Weed scientists say farmers will lose a valuable herbicide, if EPA follows through on a proposal to prohibit the use of diuron on food and feed crops.

EPA plans to end the chemical's uses on food and feed crops including alfalfa, grapes, field corn and about three dozen others, in addition to cotton, whose seed is used as cattle feed.

But the agency also said it was revoking all food tolerances for diuron except for cotton seed, which would appear to leave the door open to its continued use as feed. Diuron could be used only as a harvest-time defoliant on cotton, under the proposal. 

The proposed interim decision was made because of the cancer risk to humans and effects on wildlife. EPA is accepting comments until June 27.

Late Tuesday, the Diuron Task Force, composed of two registrants, issued a statement saying EPA's decision was based on "conservative modeling of potential drinking water concentrations and dietary exposure."

Lead members of the task force, which include ADAMA Agan and Tessenderlo Kerley, said they have "generated substantial data to support registration review of diuron and are committed to supporting the continued availability of it for farmers for the years to come."

“I would say 99% of the cotton weed management programs that I recommend to farmers include diuron at some point in the system,” said professor and extension weed scientist Stanley Culpeper at the University of Georgia. “That is about as important as it gets. So, I am very stressed out.”

EPA said in its announcement that “there are many alternative herbicides available for important use sites such as cotton, asparagus, blueberry, citrus, and non-agricultural sites.” But Culpeper said, “I would scientifically, with plenty of data to support it, suggest that is not necessarily the case.”

“Our toolbox is in trouble because of resistance and because of the current regulatory atmosphere,” he said. “We need to preserve our tools, we need new tools.”

Larry Steckel, a weed scientist at the University of Tennessee, adds, “I guess I’ve got some alternatives. But if we keep getting rid of alternatives, we won’t have any alternatives.”

Nathan DonleyNathan Donley

 But Nathan Donley, environmental health science director at the Center for Biological Diversity, told Agri-Pulse, “The resistance management argument has essentially turned into a ‘Let’s keep every pesticide on the market forever just in case’ argument. If the pest you’re targeting keeps developing resistance to pesticides, that’s not a reason to keep every pesticide approved, it’s an indication that the system is broken and needs fixing.”

In a statement issued after the EPA announcement, Donley said, “We can only hope this important, science-based decision is an indication of a new path forward where the agency routinely prioritizes the health of people and wildlife when assessing all pesticides.”

Culpeper agreed that “we have to make sure the tools that we use are safe for the user, the consumer and the environment,” but also said diuron is “critical to weed management.”

“We need to do everything we can to try to make sure we're using that tool the way it needs to be used to protect the environment, to protect the user, to protect the consumer, but hopefully still have it available to manage the most problematic pests,” he added.

In its proposed interim decision document, EPA said that from 2015 to 2019, growers applied an average of 2.3 million pounds of diuron each year on about 4.4 million total acres. Most of it, about 1.5 million pounds of active ingredient, or 65% of all diuron applied in agriculture, was to control weeds in cotton. Crops with the next highest usage include citrus, alfalfa and fallow fields.

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Dennis Rak, who owns Double A Vineyards in New York, which provides grapevines, berries, shrub willow and hops, says diuron has been an effective tool. “We use it in rotational management” to prevent resistance, he says. One big benefit is that it’s inexpensive.

“We do have alternatives, but I sure hate to lose it,” he says. 

But John Roncoroni, a University of California Cooperative Extension adviser emeritus, called diuron “only minimally important to wine grape growers.” Its potential for leaching means it cannot be used in many areas of California. “We have many other herbicides which are available to growers with much better toxicity profiles,” Roncoroni said.

Culpeper and Steckel also say the cost factor is important for growers, and they stress diuron’s effectiveness on Palmer amaranth, commonly called pigweed.

Culpeper said 1,737 growers recently responded to a survey he conducted that asked them to identify the most problematic pests they deal with — not just insects but also weeds. Palmer amaranth was listed 1,773 times. In other words, some growers listed it more than once.

And Steckel noted the herbicide’s effectiveness as a residual. Atrazine breaks down quickly, “it basically has no residual,” he says. “Diuron doesn't suffer from that.”

"Diuron is an effective herbicide which provides significant value to growers for the control of invasive and resistant weeds when used on alfalfa, cotton, citrus, and several other crops as well as non-agricultural areas," the registrants said. "It has been used for decades, and when used according to its label, the benefits of diuron far exceed any potential risks."

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