The U.S. Department of Agriculture said it would conduct an environmental impact statement investigation into the potential effects of the plant, which will keep it off the market for a year or more.

The findings, published in this week's issue of the Proceedings of the National Academy of Sciences, offer the strongest evidence yet that it will be difficult to control genetically modified plants from interbreeding with surrounding plants to create new and unanticipated hybrids.

Lidia Watrud and colleagues tracked the creeping bentgrass pollen from experimental crop fields in central Oregon. The grass, created by Marysville, Ohio-based Scotts Co., carried a gene that allowed it to resist the effects of Monsanto Co.'s Roundup weedkiller.

Several plants have been engineered to survive spraying with Roundup, known chemically as glyphosate. They carry an enzyme from bacteria that allow them to resist the herbicide.

Farmers can spray these crops freely with Roundup, killing the weeds without harming their crops.

'SUPERWEEDS' FEAR

But the fear is that the modified crops can spread the bacterial gene to surrounding plants, creating hard-to-kill "superweeds," or perhaps having unforeseen effects on other crops.

Developers of gene-modified crops argue that it would be easy to use other weedkillers to control any newly spread plants.

The EPA team collected seeds from naturally occurring grasses and from plants they grew in pots to catch any wayward pollen.

They grew the seeds and tested the new grass seedlings to see if they were resistant to Roundup. They found such resistant seedlings as far away as 13 miles, although most were much closer.

Meghan Thomas of the USDA's Animal and Plant Health Inspection Service said the agency had already decided to conduct an environmental impact statement, a process that can take a year or sometimes longer.

"Scotts has petitioned us to deregulate the product," Thomas said in a telephone interview.

"We will go out there and we will take a look at the data, we will look at the field tests. This is a perennial and it has some wild and weedy relatives and this the first perennial that has come in to us for deregulation."

Plants previously modified genetically have been annuals, which have to be planted every year. A perennial poses more questions because it propagates more easily and may be harder to control.

Creeping bentgrass, used primarily on golf courses, grows naturally in many places that have a cool season. Known scientifically as Agrostis stolonifera, it is a potential forage crop for animals and could also be used to clean up polluted soil and water.

Some grass farmers in Oregon had opposed field tests of the genetically engineered grass because they feared it could affect their crops.