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Dicamba waiting game

Article cover photo
Farmers will have to wait until harvest is complete before the effects of dicamba applications on soybean yields -- positive and negative -- are widely known. (Photo: Joseph Murphy/Iowa Soybean Association)

By Matthew Wilde, ISA senior writer

As the song from the late, great Tom Petty goes, sometimes waiting is the hardest part.

Ever since reformulated versions of dicamba were sprayed on soybean fields this year, farmers and industry officials have anxiously awaited yield results — both good and bad from on- and off-target applications.

They will have to wait some more.

“I haven’t heard any yield reports yet related to dicamba,” said Mark Licht, Iowa State University (ISU) Extension and Outreach cropping systems agronomist. “The proof will be at harvest … wait and see what happens. There’s not a lot of data on this yet.”

A similar tune was sung by about a dozen ISU weed and agronomy specialists and farmers. Many of whom planted dicamba-tolerant soybeans and used associated chemistry or have suspected injury to non-tolerant varieties. Officials cited several reasons:

  • Farmers aren’t taking time to report yields yet.
  • Fields haven’t been harvested.
  • Farmers can’t say with certainty whether dicamba-tolerant soybeans and the chemistry helped or hurt yields.

Sixteen percent of the state’s soybeans, about 1.6 million acres, were harvested as of Sunday, according to the weekly U.S. Department of Agriculture Iowa Crop Progress and Condition Report. Records indicate 94 percent of the 10 million acres planted were herbicide-resistant varieties.

The only dicamba-tolerant soybeans available this year are Roundup Ready 2® Xtend from Monsanto, genetically engineered to tolerate Xtendimax™ with VaporGrip Technoloy, a low-volatile formulation of dicamba for pre- and post-emergence use, and glyphosate. About 20 million acres were planted nationally, according to the company.

Mike Owen, an ISU weed specialist, estimates about 1 million acres were planted in Iowa, based on feedback from colleagues. Only 50,000 acres (5 percent) were treated with new and improved dicamba chemistries such as Xtendimax, Engenia by BASF and FeXapan by DuPont — all engineered to minimize volatility —reports indicate.

Yet, Owen said ISU’s field agronomists project 150,000 to 450,000 acres of soybeans statewide sustained some degree of injury from the herbicide.

“The majority of Xtend beans weren’t treated with dicamba, but still there was fairly widespread injury,” Owen said.

However, injury doesn’t necessarily translate to yield loss.

“I have not heard anything related to yields yet,” Owen added, despite conversations with farmers throughout the growing season about suspected dicamba drift damage. 

The growth regulator overloads a plant’s hormone balance and causes growth irregularities and death. Non-tolerant soybeans are very sensitive to dicamba. A very low dose — a 1/500th rate — associated with drift can interfere with development of pods and flowers, possibly causing yield loss. Leaf cupping is a classic symptom of injury.

Owen said yield loss, if any, depends on the growth stage of affected soybeans and the concentration level of the herbicide. Soybeans in the reproductive phase are more likely to sustain injury than in the vegetative stage.

“There will be yield losses but it’s very hard to predict. The weather is such a big factor,” Owen said. “Sometimes you just can’t tell what is the real reason.”

A resurgence in dicamba use occurred to battle herbicide-resistant weeds. The decades-old herbicide is prone to drift from wind, rain and temperature inversion.

Companies developed new less-volatile formulations and strict label requirements to minimize off-target injury.

Best efforts and intentions aside, it still occurred. There were 107 official dicamba-related injury investigations by the Iowa Department of Agriculture and Land Stewardship as of Sept. 15. Almost all are soybean related. State and ISU officials say there’s probably many more cases not reported.

Brock Hansen planted Xtend soybeans on 25 percent of his soybean acres and sprayed XtendiMax with VaporGrip Technology. The ISA member from Baxter said the Xtend fields were “extremely clean” and there were no drift problems. Soybeans are averaging 65 bushels per acre, he reported.

“I can’t say the yield was positive or negative,” Hansen said. “I was very nervous spraying; it can be managed. It’s a tool we need.”

Hansen said he followed label instructions and exceeded sprayer cleanout requirements. However, he believes a little dicamba remained in the booms when he sprayed some of his non-dicamba soybeans.

“It saw some wilting and cupping in a small area, maybe an acre. But it didn’t hurt the yield,” he added.

ISA member Sean Harmon raises LibertyLink® seed soybeans for Stine Seed near Ankeny and Eagle Grove. He said dicamba applied to a neighboring field drifted onto 40-50 acres of a field. Plus, water runoff containing dicamba residue killed about a quarter-acre of the same field.

Harmon isn’t sure if yields suffered in the drift area, but the soybeans could be rejected by Stine if dicamba is found in the germ plasm. Testing is being done.

“If it wasn’t for the germ deal, I wouldn’t be worried,” he said. “In my opinion, dicamba shouldn’t be sprayed at all in-season. The neighbor did everything by the book, I can’t blame him.”

ISU Extension field agronomist Joel DeJong surveyed multiple fields with dicamba drift symptoms this year in northwest Iowa. But few soybeans have been combined, limiting yield reports.

“I’m waiting to hear too,” he lamented. “It will take a while to find out.”

For media inquiries, please contact Katie Johnson, ISA Public Relations Manager at or Aaron Putze, ISA Communications Director at

For permission to republish articles or to request high-res photos contact Aaron Putze at Iowa Soybean Association | 1255 SW Prairie Trail Pkwy | Ankeny | IA | 50023 | US

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