The U.S. Environmental Protection Agency (EPA) said on Wednesday it had pulled its approval of Dow Chemical Co's new herbicide Enlist Duo as the agency studies new information regarding the product's impact on non-target plants.

The EPA approved the herbicide for sale and use in several U.S. states more than a year ago, but later found that its assessment of the product's two active ingredients was incomplete.

Enlist Duo, developed by Dow Chemical's Dow AgroSciences unit, is among a group of next generation herbicides designed to combat weeds resistant to glyphosate, a widely used herbicide in the United States and the active ingredient of Monsanto Co's Roundup.

Enlist Duo, which combines an herbicide component known as 2,4-D with glyphosate, is meant for use on corn and soybeans that have been genetically altered to tolerate it while killing surrounding weeds that can drag down crop yields.

The EPA said it discovered claims by Dow AgroSciences - the registrant of Enlist Duo - in its patent filings that the two active ingredients work better in combination, according to a court filing seen by Reuters. The EPA's study had assumed the components did not have such "synergistic effects."

"The information suggests that EPA's analysis may have understated the phytotoxicity of the product, therefore EPA can no longer be confident that Enlist Duo will not cause risks of concern to non-target organisms, including those listed as endangered," the court filing said.

Dow Chemical said it was working with the EPA to provide further assurances for Enlist and it expects a "prompt resolution of all outstanding issues."

Rival seed and agrochemical company Monsanto Co, said it is not worried that its next generation herbicide platform Xtend, which combines the herbicide dicamba with glyphosate, would be similarly targeted by EPA.

"Monsanto's patent submissions have not made claims of synergistic activity between glyphosate and dicamba. We believe we have submitted a regulatory data package that supports approval of our product," Monsanto spokesperson Sara Miller said.