax Label for Xtend Soybeans), authors Drs. Mark Loux and Bill Johnson summarize the important points of the XtendiMax (dicamba product) label for applying dicamba on Xtend (dicamba-resistant) soybeans. They cover the volatility-reducing component of the herbicide, its application rate and period of allowed application to soybean, unallowed spray additives, and allowed application parameters that include required nozzle type, allowed weather conditions, and required buffer distances between treated soybean and downwind sensitive areas. It is important to note from their presentation that the ultimate responsibility for adhering to all the guidelines and restrictions in the label falls on the applicator.

In a Nov. 21, 2016 post from the Univ. of Arkansas Extension Service (title–Explained: Dicamba and its Formulations), Dr. Bob Scott, Professor of Weed Science at the Univ. of Arkansas, discusses the four formulations of dicamba on the market. The two important formulations to become acquainted with are 1) Clarity (combination of dicamba with diglycolamine, or DGA salt), which is the same formulation being marketed by Monsanto as XtendiMax, and 2) BASF’s Engenia herbicide (pending state registrations), which is dicamba combined with sodium methylamine, or BAPMA salt. The BAPMA salt reduces the volatility of the dicamba molecule. Click here for an Engenia brochure from BASF.

In a Dec. 2, 2016 article in Delta Farm Press (Title: What You Should Know about Newly Approved Dicamba formulations), Dr. Larry Steckel, Univ. of Tenn. Weed Specialist, presents a general overview of the requirements and cautions when using labeled dicamba products. Specifically, he shows that extremely low rates of this herbicide can be detrimental to off-target plants that are sensitive to dicamba, and how this has dictated the strict dicamba herbicide label restrictions that must be followed. This requirement for additional caution when applying this herbicide has led or will lead to mandatory stewardship training in most states. He adds a final cautionary statement about the possible repercussions if proper stewardship is not followed when applying dicamba products. In a UTIA AgCast podcast entitled “New Podcast Covers 2017 Dicamba Expectations” dated Dec. 12, 2016, Dr. Steckel discusses all of the above issues with dicamba.

In that same Dec. 2, 2016 issue of Delta Farm Press, David Bennett reported on the action by the Arkansas Plant Board to apply restrictions on the use of dicamba in 2017. This action was in response to off-label applications of dicamba that led to off-target drift issues that damaged considerable acreage of midsouth crops in 2016. The matter awaits action by the governor. The specific regulations recommended by the Board include: 1) a ban on dicamba DMA salt and acid formulations; 2) prohibiting the application of dicamba DGA salt and sodium salt formulations from April 15 to September 15; 3) requiring growers to abide by a quarter-mile downwind buffer and a 100-foot buffer for other directions when applying BASF’s Engenia herbicide on dicamba-tolerant soybeans and cotton; and 4) a training requirement for anyone using DGA-based herbicides on dicamba-tolerant crops.

In the Dec. 5, 2016 issue of Southeast Farm Press, Dr. Eric Prostko, Extension Weed Scientist at the Univ. of Georgia, presents a few thoughts on the coming use of dicamba herbicides on dicamba-tolerant crops. They are: 1) AR crop technology is a new tool in the weed control toolbox and will work if properly used; 2) Auxin herbicides are not new, and AR weeds have already been identified; thus, auxin herbicides must be used as one tool in combination with already available and used weed management tools to prevent an increase in AR weeds; 3) Proper label-specified application of auxin herbicides must be followed to prevent a furtherance of the negativity associated with the use of GMO crops; and 4) as with all herbicides, all available weed management tools–e.g., different herbicide modes of action, tillage, cover crops, residual herbicides, narrow rows, crop rotation–should be used in combination with the new auxin herbicide systems.

In the Dec. 15, 2016 online issue of Southeast Farm Press, Dr. Prostko provides a concise summary of highlights from dicamba herbicide labels. They include what can and can’t be tank-mixed with the herbicide; required nozzle type, spray volume, ground speed, and spray boom height; range of allowed wind speeds for application; and amount of downwind buffer required when applying the herbicide.

The issue of auxin herbicide use on AR crops is in a state of flux. Things to look for in the near future are the status of the forthcoming Enlist Duo and Roundup Xtend herbicide labels, a schedule of required training opportunities in the various states, and further restrictions that may be forthcoming for the application of auxin herbicides.

So, as of this writing, there are soybean varieties that have resistance to both 2,4-D and glyphosate, and soybean varieties that have resistance to both dicamba and glyphosate. However, herbicide products with these combinations have not received regulatory approval, and thus are not available for use in their respective systems. Xtend soybean varieties are available and will be planted, but dicamba and glyphosate herbicides can only be applied to them in separate applications. Also remember that 2,4-D-resistant soybeans will be vulnerable to injury from dicamba off-target movement, and dicamba-resistant soybeans will be vulnerable to injury from 2,4-D off-target movement.

Even if/when use of either or both of these two auxin herbicide-resistant systems in soybeans is fully realized, they must not be relied on alone to manage HR weeds. Using a diversified approach of applying herbicides with different modes of action in combination with sound agronomic practices listed above will remain the only durable and productive weed management system for soybeans.

When writing this article, I was very careful to provide the facts as I interpreted them. However, it is very possible that I misinterpreted facts from some of the sources, which may have resulted in unintentional misstatements. Therefore, be sure to consult with your state research and extension weed specialists for clarification if in doubt.

Composed by Larry G. Heatherly, Dec. 2016, Thanks to Dr. Trent Irby, MSU Extension Soybean Specialist, for review and edits.