Roundup Ready2 Xtend (dicamba resistant) soybean cultivars have been deregulated and are approved by important international markets. And recently, the Environmental Protection Agency (EPA) accepted the label for XtendiMax with VaporGrip and Engenia.

Enlist (2,4-D resistant) soybean is not widely available due to a decision by Dow AgroSciences to curtail availability until the crop is deregulated globally. However, the 2,4-D and glyphosate herbicide combination (Enlist Duo) specifically formulated and registered for the new trait is approved by EPA, albeit not widely applied by farmers at this time.

A primary concern for these new herbicide-resistant crops and the new HG (herbicide group) 4 herbicide formulations is the issue of off-target movement and injury. The three avenues for off-target injury include movement by herbicide volatilization, movement by the drift of the spray droplets during application, and the movement onto sensitive crops via the contamination of sprayers and support equipment.

Based on Iowa State research, non-dicamba soybeans are more sensitive to dicamba than 2,4-D. Dicamba caused greater yield reduction at lower relative rates than 2,4-D. Foliar injury was observed at herbicide rates that did not cause a reduction in yield.

Drift that occurs during the reproductive development of sensitive soybeans has a greater negative impact on soybean yield based on research from the University of Missouri.

Illegal dicamba use

Roundup Ready2 Xtend soybeans have been widely planted despite the unavailability of the dicamba formulations labeled for application to the dicamba-resistant soybean cultivars. Unfortunately, decisions to use available, non-registered dicamba formulations for over-the-top applications has been common, particularly in the mid-south where more than 100,000 acres of non-dicamba resistant soybean and other crops were negatively impacted.

The question is whether the use of the new dicamba products with new formulations developed to reduce the potential for volatilization would have caused similar widespread problems.

There are three dicamba formulations that are or may be registered for use in Roundup Ready2 Xtend soybean. BASF has developed Engenia, a new salt of dicamba, which is described as having 70% lower relative volatility when compared to Clarity. Monsanto has developed Roundup Xtend (glyphosate + dicamba) (not currently registered) and XtendiMax (dicamba) with VaporGrip Technology; these formulations use the same salt of dicamba as Clarity, but include an additive (VaporGrip Technology) that reduces volatilization. DuPont will evetnually market its own brand of dicamba with Vapor Grip Technology, FeXapan, through an arrangement with Monsanto.

Volatilization issues

The potential for volatilization for these new dicamba formulations was determined from laboratory and growth cabinet experiments as well as field evaluations. The question becomes whether the volatilization data derived from experiments that represent a relatively small area is valid when farmers adopt the technology and spray the new dicamba formulations over the landscape and under variable environmental conditions.  The answer to this question remains to be seen.

The companies have developed extensive stewardship programs for farmers and applicators intended to minimize off-target movement of dicamba and 2,4-D. The goals of the programs are to increase awareness of application parameters and environmental conditions that contribute to particle and vapor drift.

The labels for the new formulations take a different approach to off-target movement than seen previously. Only nozzle types specified on the label (manufacturer, type, and size) can be used to apply the new formulations. Only products that have been tested to determine their effects on spray droplet size and thus drift can be tank-mixed with these herbicides. Non-treated buffer zones are specified when spraying adjacent to sensitive vegetation.  This information will be available on websites supported by the respective companies.

Farmer stewardship needed

Given the widespread illegal use of dicamba on the dicamba-resistant soybean, it is suggested that the programs thus far were less than successful at describing the needed stewardship concepts and gaining acceptance of the importance of these concepts. Again the question becomes how farmer adoption of the new dicamba formulations will fare when there is considerable more area treated across the landscape during widely variable environmental conditions. Will farmers and applicators use the appropriate spray tips and observe the environmental criteria described on the labels?

Lastly, the other potential problem for the new dicamba herbicides is sprayer and nurse tank contamination. Again, the herbicide labels describe the criteria to clean dicamba residues from tanks and sprayers. Research conducted at Iowa State University evaluating cleanout procedures demonstrates clearly that when specified procedures are followed, tank contamination can be significantly reduced and should minimize off-target issues. However, the studies did not assess the potential for contamination in spray lines, booms, and reservoirs where herbicide residues can result in serious off-target problems if these are not appropriately cleaned. It will be crucial for farmers and applicators to observe the equipment cleanout processes.

Overall, the important considerations to reduce off-target movement of the new HG4 herbicides are to follow the stewardship programs provided by the companies. These decisions and subsequent actions likely require time and procedures that are not simple or convenient. Importantly, these decisions and actions occur when there is limited time available for covering acres often contributing to poor decisions, especially during periods when unfavorable weather conditions limits time in the field.

Not a silver bullet

Finally, the expectations of the contributions that the new HG4 resistance soybeans will provide for the management of herbicide-resistant weeds need to be tempered by reality. While the HG4 herbicides can provide good control of many important herbicide-resistant weeds, they do not represent “the answer” to this burgeoning problem. Unless these new tools are included in a more holistic approach to weed management, it is unlikely that the benefits they provide will fully offset the potential risks that exist.

(Story written by Mike Owen, University Professor and Extension Specialist in Agronomy and Weed Science at Iowa State University. It appeared in the 2017 Herbicide Guide for Iowa Corn and Soybean Production.)