Genetically modified (GM) canola is appearing in farmers' fields where it wasn't planted, and because the plant has been engineered to resist conventional herbicides, it's tough to kill.

Agricultural scientists suspect that the plants spread through cattle manure. The seeds travel through an animal's digestive tract and are deposited on the soil, where they germinate.

"The GM canola has, in fact, spread much more rapidly than we thought it would," said Martin Entz, a plant scientist at the University of Manitoba. "It's absolutely impossible to control."

Ottawa approved GM canola in 1996, and at the time it did consider the possibility that it could become a weed. The Canadian Food Inspection Agency describes the current problem as "a nuisance" and has advised farmers to "use another chemical."