Six years ago, some Manitoba farmers began growing genetically-modified canola because of its hardy resistance to insects and disease. Many stopped seeding it when consumers rejected it. But that hasn't stopped GM canola plants from popping up where they're not wanted.

A close look at a field planted with wheat near Headingley reveals an invader – genetically-modified canola. Agronomist Gary Martens says it's bad news for farmers. "The canola itself becomes the weed because it's in a place where you don't want it and it's relatively more difficult to control than it used to be."

GM canola is sprouting on farms where it's never been seeded. Plant scientists say the seed may have spread a number of ways: by wind, insects, even animals – and there's little that can stop it.

"The genie is out of the bottle now," says Martin Entz, a plant scientist at the University of Manitoba. "There's nothing we can do. It will never go back in."

Scientists say Manitoba farmers can't expect GM canola to go away easily. In fact, even spraying the plant with herbicide repeatedly doesn't guarantee it won't come back. They say the only sure way to rid a field of GM canola is to pull it out by hand.

The spread of the seed means farmers in Western Canada can't be sure their crop is GM-free and that could hurt sales. Entz says many people blame the seed's creator for the problem. "The genes belong to Monsanto and the question farmers are asking is are they liable, or is the company liable?"

Monsanto is urging farmers to call them if they have unwanted genetically-modified canola growing on their land. The company says it will come and remove it by hand, till it under, or use a special herbicide to kill it.