PAOLA, Kansas - Farmer Mark Nelson bends down and yanks a four-foot-tall weed from his northeast Kansas soybean field. The "waterhemp" towers above his beans, sucking up the soil moisture and nutrients his beans need to grow well and reducing the ultimate yield. As he crumples the flowering end of the weed in his hand, Nelson grimaces.

"When we harvest this field, these waterhemp seeds will spread all over kingdom come," he said.

Nelson's struggle to control crop-choking weeds is being repeated all over America's farmland. An estimated 11 million acres are infested with "super weeds," some of which grow several inches in a day and defy even multiple dousings of the world's top-selling herbicide, Roundup, whose active ingredient is glyphosate.

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 U.S. farmers starting their fall harvest this month. And their use of more herbicides to fight the weeds is sparking environmental concerns.

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.

"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.

"Most of the public doesn't know because the industry is calling the shots on how this should be spun," Mortensen said.

Last month, representatives from the U.S. 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.

"It is only going to get worse," said Lee Van Wychen, director of science policy at the Weed Science Society of America.

MONSANTO ON THE FRONT LINE

At the heart of the matter is Monsanto Co, the world's biggest seed company and the maker of Roundup. Monsanto has made billions of dollars and revolutionized row crop agriculture through sales of Roundup and "Roundup Ready" crops genetically modified to tolerate treatment with Roundup.

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.

And 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.

For Monsanto, it spells a threat to the company's market strength as rivals smell an opportunity and are racing to introduce alternatives for Roundup and Roundup Ready seeds.

"You've kind of been in a Roundup Ready era," said Tom Wiltrout, a global strategy leader at Dow AgroSciences, which is introducing an herbicide and seed system called Enlist as an alternative to Roundup.

"This just allows us to candidly get out from the Monsanto story," he said.

Gilford Securities analyst Paul Christopherson last month reiterated a "sell" recommendation on Monsanto's shares, citing Monsanto's "overdependence" on glyphosate and Roundup Ready crops, calling glyphosate resistance by weeds a "big and growing phenomenon."

Monsanto officials say they are asking farmers to use different types of herbicides to fight weeds, but insist that Roundup remains effective for the majority of U.S. farmers.

Still, company spokesman Tom Helscher said weed resistance was a "wake-up call for all U.S. 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.

POURING ON THE PESTICIDES

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.

Nelson, for example, has been a fan of Roundup since Monsanto introduced Roundup Ready soybeans and corn in the 1990s. For years he needed no other herbicides for his 2,000 acres, marveling at how easily Roundup wiped out weeds. He often did not even use the full concentration recommended.

Now Nelson uses several pesticides and sprays his fields multiple times to try to control waterhemp, which can grow eight-feet tall and can be toxic to livestock.

He uses the maximum amount of Roundup along with other herbicides including one known as 2,4-D, which some scientific organizations have deemed a cancer risk.

"Just spraying Roundup was so easy," he said. "There is no ease anymore."

In Ohio, the nightmare weed for farmer John Davis is "marestail," an annual weed that grows well in key crop-growing areas of the U.S. Midwest and which is resistant to glyphosate and other herbicides.

"I see marestail in my sleep," said Davis, president of the Ohio Corn Growers organization. "I have spent a significant amount of dollars trying to control marestail until I realized I was not going to control marestail."

Davis calls the weed resistance problem a "major economic blow" to his farming operation.

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.

ENVIRONMENTAL PROBLEMS

Then there are the environmental woes. A U.S. 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 U.S. Department of the Interior.

The USGS said more than 88,0000 tons of glyphosate was used in 2007, up from 11,000 tons in 1992.

"This is a big problem that actually does threaten the ability of nations to feed their people. it needs a fair amount of research and studies dedicated to it," said Iowa agronomist Bob Streit.

Streit is among a group of scientists who believe glyphosate is actually harming the plants it is supposed to protect by tying up nutrients in the soil the plants need. The group has lobbied regulators to rein in use of glyphosate.

The Environmental Protection Agency has started a review of the safety and efficacy of glyphosate and is considering the arguments of critics and the findings of the USGS study.

"EPA considers all relevant information in its review," said an EPA spokesperson. "We will be evaluating it as part of the glyphosate review."

EPA plans to propose a decision in 2014 and issue a final registration review decision for glyphosate in 2015.

For Monsanto, the weed resistance problem is more significant than the recent concerns raised about possible insect resistance developing to Monsanto's corn seed, said Gabelli & Co analyst Amon Wilkes.

Wilkes remains bullish on Monsanto's prospects. While he sees competition to Roundup as a "potential problem," he noted the company has been moving to introduce new products.

"You always have to be continually innovating. Monsanto is doing that."

Monsanto insists that the Roundup Ready crops and herbicide system "has long-term value" and that any rivals will also run the risk of triggering weed resistance.

"The benefits of glyphosate-tolerant crops have been real for farmers and the environment," said Monsanto's Helscher.