By Bob Hartzler and Michael D.K. Owen, Extension Weed Specialists

The EPA recently approved a new, low-volatile dicamba formulation — M1768 (Xtendimax with VaporGrip Technology) for dicamba-resistant soybean cultivars. While we recognize the benefit this technology provides in managing the growing herbicide resistance problem, we have concerns regarding the risks for non-target plant injury associated with an anticipated expanded use of dicamba.

All herbicides have risks, especially when used over large areas of the landscape. Synthetic auxin herbicides (Group 4) can cause significant plant injury at fractions of normal use rates, resulting in a higher risk of off-target injury than with most herbicides. The dicamba formulations that are approved, or are pending approval, for use with Roundup Ready 2 Xtend soybean have lower vapor pressures than earlier dicamba formulations. 

The lower vapor pressure reduces, but does not eliminate, the potential for off-target movement via volatilization. Label restrictions regarding application (nozzle type, PSI, boom height, buffer zones, etc.) will reduce risks associated with particle drift. 

However, we are concerned that these advancements in formulation and application technology will be offset by: 

1. The increased acres treated.
2. The higher use rates.
3. Applications made later in the growing season with higher temperatures. 

In addition, the risk of crop injury from dicamba residues remaining in sprayers when moving from Roundup Ready 2 Xtend soybean fields to non dicamba-resistant soybean fields cannot be overstated.

In order to successfully incorporate dicamba-resistant soybean into Iowa’s crop system, everyone involved with this technology must acknowledge the high risks associated with expanded dicamba use. Applicators must become familiar with, and follow, the application practices specified on the new product labels. Several of the restrictions involve significant changes in typical application procedures (e.g., maximum boom heights above the target or crop, buffer zones around sensitive vegetation, etc.). The widespread injury to soybean, horticultural crops and other sensitive plants that occurred during 2016 in Missouri and other states due to illegal applications of dicamba on Roundup Ready 2 Xtend soybean illustrates the risks when the proper stewardship is not practiced.

This article has focused on dicamba, but we have similar concerns regarding 2,4-D use on Enlist crops. Factors that reduce, but do not eliminate, non-target risks associated with the Enlist weed management system include:  

1. The choline salt and other amine formulations of 2,4-D are non-volatile under typical field conditions. 
2. Soybean has a higher tolerance to 2,4-D than dicamba.
3. Enlist crops are likely to be planted on fewer acres than Roundup Ready 2 Xtend soybean.  

Although the risk of injury to agronomic crops is less with an increase in use of 2,4-D compared to dicamba, there are many 2,4-D sensitive species in the Iowa landscape (e.g. grapes, tomatoes, landscape plants). Proper stewardship of this technology will be just as important as with the dicamba-based weed management system to prevent off-target injury.

Failure to successfully manage off-target movement of dicamba and 2,4-D poses a serious threat for Iowa agriculture. It is important to recognize that the costs associated with negligent applications of these products will not be limited to the crops damaged from off-target movement. Widespread injury associated with off-target movement is likely to result in greater restrictions or a loss of these technologies, and additional restrictions on alternative products.