Researchers at Sweden's Lund University and the Danish Technical University found transgenic plants growing in a field planted with GM rapeseed more than 10 years ago, according to their report in Biology Letters.
Although measures were taken in the years following the GM trial to remove 'volunteers', or plants that grow spontaneously from seed in the soil, some appear to have been overlooked, said the authors. Fifteen out of 38 sample seedlings tested positive for the genetically modified trait of herbicide tolerance 10 years after the trial had ended.
Their findings are in contrast to previous studies. "In general, studies suggest that the majority of seeds disappear from the seedbank within two years," they write.
Tighter measures to contain GM crops may be needed, concludes Dr Tina D'Hertefeldt and her colleagues.
"This finding of volunteers, despite labour intensive control for 10 years, supports previous suggestions that volunteer oilseed rape needs to be carefully managed in order for non-GM crops to be planted after GM crops."
The news comes as several food producers call for a re-think on European resistance to planting genetically modified crops. The prices of non-GM cereals are now significantly higher - about $50 per tonne for corn - than GM varieties and higher raw material costs are affecting margins in the industry.
But environmental groups such as Greenpeace maintain that consumers are still against the widespread introduction of GM crops, with recent studies further raising fears of a reduction in negative environmental effects such as a reduction in biodiversity and increasing resistance to herbicides.
Dr D'Hertefeldt says there is a need for more discussion about how to contain GM oilseed rape in particular. The plant is known to have especially long seed dormancy. A larger French study, published at the same time as the Swedish research was submitted, also found survival of volunteer plants for eight years after a GM trial.
"I would expect the same to happen in a commercial field too," she told FoodNavigator.com. "It may even be more prevalent as the trial had very stringent regulations, and higher controls than a farmer would probably carry out."
For the food industry, the question is how many volunteer plants you would get in a field being grown with conventional oilseed rape after a GM crop. "If you had a high number, you could get above the threshold for labeling GM ingredients," she told FoodNavigator.com.
Rapeseed, also called canola in North America, is the fourth most commonly grown GM crop in the world, after soya, maize and cotton. Rapeseed oil is increasingly used in the food industry.
"I think for oilseed rape we may have to be aware that there will always be some contamination and therefore we may need labeling to tell the consumer," added D'Hertefeldt.
Long-term persistence of GM oilseed rape in the seedbank
Authors: Tina D'Hertefeldt, Rikke Jørgensen, Lars Pettersson