An estimated 11 million acres in the US are infested with “super weeds”, some of which grow several inches a day and defy even multiple dousings of the world’s top-selling herbicide, Roundup.

The problem's gradual emergence has masked its growing menace. Now, however, it is becoming too big to ignore. The super weeds boost costs and cut crop yields for US farmers starting their fall harvest this month. And their use of more herbicides to fight the weeds is sparking environmental concerns, a report said.

With food prices near record highs and a growing population straining global grain supplies, the world cannot afford diminished crop production, nor added environmental problems, expert said.

"I'm convinced that this is a big problem," said Dave Mortensen, professor of weed and applied plant ecology at Penn State University, who has been helping lobby members of Congress about the implications of weed resistance.

Last month, representatives from the US Environmental Protection Agency, the Department of Agriculture and the Weed Science Society of America toured the Midwest crop belt to see for themselves the impact of rising weed resistance.

At the heart of the matter is Monsanto, the world's biggest seed company and the maker of Roundup.

Monsanto has made billions of dollars and revolutionised row crop agriculture through sales of Roundup and "Roundup Ready" crops genetically modified to tolerate treatment with Roundup whose active ingredient is glyphosate.

The Roundup Ready system has helped farmers grow more corn, soybeans, cotton and other crops while reducing detrimental soil tillage practices, killing weeds easily and cheaply.

But the system has also encouraged farmers to alter time-honored crop rotation practices and the mix of herbicides that previously had kept weeds in check.

Now farmers are finding that rampant weed resistance is setting them back - making it harder to keep growing corn year in and year out, even when rotating it occasionally with soybeans. Farmers also have to change their mix and volume of chemicals, making farming more costly.

Company spokesman for Monsanto, Tom Helscher, said weed resistance was a "wake-up call for all US farmers."    

"We have a shared responsibility and we're committed to working with farmers to take the steps necessary to insure that glyphosate continues to be an effective weed control tool for many years to come," Helscher said in a statement.

To fight superweeds, farmers are using stronger dousings of glyphosate as well as other harsh chemicals that have sparked concern among environmental and public health groups.

Some farmers have resorted to hiring crews to weed fields by hand, and some are returning to tilling their fields, a practice that contributes to soil erosion.

"We are at a disturbing juncture," said Margaret Mellon, director of the food and environment program at the Union of Concerned Scientists. "The use of toxic chemicals in agriculture is skyrocketing. This is not the path to sustainability."    

Penn State's Mortensen said farmer efforts to control resistant weeds are estimated to cost nearly $1 billion a year and result in a 70 percent increase in pesticide use by 2015.

Since Monsanto introduced its glyphosate-resistant crops, 21 weed species have evolved to resist the herbicide, up from none in 1995. The list is growing by one to two species per year, Mortensen said.

Farmers and crop experts say that when superweeds take root in farm fields, yield reductions of 1-2 bushels an acre are common, even with extra pesticide doses.

With soybeans at more than $14 a bushel, a 1,000-acre farm might lose more than $20,000 to weeds on top of the costs of the added pesticides.

A US government study released last month gave evidence that glyphosate is also polluting the air and waterways. The chemical was found in waterways through Mississippi and Iowa, according to the report issued in August by the U.S. Geological Survey Office, a part of the US Department of the Interior.

The US Geological Survey said more than 88,0000 tons of glyphosate was used in 2007, up from 11,000 tons in 1992. - Reuters