The U.S. Department of Agriculture is considering whether corn and soybean seeds genetically modified to resist the weed killer 2,4-D should be commercially available.

2,4-D is approved for use by both gardeners and farmers, but is controversial as the main ingredient in Agent Orange, the herbicide widely used during the Vietnam War.

The pesticide 2,4-D, once sold throughout Canada under brand names such as Killex, is prohibited for lawn care in most provinces east of Manitoba and in Alberta as part of a much broader prohibition on lawn care products.

The chemical is widely used here in agriculture and forestry and has been approved by Health Canada for general use. GMO seeds are also controversial for many consumers, though attempts to have GMO foods labelled have failed.

Agent Orange contained a deadly dioxin that is not part of the 2,4-D formulation.

In the U.S., 2,4-D, sold by Dow AgroSciences and other companies, is the third most-used weed killer.

But GMO seeds modified to withstand the chemical are permitted only in tightly controlled field trials under current USDA rules.

The U.S. federal agency is looking at deregulation to allow commercial development of 2,4-D resistant seeds as the result of a request from Dow Chemical. It has opened the issue for public comment for 45 days.

EPA also to review 2,4-D

The Environmental Protection Agency is conducting a separate review of the herbicide.

Agent Orange has not been produced since the 1970s, but the dioxin in the herbicide is implicated in birth defects in Vietnam and cancer among veterans of the Vietnam War.

Canadian soldiers and hydro workers from New Brunswick, Ontario and British Columbia were also exposed to the toxic chemical when it was used to clear brush in the 1970s.

2,4-D has been controversial here since Quebec banned its use in golf courses and home lawn care, defending the ban before NAFTA after a challenge from Dow. Many other provinces followed suit.

A USDA draft environamental impact statement says approving GMO corn and soybean seeds resistant to 2,4-D is likely to increase use of the herbicide.

One potential impact may be an increase in the number of weeds resistant to 2,4-D use.

Weeds resistant to Roundup

Dow AgroSciences has asked the U.S. Animal and Plant Health Inspection Service, part of the USDA,  to deregulate one variety of GMO corn and two varieties of soybeans. All three seeds have immunity to multiple weed killers.

Farmers are keen on alternatives as weeds have become immune to Monsanto's Roundup. But skeptics are concerned that use of the new seeds and 2,4-D will only lead to similar problems as weeds acquire resistance to that chemical too.

"It's just so clear. You can see that you have this pesticide treadmill effect," said Bill Freese, a chemist with the Washington, D.C.-based Center For Food Safety, which promotes organic agriculture.

Freese also raised concerns about 2,4-D's tendency to drift beyond the area where it is sprayed, threatening neighbouring crops and wild plants.

The APHIS has already determined that the seeds do not pose a “plant pest risk” to other agricultural crops or other plants or plant products. 

With files from the Associated Press