Investigation Looking at Weed Resistance Issues Related to Herbicide-tolerant Crops


by Pam Smith, Crops Technology Editor

DECATUR, Ill. (DTN) — Herbicide-resistant weed worries continue to build beyond the farm field. The Office of Inspector General (OIG) for the Environmental Protection Agency announced Friday it will investigate how EPA has managed weed resistance issues related to herbicide-tolerant crops EPA has approved.

The OIG will be looking at processes, practices and alternatives the EPA has provided to delay herbicide resistance, according to a memorandum made public this week. OIG will look for evidence that EPA independently collects and assesses information and works to mitigate herbicide resistance occurrences. It will also be asking what steps EPA has taken to determine and validate the risk to human health and the environment from herbicides it has approved to combat herbicide-resistant weeds.

Every federal agency within the executive branch has an inspector general, an independent investigative body designed to detect fraud, waste and abuse within that agency. The EPA Office of the Inspector General is charged with performing audits, evaluations and investigations of EPA and its contractors.

Farmers and the agriculture industry have already seen clues of Washington's increased concern regarding weed resistance to herbicides. In 2014, EPA included lengthy herbicide resistance-management language on the label for Dow AgroScience's Enlist Duo herbicide, a new formulation of 2,4-D and glyphosate. The Enlist Duo label encourages growers to use broad-spectrum soil-applied herbicides as a foundation weed control program and rotate use of the chemistries, among other measures aimed at slowing weed resistance.

Both Enlist and Monsanto's new Xtend Weed Control System, which includes traits that offer tolerance to dicamba and glyphosate herbicides, have faced heavy criticism from environmental groups claiming additional use of the chemistries will contribute to more weed resistance. The specific dicamba formulations to be used with Xtend crops have not yet received EPA approvals. It's expected the new dicamba formulations will have similar resistance-management language recommended or required as part of their use labels.

Purdue University weed scientist Bryan Young told DTN there's no question times have changed regarding weed resistance. “Back 10 to 15 years ago, I didn't get the impression from the EPA that weed resistance to herbicides fell within their primary directive. Instead, I believe the EPA was focused on their responsibility to make sure that individual herbicide products don't pose any undue risk to humans, off-target organisms, or the environment when applied according to the label,” said Young.

Since then, the increase in weeds developing resistant populations to many common herbicides has shown herbicide applications need to be considered as a whole and not just on an individual basis, he added. “There would have been an outcry of farmers citing their freedom to operate was compromised if the EPA limited glyphosate applications or required tank-mixtures with glyphosate before we had widespread resistance to glyphosate,” he stated.

“Now there's no denying the impact of herbicide-resistant weeds, and everyone who's involved with applying herbicides should be managing against weed resistance to all herbicides. Herbicide-resistance management requires the effort from herbicide applicators, retailers, basic manufacturers, academia and regulatory agencies, not just the EPA,” he said.

Young added that the EPA is being pulled in many directions because the need to integrate several herbicide modes of action at various times during the season, to prevent or manage resistance, doesn't always line up with the ideals of reducing the pesticide load on the environment. “At the end of the day, I hope the OIG finds that the EPA procedures and decisions were based on sound science,” he said.

The first documented cases of weed resistance date back as far as 1957. However, as farmers increasingly relied on single herbicides, the intense selection pressure allowed rare, unique individual weeds to survive and pass their genetics of resistance along to other weeds.

The ability of aggressive weeds such as Palmer amaranth, horseweed and waterhemp to withstand glyphosate has taken the weed resistance issue to new levels. In total, 16 weeds in the U.S. are now confirmed resistant to glyphosate. Globally, weeds have evolved resistance to 22 of the 25 known herbicide sites of action and to 160 different herbicides. Herbicide-resistant weeds have been reported in 86 crops in 66 countries, according to The International Survey of Resistant Weeds ( The increase in weed populations known to resist herbicides from multiple sites of action is also growing and is of particular concern since no new herbicide sites of action have been commercialized since 1980.

In the memo, OIG said it plans to use government auditing standards to conduct the project. The anticipated benefit of this effort is a greater understanding of herbicide resistance, which will lead to an enhancement of EPA's herbicide resistance management and oversight, the memo added.

Preliminary Research and Project Notification memorandums inform EPA officials about the status of Office of Inspector General preliminary and project research that may result in a full audit or program evaluation report. To see the memorandum, go here:…

For more information on how to manage resistance weeds go to:



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