The researchers did not use genetically modified canola, but said the new breed was different enough from conventional strains for their study, published in Friday's issue of the journal Science.

Experts said the study will help in the debate over whether genetically engineered plants can spread their modifications to other, surrounding plants.

Those who oppose the genetic modification of food want guarantees that crops they believe to be conventional really are, and there are also fears that superweeds that resist herbicides could result from the spread of pollen from genetically engineered plants.

In addition, there are issues about the licensing and patenting of modified crops, and companies that sell GM seeds want to ensure that their profits do not literally scatter on the winds.

REAL WORLD DATA

"These are real-world data that can be used for real-world decisions," Paul Rayner, an agronomist at the University of Georgia, told Science.

Starting in 2000, Australian farmers planted canola bred - not genetically engineered - to resist certain weedkillers.

"The canola we used was a conventionally bred variety produced by Pioneer Seeds. It was released for sale to farmers for the first time in the 2000-2001 growing season," Rieger said in an interview conducted by e-mail.

"We took advantage of the fact that this was the first year this crop was released and therefore had not contaminated the surrounding area with these unique genes."

The farmers planted the crops under normal conditions.

The researchers looked at more than 700,000 seedlings from neighboring fields that had not been planted with the new canola.

The ability to resist the effects of the weedkiller spread to 63 percent of the neighboring fields planted with normal canola. But only a few of the fields had any more than 0.03 percent of plants that carried resistance.

Being close did not make a difference. The pollen seemed to have scattered randomly over neighboring fields, perhaps because both wind and insects, mostly bees, pollinate canola, they said.

They said their study was large, included three Australia states landscape and also involved big fields.

"This study demonstrates that cross-pollination between commercial canola fields occurs at low frequencies but to considerable distance," they concluded.

Rieger believes the findings apply to genetically modified crops. "Research has shown that GM and non-GM crops behave in the same manner," she said.

Canola was an interesting crop to experiment on, because it reproduces using self-and cross-pollination.

"If it was only self-pollinating the movement of pollen would not be an issue," she said.