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Avoid Postemergence Dicamba Applications in Iowa Soybeans, Say ISU Weed Scientists

ISU weed scientists instead recommend use in preemergence situations.

Iowa State University (ISU) weed scientists are recommending Iowa farmers avoid using dicamba designed for dicamba-tolerant soybean systems on a postemergence basis. Instead, they told those attending last week’s ISU Integrated Crop Management conference they’re recommending it only be used on a preemergence basis.

“What we decided is we will recommend the pre- and the burndown type of applications, but we will not recommend the postemergence applications,” says Mike Owen, ISU Extension weed specialist. “We are fully aware this limits the utility of the technology.

“We also looked at changes mandated by the EPA (Environmental Protection Agency),” adds Owen. “None of those changes do anything to address volatilization. They will help with particle drift and with spray tank contamination. But they summarily ignored the volatilization issue that we believe exists with dicamba products.”

Iowa’s Relationship with Dicamba

Dicamba isn’t a new herbicide for Iowa farmers. “Iowa has had a love-hate relationship with dicamba,” says Bob Hartzler, ISU Extension weed scientist. 

At times, dicamba was applied on up to 80% of corn acres in northwest Iowa, due to the area’s high pH soils. Atrazine was limited as a weed control tool on high-pH soils, so dicamba was a good alternative.

One bright spot in 2017 was dicamba’s efficacy on weeds, says Hartzler.

“Dicamba has never been a strong pigweed herbicide, so we wondered how effective it would be on waterhemp,” he says. “We were all impressed with how well it performed. We thought there would be more failures than what we saw. 

“With timely applications, it would have worked better,” he adds. “We saw pictures of people killing big waterhemp (taller than the 4-inch height stated on the label). That is not good management.”

What’s Changed

What’s different now is the dicamba formulations designed for application on dicamba-tolerant soybeans are applied later in the season compared with corn. Historically, dicamba applications on corn occurred prior to soybean emergence or in the early vegetative stages. If off-target movement did occur, soybeans were at growth stages much less susceptible to have yields impacted, Hartzler says.

This year changed matters. So far in 2017, the Iowa Department of Agriculture and Land Stewardship (IDALS) has had 271 Iowa off-target herbicide cases reported, 175 of which involved growth regulator herbicides that include dicamba.

“It was the first time for IDALS that there have ever been over 200 cases in a year,” says Hartzler. 

Not Enough 

Three dicamba formulations marketed in 2017 as low in volatility matched with dicamba-tolerant soybeans are:

  • BASF’s Engenias FeXapan Plus Vapor Grip Technology
  • Monsanto’s Xtendimax with Vapor Grip Technology
  • Dupont

Manufacturers have pegged the off-target dicamba movement within dicamba-tolerant systems is due to factors like:

  • Incorrect nozzle selection or boom height 
  • Excessive wind speed or wind direction toward a sensitive crop 
  • Insufficient buffer 
  • Spray system contamination 
  • Use of unregistered product 

Hartzler agrees that in many cases, off-target movement is due to factors like these. Still, volatility has played a role, too, he says. 

He cites a 2017 University of Missouri evaluation of soybean plant injury for volatility following application of Engenia, Xtendimax, and the older, more volatile Banvel formulation of dicamba. These chemicals were sprayed in geographically separate areas. Air samples were taken and indicator plants were placed at the following time intervals following applications of the three dicamba formulations: 

  • 0 to 2 hours
  • 2 to 8 hours
  • 8 to 16 hours
  • 16 to 24 hours
  • 24 to 72 hours

Visual injury symptoms were recorded 21 days following application. 

Not surprisingly, Banvel damaged the most soybeans in all six cases. But the injury gap between it and the formulations marketed as low in volatility weren’t as much as Hartzler expected.  

In the 24-to 72-hour window, for example, 22% of plants treated with Banvel showed visual injury 21 days after application. Soybeans with injury from Xtendimax, though, weren’t far behind with 18% injury. Engenia had the lowest rate of injury at 11%. 

“There was just a 20% (damage) reduction with Xtendimax compared with Banvel,” says Hartzler. “I say that is not enough.” 

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