‘Perfect storm’ links to dicamba injury

July 16, 2020

Injury related to dicamba usage appears to be more prevalent this year than in prior years according to Iowa State University agronomy professor and Extension weed specialist Bob Hartzler.

 “For the first three years of dicamba-resistant soybeans, it was pretty easy to figure out where the dicamba injury came from, you could see the gradient in the fields showing severe injury and then it would fade away across the field,” Hartzler said. “This year, a significant percentage of damage in fields is nearly fence row to fence row.”

Hartzler says the U.S. Court of Appeals' decision to cancel the registration of three dicamba products could have rushed applicators to spray the product based on fears it wouldn’t be available for use in 2020.

He noted that 71% of the state’s soybean crop was planted by May 10, and crops were reaching the state appropriate for post applications at the time of the June 3 ruling.

Hartzler and Prashant Jha, an associate professor and Extension weed specialist at Iowa State University presented a variety of factors that led to more widespread injury this year in a blog post. You can read more here.

More likely, Hartzler and Jha said, it was environmental factors responsible for more widespread injury this year than the ruling itself.

He noted conditions in North Central Iowa in the two weeks following the court’s ruling where wind speeds during the day averaged greater than 12 miles per hour for 10 of the 14 days, resulting in a total of 40 hours during that time when daytime wind speeds were between 3 and 10 miles per hour. Daily high temperatures exceeded 85 degrees on five of those days.

“Weather conditions limited opportunities to get fields sprayed, resulting in large quantities of dicamba being applied in a small time period,” Hartzler said.

Limited rainfall during that time also left dicamba on soil and foliar surfaces for extended times where it is prone to volatilization during hot periods, he noted.

Widespread injury

As a past member of the national dicamba task force for the American Soybean Association in 2017, Iowa Soybean Association (ISA) member Wayne Fredericks is concerned about the widespread cupping and injury he’s seeing in his fields this year.

He planted Enlist E3TM, glyphosate, glufosinate and 2,4-D tolerant (dicamba sensitive) soybeans this year as a way to limit as best he could the drift and volatility concerns associated with using the dicamba product. But now he’s seeing injury in his fields, despite most of his neighbors considering his fields during herbicide application.

 “And now, here we sit, looking at all this cupping,” Fredericks said.

Weed scientists in the Delta region coined the term ‘atmospheric loading’ to describe the problems in their region experienced with dicamba movement. Atmospheric loading refers to so much dicamba moving into the atmosphere that it makes it perhaps impossible to identify the specific application source resulting in injury to a field.

Fredericks has talked to Hartzler and other experts. He thinks atmospheric loading could be playing a part in the injury he’s seeing in his fields and in those around him.

Or, it could be a ‘perfect storm’.

“There were poor weather conditions during application and then hot, dry conditions. It’s tough to pinpoint the real culprit,” Fredericks said.

Overlapping usage

Corn and soybean acres in Iowa were planted at a record pace due to ideal planting conditions. In many areas, Hartzler says, there was an overlap between dicamba being sprayed on both corn and soybeans, resulting in a larger use throughout the state.

Hartzler says the use of dicamba in corn has increased in recent years due to the spread of waterhemp biotypes resistant to Group 5 (atrazine), Group 9 (glyphosate) and Group 27 herbicides (HPPD inhibitors). There have also been reports of dicamba being used for late post-emergence applications after previous treatment performed poorly, he said.

Hartzler identified cases in which dicamba rates were increased to the full label rate for post applications as resistance became more common to other products in Groups 5, 9 and 27.

He noted that marketing programs in 2020 resulted in movement from Status (dicamba + diflufenzopyr) to straight dicamba products. The diflufenzopyr in Status allows dicamba rates to be reduced by 50%, and the switch to straight dicamba products resulted in more dicamba per acre being applied.

ISA District 1 Director Brent Swart in Spencer says he’s not seeing cupping in his dicamba-tolerant soybeans, but injury does appear to be more widespread this year.

 “We’re seeing more this year versus last year as far as symptomology across whole fields,” says Swart, a farmer and Pioneer field agronomist.

He says the severity of injury appears to be different from field to field.

“In most cases it’s hard to say it came from a certain neighbor or direction because it could have come from any direction based on what we’re seeing,” Swart says.

As of now, it’s also difficult to determine how much yields might be affected by the injury.

“Everyone’s talking about options they have for next year to control weeds and what products are going to be available or not available,” Swart said. “But there’s a lot of that story yet to write based on what happens in the next few months.”

Ed Anderson, ISA’s senior director of research, says the widespread injury makes it difficult to pinpoint the specific source that caused the injury. This finding, he says, shows the need for an integrated weed management approach.

“Weeds are still our No. 1 challenge to production, and nature finds a way,” Anderson said. “I have grave concerns about over-the-top use of dicamba on corn and transgenic soybeans, where constantly exposing weed populations to the same chemistries and modes of action will increase the selection pressure for herbicide-resistant weeds.”

Moving forward

An integrated weed management approach includes different herbicides, including pre-plant and post-emergence applications and agronomic practices like timing corn and soybean planting, plant populations, row spacings and even the use of cover crops.

“Integrated weed management involves thinking holistically about your production operation,” Anderson says.

Hartzler says cover crops offer a potential solution to weed management.

“For people who haven’t used cover crops, it takes quite a bit of a shift in mindset,” Hartzler said. “But there are a lot of other benefits to using cover crops. As we struggle in managing weeds, that will help push people toward cover crops and help deal with some of the water quality issues in the state.”

There’s also an opportunity to research weed seed control, which Jha is undertaking at Iowa State University.

“Weed seed control won’t eliminate the need for herbicides, but it should allow reductions in how much herbicides are used and reduce the speed of resistance to those products,” Hartzler said.

For more information about resistance-related topics, check out the Iowa Pest Resistance Management Program and the United Soybean Board’s Take Action program.