The US Environmental Protection Agency is considering an emergency request by Texas regulators to allow cotton farmers to deploy a controversial herbicide, marking a new front in the war on so-called "super weeds."

The Texas Department of Agriculture has asked the EPA for an exemption to permit growers to spray fields this summer with propazine - a chemical little used in US agriculture - to control an invasive plant known as palmer amaranth, or pigweed.

Pigweed, which can grow 3 inches a day, is one of several "invaders" that have developed resistance to the nation's dominant weed killer, glyphosate, sold by Monsanto as Roundup.

At the behest of the state's cotton growers, Texas is asking the EPA to let farmers spray propazine, the active ingredient in the herbicide Milo-Pro, on up to 3 million acres, or nearly half of the state's estimated cotton acreage.

The state is the nation's largest cotton producer, accounting for 33% of last year's crop, which was valued at $5.2 billion, according to the US Agriculture Department.

The Center for Food Safety, a non-profit advocacy group, and other environmental organizations oppose the proposal on the grounds that propazine poses potential risks to human health. The substance has been identified by the EPA as a possible human carcinogen and is a restricted-use pesticide requiring a license to purchase and apply.

Propazine is closely related to atrazine, a herbicide that is used by many US corn growers but banned in the EU. Its critics cite studies indicating it can interrupt sexual reproduction in frogs, and result in potential human reproductive problems.

EPA itself says that propazine's similarity to atrazine suggests it may cause disruptions to hormonal systems in rats, and has the potential to leach into groundwater or reach surface waters by runoff.

Milo-Pro, produced by Iowa-based Albaugh, is currently approved by the EPA only for use on grain sorghum crops in Texas, Oklahoma, New Mexico, Colorado and Kansas.

Propazine's EPA registration was canceled in 1988 due to failure of chemical companies to provide data for a groundwater monitoring study, but a new registration was issued a decade later.

The EPA has begun seeking public comment on the request and is expected to rule within 50 days.

Farmers for a decade have been fighting weeds that have developed resistance to glyphosate, especially in the South, where a longer growing season and warm climate have made it the battle's front line.