Dow Chemical has indicated it will fight plans by the US Environmental Protection Agency (EPA) to suspend its registration of the chemical group’s Enlist Duo herbicide.

In late November, EPA said it would ask a federal appeals court time to vacate the agency’s own approval of the Dow AgroSciences herbicide Enlist Duo to give it more time to evaluate new information regarding the product’s toxicity to neighboring crops.

Just over a year ago, the environmental watchdog had approved the herbicide as a weapon in the battle against the so-called superweeds which have become resistant to glyphosate, the main ingredient in Monsanto’s universal herbicide Roundup.

Following the herbicide’s approval, EPA was sued by two environmental groups in a case still pending. The crop protection agent was already controversial because it combines glyphosate with 2,4-D, a component of the defoliant Agent Orange used during the Vietnam War.

Dow’s response filed with the court last week, which the three-judge panel will take into consideration when making a decision, is that EPA can review the product, but has no right to cancel the registration without due process. The US group has told customers that it still expects Enlist Duo to be available for the 2016 growing season.

The chemical group’s lawyers added that Dow has a legal property interest in the herbicide’s registration, and this cannot be revoked without due process.

As Enlist Duo is not on the market yet, said a delay would not dramatically affect growers in the 15 states where the herbicide is already approved, but some are said to be eager to use it to hem-in the rapid growth of glyphosate-resistant giant ragweed and water hemp.

EPA contends that Dow did not provide sufficient information upfront about the potential synergistic effects of the two active ingredients in Enlist Duo, glyphosate and 2,4-D, meaning that the mixture of the two might be more toxic than the sum of their ingredients individually.

According to US reports, the agency studied the two chemicals in the herbicide and initially concluded that the combination was not synergistic, i.e. that the toxicity of the two ingredients combined was not greater than the effects of the individual compounds. However, Dow apparently tripped over its own statements to the US Patent Office in a patent claim meanwhile dropped, that the combination would indeed be synergistic.

EPA is said to have discovered the filing and asked to review Dow's data about the herbicide's synergies, saying it could “no longer be confident” that even if used according to the approved label, the product would not endanger  non-target organisms.

Dow is also at odds with an EPA proposal to discontinue chlorpyrifos, an ingredient in pesticides sprayed on crops including soybeans, citrus fruits, grapes and nuts. It maintains that the chemical has been thoroughly tested for health, safety and environmental effects.

The agency is currently seeking public comment on its proposal, and expects to issue a final rule in about a year.

While EPA said there do not appear to be risks from exposure to chlorpyrifos in food, it said it has determined that safe levels of chlorpyrifos may be exceeded in parts of the US for people whose drinking water is derived from some small vulnerable watersheds where the chemical is heavily used.

Dow said it “remains confident that all US tolerance issues relating to the continued use of chlorpyrifos can be readily resolved with a more refined analysis, adding that it believes EPA's projections of residues in drinking water are based on “unrefined” mathematical modeling that is “inconsistent with extensive real-world monitoring.”

Together with other environmental advocates, the US Natural Resources Defense Council, one of the groups involved in the Enlist Duo lawsuit, has filed a suit seeking a national ban on chlorpyrifos as it interferes with brain development in children. The pesticide was phased out for most household uses in 2000.