Close on the heels of the Environmental Protection Agency's (EPA) approval of Dow Chemical's controversial new weed killer Enlist Duo, the Natural Resource Defense Council (NRDC) is suing the US authority.

The NRDC contends that the herbicide, which contains glyphosate and 2,4-D, will further deplete the monarch butterfly population and it is also a risk to human health.

Enlist Duo has aroused concerns in particular because 2,4-D was a component of the Vietnam War as a defoliant Agent Orange, which has been linked to serious health conditions, birth defects and deaths.

Developed by Dow as an answer to severe weed resistance problems that are limiting crop production around the US, the crop protection agent is designed for use in controlling weeds in corn and soybeans genetically-engineered (GE) to tolerate 2,4-D and glyphosate.

Enlist corn and soybean will be sold along with the herbicide, as soon as the 2015 planting season.

For the first time, the EPA has issued first time restrictions as part of the approval of Enlist Duo, a procedure it said could be a template for future approvals of herbicides for genetically modified crops (GMO).

At the same time, the agency has defended its decision to approve the herbicide, saying its scientists used "highly conservative and protective assumptions" to evaluate human health and ecological risks for the new uses of 2,4-D in Enlist Duo.

"The assessments confirm that these uses meet the safety standards for pesticide registration and, as approved, will be protective of the public, agricultural workers, and non-target species, including endangered species," EPA said.

Under the restrictions, Dow is required to track and report on weed resistance to the product. The EPA has issued a 30-foot in-field "no spray" buffer zone for application areas and banned Enlist Duo from being used when wind speeds are above 15 miles per hour.

The agency said the approved formulation contains the choline salt of 2,4-D which is less prone to drift than the other forms of 2,4-D.

Initially, Enlist Duo will be allowed only in Illinois, Indiana, Iowa, Ohio, South Dakota and Wisconsin. EPA will take public comments until Nov. 14 prior to deciding on approving the product for use in Arkansas, Kansas, Louisiana, Minnesota, Missouri, Mississippi, Nebraska Oklahoma, Tennessee and North Dakota. Up to now, it said it has received over 400,000 comments.

EPA will review its approval of Enlist Duo in six years rather than the usual 15 years.

The Natural Defense Council said its lawsuit is an effort to prevent EPA from allowing Dow to sell Enlist Duo until further tests are done and other agencies are included in the process. According to Sylvia Fallon, a scientist at NRDC, glyphosate has wiped out milk weed, which is a necessary staple for the monarch butterfly and the agency has not considered this impact.