Farmers who have grown genetically modified oil-seed rape on their land as part of the government's trials have been warned not to grow conventional oil-seed on the same land this autumn for fear of contaminating it.The Department of Environment issued the warning yesterday to prevent farmers growing crops that might become unsaleable in Britain because they contained too much GM material.In a statement the department said research showed that seeds persisted in the soil in greater quantities than previously thought<a href="http://politics.guardian.co.uk/green/story/0,9061,1006219,00.html">Guardian Unlimited Politics - Special Reports - Alert on spread of GM seeds</a>