Arkansas soybean farmers fighting herbicide-resistant pigweed got good and bad news Wednesday.
On one hand, word came that a federal court had cleared the way for use of Dow Chemical's herbicide Enlist Duo, a technology eagerly awaited by farmers.
On the other, a University of Arkansas researcher confirmed that experiments conducted in a greenhouse have shown pigweed can develop resistance to another much-anticipated herbicide, Monsanto's Xtend.
The research results, announced Sunday on Twitter by Bob Scott, a UA extension weed specialist, produced a flurry of inquiries from farmers and Monsanto officials, Scott said.
"I think it was the tweet heard around the world," Scott said Wednesday.
Scientists consider herbicide-resistant pigweed the top weed threat to Arkansas's $1.7 billion soybean crop, which likely prompted the strong state reaction to the news. The weed, more formally known as Palmer amaranth, can wipe out entire fields of soybeans in extreme cases and more frequently cuts yields by 10 to 20 percent, Scott said.
Resistance in pigweed isn't new, but scientists and farmers are worried they are running out of herbicides to kill it.
Farmers had been hoping to use Enlist Duo, and the U.S. Environmental Protection Agency approved it in 2014. Environmental groups and others sued to block the herbicide's release. In November, the EPA asked the 9th U.S. Circuit Court of Appeals to revoke its approval, saying Enlist Duo, which contains glyphosate and a new formulation of 2,4-D, might be more toxic than previously believed.
On Monday, the San Francisco-based court rejected the EPA's request without explaining its decision.
An EPA spokesman said Wednesday that the agency was "reconsidering" Enlist Duo's registration.
"The agency is awaiting new data from the manufacturer and will review that information to determine next steps," the spokesman said in an email. "Meanwhile, Enlist Duo remains in effect, and the product can continue to be sold at this time."
Dow Chemical said in a statement that it would "continue to work cooperatively" with EPA on Enlist Duo.
"As a result of the [court] decision, the current U.S. registration for Enlist Duo remains fully intact for all labeled uses," a Dow spokesman said.
Terry Walker, director of the State Plant Board, said Arkansas growers would be able to use Enlist Duo with genetically modified Enlist soybeans this year.
"If it's properly registered with the EPA, then it's legal to use in the state," Walker said.
He said farmers must still comply with state regulations designed to prevent Enlist Duo from drifting from their fields and damaging neighboring crops and other plants.
Scott said farmers must remain careful not to overuse any one type of herbicide or resistance will develop relatively quickly.
The UA researchers, working in a greenhouse, applied the herbicide dicamba to pigweed collected from fields, Scott said. Monsanto's Xtend is made up of dicamba and Roundup, which is no longer effective against pigweed.
In the UA experiment, applications of dicamba killed the first two generations of pigweed, Scott said. The third was resistant, he said.
"This is the inevitable result of using a single effective mode of action to control a given pest," Scott said.
The term "mode of action," or mechanism of action, refers to the way a herbicide works inside a plant to kill it.
A Monsanto spokesman said the company was aware of Scott's tweet on dicamba resistance.
"We support this type of research as it helps weed scientists understand mechanisms by which plants can develop tolerances to herbicides," the spokesman said in an email, adding that the company recommends using chemicals with multiple mechanisms of action.
Xtend soybeans have received federal approval, but the Xtend herbicide has not yet been cleared by EPA.
Asked if Monsanto hoped to release the Xtend system in 2016, the spokesman said only that the herbicide was "in the late stages of review by the EPA."
Scott said any new herbicides must be affordably priced. Otherwise, he said, farmers already pressured by low commodity prices would be less likely to buy multiple herbicides with differing modes of action.
"If we don't [use multiple modes of action], we have shown that pigweed will have the ability to develop resistance to dicamba ... and every other chemical that comes out of the gate," Scott said.
Rotating corn or rice with soybeans is the best way to control pigweed, he said. Multiple herbicides are still effective against pigweed in rice and corn, Scott said.
Another option would be to plant LibertyLink soybeans and use Liberty herbicide along with the chemicals Dual or Zidua with metribuzin, he said. The latter chemicals will prevent development of resistance to Liberty, Scott said.
Using Dual or Zidua with metribuzin also will hinder resistance to Enlist Duo, Scott said. In addition, Liberty can be used on Enlist soybeans, he said.
Business on 01/28/2016