WINTERSET, IOWA (Reuters) - A U.S. government program designed to convert farmland to wildlife habitat has triggered the spread of a fast-growing weed that threatens to strangle crops in America’s rural heartland.

The weed is hard to kill and, if left unchecked, destroys as much as 91 percent of corn on infested land, according to the U.S. Department of Agriculture (USDA). It is spreading across Iowa, which accounts for nearly a fifth of U.S. corn production and in 2016 exported more than $1 billion of corn and soy.

The federal Conservation Reserve Program pays farmers to remove land from production to improve water quality, prevent soil erosion and protect endangered species.

The destructive weed - Palmer amaranth – has spread through seed sold to farmers in the conservation program, according to Iowa’s top weeds scientist, Bob Hartzler, and the conservation group Pheasants Forever.

“We are very confident that some of these seed mixes were contaminated,” Hartzler said.

Hartzler, an Iowa State University agronomy professor, said one seller was Allendan Seed Company, the state’s largest producer of local grass and wildflower seeds for conservation land.

In written responses to questions from Reuters, Allendan said it was “possible that pigweed seed ... was present in some mixes.”

Palmer amaranth is a type of pigweed. Allendan did not confirm it had found the seed in any of its supplies. It said outside labs that the firm hires to test seed quality had been unable to distinguish Palmer amaranath from other pigweeds.

The company said it started using a new DNA test in February to check its seed for Palmer amaranth.

Many farmers joined the conservation program in the past year as prices for their crops tanked amid a global grains glut. The weed can be killed, but the cost of clearing it would be another hit to the cash-strapped farming community in the United States, the world’s top corn supplier.

The program is managed by the Natural Resources Conservation Service (NRCS) and the Farm Service Agency (FSA), units of the USDA. NRCS officials have acknowledged that contaminated seed mixes for conservation land have spread Palmer amaranth.

Slideshow ( 8 images )

In another state, Minnesota, authorities are also investigating whether the conservation program inadvertently introduced the weed to that state.

Keith Smith, a corn and soybean farmer in Gladbrook, Iowa, said he yanked Palmer amaranth out of land he set aside in the conservation program after finding the weeds last year.

He doused them in diesel and torched them with old tires.

Smith now regrets joining the program.

“I thought I’d help out the Earth,” he said.

2016 Conservation Reserve Program - here

ONE PLANT, HALF A MILLION SEEDS

The NRCS and FSA denied responsibility for the infestation because they do not supply or test the seed that farmers use to turn cropland into a refuge for wildlife. Landowners are responsible for finding their own seed.

None of the companies or organizations involved in the program should be blamed, said Jimmy Bramblett, the NRCS’s deputy chief of science and technology. “It’s just something that happened,” he said.

The NRCS is nonetheless considering giving financial assistance to Iowa farmers to help control the weed and is working with the farming community and other government agencies to control it, Bramblett said.

Slideshow ( 8 images )

Palmer amaranth, which is native to the southwestern United States, grows up to 2 inches (5 cm) a day and can reach a height of 10 feet. It produces up to 500,000 seeds the size of a pepper grain, which travel easily on the wind, in manure or stuck to farm equipment and vehicles.

Midwest farmers now face increased costs for the herbicide and labor to eradicate the weed. Fighting Palmer amaranth has doubled or tripled annual herbicide and labor costs to between $60 and $80 per acre for cotton farmers in Georgia, said Stanley Culpepper, a weed science professor for the University of Georgia.

Iowa farmers currently spend between $35 to $40 per acre on herbicides, Iowa State University research shows. If Palmer amaranth is firmly established, costs could increase by up to 50 percent, Hartzler said.

Corn and soybeans can compete better with weeds than cotton plants, so the expense of controlling it could be less than on cotton farms.

DETECTIVE WORK

Palmer amaranth first arrived in Iowa in 2013 but exploded across the state last year, spreading from 5 to 48 of the state’s 99 counties, according to Iowa State University.

In at least 35 of those counties, the weed was found on land in the conservation program.

The rapid rise in the incidence of the weed came after landowners in Iowa signed more contracts to put fields into the program than any other state - 108,799 out of the 637,164 total U.S. conservation program contracts, according to the USDA.

An Iowa landowner contacted Iowa State’s Hartzler after Palmer amaranth infested 70 acres of farmland he planted with the conservation seed mix.

“The Palmer amaranth was uniformly distributed across those 70 acres, so that was a good sign that it came in the seed,” Hartzler said.

Hartzler said he and his intern found the tiny black Palmer amaranth seeds in samples they took from seed bags the landowner purchased from Allendan.

He then grew some of the seeds in a greenhouse, he said, and they produced Palmer amaranth.