The USDA has oversight over the seeds, not the herbicide.
The public has 45 days to comment on its report. The Environmental Protection Agency is conducting a separate review of 2,4-D, although it previously found it safe to use.
Some corn and soybean farmers have eagerly anticipated a next generation of herbicide-resistant seeds as weeds immune to Monsanto's Roundup, known generically as glyphosate, become more common. Most corn and soybeans grown in the U.S. are genetically engineered, usually with the Roundup resistant trait.
But some scientists and environmentalists regard the development with alarm, noting 2,4-D can easily drift beyond the area where it is sprayed, threatening neighboring crops and wild plants.
The USDA's plant inspection agency found the greatest risk from the seeds, developed by Dow AgroSciences, was that increased use of the herbicide could hasten the evolution of weeds resistant to it. But it said resistance was inevitable because 2,4-D, sold by Dow AgroSciences and other companies, is the third most-used weed killer in the nation.
The herbicide has had limited use in corn and soybean farming because it becomes toxic to the plants early in their growth. The new seeds would allow farmers to use 2,4-D throughout the plants' lives.
The EPA will look at the impact of expanded use of 2,4-D in a report expected to be released for public comment in the coming months. The EPA and APHIS are expected to make final decisions simultaneously on use of the chemical and seeds. That could happen in the spring or early summer.
Dow AgroSciences has asked APHIS to deregulate one variety of corn and two varieties of soybeans. Both soybean varieties resist 2,4-D, but they differ in their immunities to other herbicides. All three seeds have immunity to multiple weed killers.
APHIS said farmers could help deter the development of 2,4-D resistance by using a variety of means to fight weeds and not relying solely on it.
Scientists do not believe 2,4-D was responsible for the health problems in Vietnam veterans that have been linked to Agent Orange. Instead, they have focused on dioxin, a cancer-causing contaminant found in another ingredient known as 2,4,5-T. EPA banned 2,4,5-T in 1985.