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A recent study found that the herbicide dicamba can have unintended consequences for some weedy plants.
The controversial chemical can travel miles away from the crop it's been applied to. That movement, known as dicamba drift, can damage nearby crops, plants and trees.
Some plants curl up and show signs of damage when exposed to those low doses of dicamba, such as oak trees, tomatoes and grapes. But according to research from the University of Michigan, a common weed called wood sorrel produces more flowers when exposed to small amounts of the chemical.
These flowers are also smaller, said Gina Baucom, University of Michigan ecologist who led the study.
"We find that really interesting," Baucom said. "We want to know what that means. Does it mean that the smaller flowers are producing reduced nectar or pollen for the pollinators?"
Baucom said her team first decided to study the impacts of dicamba drift on weeds because of its possible impact on insects.
"One of the important places for pollinators and natural organisms to get nutrition in agricultural fields are these areas around the crop and beside the crop that have natural weeds growing," she explained, adding that it could be problematic if dicamba drift were killing those plants.
Dicamba has a long history in agriculture, but scientists are still working to understand how it can affect surrounding plant and wildlife communities. Though the University of Michigan study focuses specifically on one weedy plant, the findings add to a growing body of research showing that the widely used agricultural herbicide may be incrementally changing ecosystems across the U.S.
A brief history on dicamba
Dicamba was first used in the 1960s as an herbicide for corn and for applications in soybean fields before plants began to sprout. The chemical wasn't directly applied to soybean plants until decades later, once dicamba-tolerant soybeans hit the market in 2016.
As more farmers began using dicamba, its tendency to drift on the wind became more apparent. Neighboring farms that didn't plant dicamba-resistant soybeans began seeing severe damage to their crops.
Bob Hartzler, a retired weed scientist from the University of Iowa, calls the widespread adoption of an herbicide that's lethal to non-resistant soybeans a "big black eye for agriculture."
"It was frustrating to me that so many in agriculture defended its use and tried to blame everything under the sun for the problems other than what the science showed was really the issue," said Hartzler, who was not involved in the study.
Dicamba turns into a vapor when exposed to temperatures above 85 degrees Fahrenheit. Those vapors are able to travel several miles, sometimes days after dicamba is applied to a field.
"In most cases, you're not killing plants with that drift, but you are altering those plants," Hartzler said.
The growth-regulating herbicide changes a plant's hormones, causing rapid, uncontrolled cell growth. In the normal dose that farmers are instructed to apply, most weeds die —including the wood sorrel in the University of Michigan study.
But when some plants are exposed to lower doses of dicamba, they are more likely to show a beneficial reaction, a biological phenomenon known as a hormetic response.
The University of Michigan researchers planted a field with wood sorrel, also known as oxalis, and treated half of the field with low doses of dicamba for two years. The wood sorrel exposed to dicamba flowered more and sometimes produced more seeds.
Importantly, however, wood sorrel did not show signs of becoming stronger over successive generations.
"That's good news because it means that we're not going to, over time, select for highly vigorous plants that can respond in a hormetic way to eventually have monster weeds," said Baucom.
Which means wood sorrel isn't stronger per say, nor is it necessarily a threat to farmers or homeowners. But Baucom said the changes within the plant could alter the surrounding ecosystem. Plants exposed to dicamba in the study showed lower rates of pollinator visitation, for instance, likely because these plants had more flowers for insects to visit.
That is one of many unknowns that experts are still researching.
Continuing research
Those potential, broader effects across landscapes have yet to be studied, according to Aaron Hager, a professor of crop sciences at the University of Illinois Urbana-Champaign who was not involved in the study.
"We knew there was going to be issues of off-target movement," Hager said. "Nobody really could predict how extensive it was going to be."
Hager said other factors should be taken into account when studying the effects of herbicides on plants.
"What happens now if a pathogen and dicamba are in there together? Is there any kind of an additive effect?" said Hager.
Baucom, the University of Michigan researcher, agrees there's more to be studied, including how plants like wood sorrel react to dicamba after being exposed to it for several years and how the herbicide affects pollinator health.
This study, she added, is a starting point for future investigation.
"It's really just an example of how the use of an herbicide can influence ecological interactions," Baucom said. "That's one of the things that my lab is now interested in: what are the downstream consequences of using herbicides in fields that then can influence dynamics in natural populations?"
This story was produced in partnership with Harvest Public Media, a collaboration of public media newsrooms in the Midwest and Great Plains. It reports on food systems, agriculture and rural issues.
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