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Dicamba – A New Learning Curve

University of Minnesota Extension, Stearns County News
August 16, 2017         
Source:  Dan Martens, Extension Educator
University of Minnesota Extension
Stearns, Benton & Morrison Counties


Dicamba – A New Learning Curve
By Dan Martens, University of Minnesota Extension

FOLEY, Minn. (08/11/2017) — Dicamba herbicide is getting a fair amount of attention in Ag Crop news this summer. Dicamba is the common chemical name of a growth regulator herbicide that has been around farms for at least 3-4 generations. It is in many products use to control dandelions and other weed in lawns. So what’s different now?

During the last couple of years, soybean varieties were approved that were genetically modified to tolerate dicamba. New formulations of dicamba were approved this year for use on dicamba tolerant soybeans. They are supposed to be less volatile and less likely to move off fields. It is illegal to apply other formulations of dicamba to these soybeans. The new dicamba products have detailed directions for minimizing risk to other fields. This works is also being done with cotton in the south.

Using dicamba on tolerant soybeans offers a significant opportunity to control weeds, like tall waterhemp, that are resistant to other common broadleaf herbicides, or soon will be. Herbicide and seed companies are working on other herbicide products and crop varieties that are tolerant to them.    

Spraying more land with dicamba increases the risk that soybeans and other crops that are not tolerant to dicamba will be injured by drift, volatilization, tank contamination, or application errors. Extension Weed Scientist Jeff Gunsolus says that dicamba could volatilize for up to 5 days after applications. In low dose exposure situations, leaf symptoms might not be seen for 14 to 21 days. Temperature inversion layers are an important concern, along with wind and other factors. Jeff points out that “less volatile” does NOT mean “not volatile.”

Minnesota Department of Ag is the lead agency for investigating dicamba injury issues. MDA is responsible for permits to sell and use pest management products. Calling MDA does NOT automatically result in a litigation process. MDA’s primary goal is determine whether dicamba is the problem; and then why or how. This is the key to learning how to use the product effectively with less risk to non-target crops. MDA can be called with concerns at 651-201-6333.

Weed Scientist Jeff Gunsolus shared recently that farmers and agronomists need to consider a broad range of weed management options including, but not limited to - Liberty Link technology, pre-emergent herbicides, earlier applications of pre and post emerge products, cultivation, crop rotations that allow more diversity in weed control practices and products; and yes, even hand weeding.

The most common soybean injury symptom from dicamba is cupping of leaves or leaves that are more “dish-shaped” than flat. Other kinds of stress can cause some cupping.

Several articles related to this topic can be found by doing an Internet search for “Minnesota Extension Crop News.” These include, Response of soybean yield to dicamba injury, Assess and documenting yield loss due to dicamba injury, and others that include pictures of typical symptoms. MDA also has information on their websites. Agribusinesses are also working hard on these issues. THE GOAL IS TO GET IT RIGHT. You’re welcome to call the Extension Office for any of this information. Articles are posted related to late season weeds and insect problems also.

LATE SEASON WEED ISSUES. The answer is that there are probably NOT herbicide solutions to problem weeds in crop fields at this point in the season. Poor control of large weeds, crop injury potential, and pre-harvest intervals become limitations. Some people are hand weeding to prevent seed production in fields. Some people might till down small patches of weed in fields to prevent seed production. Combines do a great job of spreading weed seed; and that should be avoided.

I looked at a soybean field infested with second generation THISTLE CATERPILLARS in northeast Benton County on Tuesday, August 8. The treatment threshold for the stage of “blooming and pod-fill” is 20% leaf damage and 35% for the “pod-fill to maturity” stage. This should be evaluated from top to bottom in the canopy. There are protocols to follow for evaluating fields. The goal is to consider care of bees and other beneficial insects in the context of taking appropriate care of the crop.

Please make safety a priority as we begin and prepare for some of the fall harvest work.


PHOTO CAPTION: Leaf cupping on soybeans that can be caused by dicamba herbicide. Photo by Aaron Hager, U-Illinois.

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