A controversial new biotech corn developed by Dow AgroSciences, a unit of Dow Chemical, will be delayed at least another year as the company awaits regulatory approval amid opposition from farmers, consumers and public health officials.

Dow AgroSciences officials said Friday that they now expect the first sales of Enlist for planting in 2014. Previously officials had set the 2013 planting season as a target, but U.S. farmers are already buying seed for planting this spring, and Dow has yet to secure U.S. approval for Enlist.

Dow wants to roll out Enlist corn, and then soybeans and cotton to be used in combination with its new Enlist herbicide that combines the weed-killers 2,4-D and glyphosate. The Enlist crops are genetically altered to tolerate treatments of the Enlist herbicide mixture. The hope is that Enlist will wipe out an explosion of crop-choking weeds that have become resistant to glyphosate alone.

Opponents have bombarded Dow and U.S. regulators with an array of concerns about Enlist, which is intended to replace Monsanto's successful Roundup Ready system. Genetically altered Roundup Ready corn and soybeans now dominate the U.S. corn and soybean market.

But as Roundup Ready crops have gained popularity, millions of acres of weeds have developed resistance to Roundup herbicide, causing farmers to use higher quantities of Roundup and other herbicides to try to beat back the weeds.

Critics warn that adding more herbicides to already resistant weed populations will only expand and accelerate weed resistance. Some have likened the problem to a "chemical arms race" across farm country.

"Weed resistance to chemical herbicides is one of the biggest problems farmers now face, and that is a direct result of converting so much of our farmland to herbicide-resistant GE (genetically engineered) crops," said Marcia Ishii-Eiteman, senior scientist at Pesticide Action Network. "We need to get out of this futile chemical arms race fast."

Earlier this month, Kansas State University scientists said they have found evidence that some more weed types have developed resistance to glyphosate. Researchers said they sprayed two common weed types, Waterhemp and Palmer amaranth, with up to four times the typical field use for glyphosate and the weeds would not die.

Next month the Weed Science Society of America will examine the weed resistance problems at a meeting in Baltimore.

Dow's Enlist herbicide is also controversial because 2,4-D, or 2,4-Dichlorophenoxyacetic acid, was one of the ingredients in Agent Orange, the Vietnam War defoliant that was blamed for numerous health problems suffered during and after the war. Although the main health effects of Agent Orange were blamed on the other component of the mixture (2,4,5-T) and dioxin contamination, critics say 2,4-D has significant health risks of its own.

Several medical and public health professionals have sent a letter to the U.S. Department of Agriculture warning of health threats that could accompany an increase in 2,4-D use.

But Dow officials said Friday that its product is needed soon as market research shows that cropland acres infested with glyphosate-resistant weeds increased 80% over the past two years.

As it awaits regulatory approval, Dow said it would showcase the Enlist system in five technology centers established in the U.S. Midwest and U.S. South to train farmers and seed sellers on Enlist's application and management. It also said it will offer more than 100 small Enlist field plots at seed company and retail locations and it is hoping to also set up on-farm "experience plots" to demonstrate the product.

Dow said it plans on receiving U.S. regulatory approval this year and will "ramp up" seed production and its supply of Enlist herbicide to support commercial sales starting in late 2013 for 2014 planting. Canada granted regulatory approval in October.

"We are committed to introducing this technology responsibly and sustaining it for the long term," Dow AgroSciences U.S. commercial leader Damon Palmer said in a statement.

Critics said they hoped the U.S. Department of Agriculture and Environmental Protection Agency are taking a hard look at the potential problems associated with Enlist.

"Those of us who have concerns about this are all kind of holding our breath... wondering one way or other what is going to happen with this," said Doug Gurian-Sherman, senior scientist in the food and environment program with the Union of Concerned Scientists.

Gurian-Sherman said Enlist represents a "head-in-the-sand" approach to weed resistance. As well, an array of health and environmental concerns associated with 2,4-D deserve strict regulatory scrutiny, he said.

Many also worry that the new biotech crops will contaminate conventional and organic crops.

"There are just some huge questions that Enlist and some of these other crops have," he said.

The USDA has received more than 450,000 comments opposing approval of the 2,4-D tolerant cropping system, according to the Center for Food Safety, which opposes Enlist and has threatened to sue the government if it is approved.