Dial in Dicamba

New rules make for a narrow spray window.

Pamela Smith
By  Pamela Smith , Crops Technology Editor
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The need to cover many acres in a short period of time coupled with specific spray requirements has Tyler Young making plans and backup plans this year, Image by Pamela Smith

Tyler Young’s weed-control strategy this year is a battle plan filled with what-if contingencies.

It’s not unusual for the Gibson City, Illinois, farmer to enter the planting season with a complicated playbook of herbicide mixtures. He actually diagrams modes of action on field maps in an attempt to keep resistance issues at bay. He has looked forward to adding new trait technologies to his weed-control lineup, because he understands the necessity to keep weeds confused.

However, Young hesitated in 2018 when it came to deploying dicamba. There was simply too much uncertainty around the new formulations when he was booking seed last year.

“I will be using the Xtend trait this season with the expectation that I will need to spray dicamba,” Young says. “But, I say that knowing the window for application will be extremely narrow and staying on label a challenge, and, that I need a backup plan if I don’t get sprayed--not to maintain tabletop cleanliness but to try to avoid letting resistant weeds from going to seed.”


On Oct. 31, 2018, the United States EPA announced its decision to extend the registration of the dicamba products Engenia, FeXapan and XtendiMax for use in dicamba-tolerant crops (Xtend technology) through Dec. 20, 2020. However, additional label changes were made to an already lengthy set of spray requirements (see “New Dicamba Spray Requirements,” below, for changes from last year).

University of Tennessee weed scientist Larry Steckel counts nearly two dozen actions that spray applicators now need to carry out to make a legal over-the-top dicamba spray application in a tolerant crop. “We’ve never seen the complexity of labels like this before,” Steckel explains. “But, I also think applicators need to anticipate that any new herbicide registered in the future is likely to come with complex labels.

“If growers and applicators want to retain these products, they need to do everything possible to stay within the regulations,” he continues. “They are not merely suggestions. They are the law.”

At press time, some states were still eyeing the possibility of enacting even tougher borders, cutoff dates and temperature restrictions with regard to postemergence dicamba applications that go beyond the newly revised federal labels.


One change that hits home for Young is the new 57-foot omnidirectional buffer requirement that was included on the revised labels. Two of the three counties he farms are on the EPA list where threatened and endangered species, and critical habitat may exist. There were 218 counties in 24 states on that list at press time.

What that means is that in addition to the 110-foot downwind buffer when spraying near sensitive areas, Young (and anyone else spraying in these specific counties) must also leave a 57-foot buffer around all the other sides of that field.

“My mapping just got more complicated,” Young says, noting that he started contacting neighbors and learning the location of susceptible fields in early winter. According to EPA, nonsensitive areas may be included in the buffer distance calculations when directly adjacent to treated field edges. That includes roads, paved or gravel surfaces and mowed and/or managed areas adjacent to fields, such as rights-of-way. Planted agricultural fields containing corn, tolerant cotton and tolerant soybeans count, as do areas covered by the footprint of a building, silo or other man-made structure with walls and/or a roof.

That 110-foot downwind buffer is not intended to protect downwind sensitive crops and plants from off-target dicamba exposure, though, says University of Illinois weed specialist Aaron Hager.

“It is only intended to protect other sensitive areas, such as water bodies and endangered species habitat,” he adds. “It’s a vague ‘do not spray’ if the wind is blowing toward neighboring sensitive crops or residential areas. It is the responsibility of the applicator to make that decision.”


The new labels also require applicators to consult and record that they have consulted a sensitive-crop registry before spraying. For many, that will mean turning to FieldWatch, a free, national, map-based registry of sensitive crops and beehives available in 21 states.

This year, for the first time, row-crop growers can register non-dicamba-tolerant crops on a new registry called CropCheck (cropcheck.org). All FieldWatch maps include a time-and-date stamp for documentation. An app also allows applicators to view specialty crops and beehives within 10 to 15 miles of their location on their mobile devices. Applicators are still responsible for checking their surroundings for dicamba-sensitive crops and sites that may not be listed.


The fact that adding AMS (ammonium sulfate) or UAN (urea-ammonium nitrate) to the spray solution can increase dicamba volatility twentyfold has been a key point in applicator dicamba training this winter. Now, weed scientists are finding strong evidence that glyphosate in the tank is also altering the spray pH and possibility causing the mixture to move. The data was strong enough to cause EPA to issue a warning on the new dicamba label.

“I think, as a researcher, we all knew that Roundup [glyphosate] would lower the pH some. My surprise is how much it is really reducing it,” says Tom Mueller, the University of Tennessee weed scientist working on this project.

“We have an extensive data set from field and lab studies that shows that XtendiMax and Engenia alone are really relatively low in volatility. But, when we add Roundup to the tank, it is lowering the pH below the 5.0 stipulated on the label--and, it is causing more emissions,” Mueller says.

“Right now, we have no buffer or additive to change that,” he adds. Mueller understands farmers will not like the thought of two separate applications if they desire to use both herbicides.

“But, I think they need to consider it, particularly if they are in areas where they know sensitive crops exist,” he says.


Young’s big battle is waterhemp, which has shown the ability to resist multiple herbicides. He will be leaning on a three-mode preemergence product this spring and praying for rain to activate.

“Waterhemp can germinate within a day of tillage and quickly grow 4 inches or more in height,” he says.

“At that stage, I can only make an aggressively growing, resistant weed mad with Flexstar or Prefix. Dicamba gives me a shot at control if I can work within the spray window and still get the weed early and small,” he says.

Residual herbicides and 15-inch-row soybeans will help, but, a fallback plan is needed, he figures. “I hate to say it, but, we’re also looking for tractors that will fit a row cultivator--especially for those edges that have spoiled up in weed seed,” he says.

New Dicamba Spray Requirements*:

> Only certified applicators may apply or mix dicamba.

> Soybean application allowed up to 45 days after planting or to R-1 growth stage, whichever comes first.

> Cotton application allowed up to 60 days after planting and limited to two over-the-top applications.

> Applications allowed from one hour after sunrise to two hours before sunset.

> In counties where endangered species may exist, the applicators must leave a 57-foot buffer around remaining sides of field. A 110-foot downwind buffer remains for all fields adjoining sensitive areas.

> Test spray solution to make sure it remains above pH of 5.0.

> Records must be created within 72 hours of application and now include target crop-planting date, the buffer distance calculation and a record of the time during the application that spraying was stopped due to shifting wind directions or wind speeds.

> Applicators must check registry such as www.driftwatch.org for the presence of nearby sensitive crops or sites.

*List is of label changes for 2018–2020. Check for additional federal and state requirements and changes prior to spray application.


Pamela Smith

Pamela Smith
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