• While dicamba is not a new herbicide, applications to soybeans in-crop is a new use pattern.
• Additional steps need to be taken with tank mix partners to ensure effective control, because some negative interactions may be noted when mixing with dicamba.
• Potential negative interactions from combinations of dicamba plus Group 1 herbicides (Select Max®) or group 9 herbicides (Roundup PowerMax®) can be overcome by managing rate and using adjuvants to increase herbicidal activity.

This years' launch of Roundup Ready 2 Xtend® soybeans marks a new era in weed management.  Growers have been looking forward to this new tool to help them manage their hard to control and glyphosate-resistant weeds.  While we have been using dicamba in corn acres for decades, this new in-crop use for soybeans carries a few additional considerations to make sure our tank mix partners are as efficacious as planned.

As we all know, XtendiMax™, Engenia™, and FeXapan™ are the only dicamba products that are labeled for over the top use in dicamba-tolerant soybeans.  As discussed in a previous article, you must visit the manufacturer website to determine which products can be tank mixed with each respective dicamba herbicide.   While there is an abundance of choices, the group 1 and group 9 herbicides need additional attention to ensure success. 

Group 1 Herbicides 
Group 1 herbicides are most frequently used to manage volunteer corn that emerges in our soybean crop.  One of the most commonly recognized herbicides in this group that is approved for use with the new dicamba products is Select Max®.  The concern with dicamba + group 1 herbicide tank mixtures is the potential for less than complete control of volunteer corn.  Research has shown a decrease in clethodim (active ingredient in Select Max®) activity when it has been tank mixed with dicamba.  Does this mean we are likely that our soybean fields will be full of volunteer corn?  Absolutely not—especially if we actively take measures to prevent this from happening.  There are two actions we can take to avoid reduced activity: be conscious of rate integrity and use proper adjuvants to gain complete control.  These products have a wide range of use rates, and by keeping the use rate above the minimum required to manage the volunteer corn will help offset some of the potential negative interactions when tank mixed with dicamba.  Also, the use of an adjuvant will help the group 1 herbicides to effectively spread or penetrate the volunteer corn leaf surface allowing for greater control.  Depending on the dicamba product used, FS AquaPrime™ or FS COC Supreme™ will enhance group 1 herbicide activity.  A drift reduction adjuvant (DRA) is also required when using a group 1 herbicide with dicamba.  The FS system offers FS Intention™ to satisfy this requirement while effectively managing particle drift.

Group 9 Herbicides
Group 9 herbicides include the active ingredient glyphosate which is most commonly recognized as Roundup PowerMax®.  With some grass species, control from glyphosate has been negatively impacted when tank mixed with dicamba products.  To avoid this possibility, we need to make sure that we don't decrease the rate of glyphosate so much that it becomes ineffective.  However, with the new dicamba formulations, we are limited to a maximum glyphosate rate of 1.125 lb. ae/a (32 fl. oz/a Roundup PowerMax®).  This maximum use rate should be sufficient to effectively manage our common grass weeds we see in our soybean fields.  In addition to managing the use rate of glyphosate, we must use the proper adjuvant to ensure success.  The primary adjuvant previously used to maximize glyphosate activity was AMS.  AMS would condition water by managing positively charged metal ions that would otherwise bind with glyphosate and reduce activity.  AMS usage with XtendiMax™, Engenia™, and FeXapan™ is not allowed, as it enhances the potential for volatilization of dicamba.  FS offers a non-AMS based water conditioner named FS Certin™ to increase glyphosate activity and manage positively charged metal ions in water without increasing volatility of dicamba.  A DRA is also required when mixing glyphosate with the new dicamba formulations.

Overall, there are many benefits to this new herbicide technology.  With all new technologies there is a small learning curve to making them successful within our operation.  By recognizing these small hurdles and acting on them prior to application can help ensure all goes smoothly and the desired result is achieved.

Contact your FS Crop Specialist for your agronomic information.