WASHINGTON -- A new kind of genetically modified crop under the brand name of "Enlist" -- known by its critics as "Agent Orange corn" -- has opponents pushing U.S. regulators to scrutinize the product more closely and reject an application by Dow AgroSciences to roll out its herbicide-resistant seeds.

The corn has been genetically engineered to be immune to 2,4-D, an ingredient used in Agent Orange that some say could pose a serious threat to the environment and to human health. Approval by the United States Department of Agriculture and Environmental Protection Agency would allow farmers to spray it far and wide without damaging their crops, boosting profits for the agribusiness giant.

Dow and its allies have insisted that their product is well tested, while industry regulators have so far overlooked critics' concerns.

"This is going to be a solution that we are looking forward to bringing to farmers," Dow's Joe Vertin told Reuters.

More than 140 advocacy groups have participated in a letter writing campaign calling on U.S. Agriculture Secretary Tom Vilsack to reject Dow's regulatory application for the herbicide and herbicide-resistant crops, submitting more than 365,000 missives ahead of a public comment period that ends April 27.

"The scientific community has sounded alarms about the dangers of 2,4-D for decades," wrote opponents in their letter to Vilsack. "Numerous studies link 2,4-D exposure to major health problems such as cancer, lowered sperm counts, liver toxicity and Parkinson's disease. Lab studies show that 2,4-D causes endocrine disruption, reproductive problems, neurotoxicity, and immunosuppression."

Some farmers have argued that the new herbicide, a combination of 2,4-D and glyphosate -- the active ingredient in Monsanto's bestselling Roundup weed killer -- is necessary to combat weeds that have become resistant to glyphosate alone.

Glyphosate has also come under considerable public scrutiny in the wake of scientific findings that demonstrate the chemical causes birth defects in the embryos of laboratory animals. Health professionals contend that 2,4-Dichlorophenoxyacetic acid (2,4-D), an ingredient in the Vietnam War-era defoliant that's been blamed for public health problems both during and after the war, poses its own risks.

Agricultural consultant Steve Savage accused Dow's opponents of resorting to scare tactics, writing on his blog, Applied Mythology, that what the victims of Agent Orange "don't deserve is to have their tragedy exploited in an irresponsible way."

While most the the public health problems associated with Agent Orange have been attributed to a different ingredient, (2,4,5-T), as well as to dioxin contamination -- a number of studies have indicated that 2,4-D has significant health risks, too, according to Wenonah Hauter, executive director of Food & Water Watch, and Mae Wu, a health attorney at the Natural Resources Defense Council.

"Many studies show that 2,4-D exposure is associated with various forms of cancer, Parkinson's Disease, nerve damage, hormone disruption and birth defects," said Hauter in a statement. "USDA must take these significant risks seriously and reject approval of this crop."

Thirty-five medical and public health professionals have signed on to a letter to the USDA warning of health threats that could accompany the huge an increase in 2,4-D use that is expected to result from approval of the genetically engineered seed.

It isn't just scientists who have concerns.

"Farmers are on the front lines of this potential chemical disaster," said Iowa conventional corn and soybean farmer George Naylor in a statement. He added, "I'm also very concerned about the further pollution of the air and water in my community."