People are being warned about cross-pollination, with claims that GM crops cross pollinate other crops and wild plants. Pollen from a GM crop might come into contact with nearby plants and weeds (but not with other crops because of the 'buffer-zones'). But for cross-pollination to be successful, the plants would have to be compatible and flowering at the same time. Bear in mind: the nearest wild relatives that GM-maize could cross with are 5,000 miles away in the Americas. Sugar-beet is harvested before it flowers and so before it sheds any pollen. Before any GM crop is planted in the open, the likelihood of cross-pollination is carefully assessed by the Government's Advisory Committee on Releases to the Environment. Every GM crop has already gone through years of testing in both laboratory and greenhouse – they're not just being plonked outside without any thought!
As for GM oilseed rape there's very little chance that it will cross-pollinate other oilseed rape crops because its pollen is heavy and sticky and doesn't travel too far from the crop itself. Research in France has shown that 97.5% of pollen falls to the ground within 1 metre of the edge of the field and 99.8% within less than 25 metres. Nor does pollen spread necessarily lead to cross-pollination. If pollen does find its way to an adjacent crop of oilseed rape, this crop must be in flower and can't be pollinated if already fertilised. Oilseed rape is mainly self-pollinating, which means pollen from a particular flower fertilises that same flower. The remaining flowers rely on pollen from adjacent flowers or plants in that field and rarely on pollen from another field.
Oilseed rape can in theory pollinate related species such as charlock or wild radish, but oilseed rape prefers to pollinate oilseed rape and charlock prefers charlock. Although crosses between oilseed rape and wild radish are theoretically possible, there is little chance of this happening in reality. Even if cross-pollination was successful it would produce few seeds and the hybrids would be weak and unlikely to produce vigorous, fertile plants. The hybrids would not persist "forever and ever" as some anti-GM campaigners have suggested. Genetics simply doesn't work that way.
Current conventional crops that are herbicide-resistant, which we've been growing for decades, have just as much chance of cross-pollinating organic crops and wild relatives but they don't and they're not likely to, and nor are GM varieties.
A.F. Raybould and A.J. Gray (1993) Genetically Modified crops and hybridisation
with wild relatives: a UK perspective. Journal of Applied Ecology, 30, 199-219.
D. Bartsch, et al. (1999). Impact of gene flow from cultivated beet on genetic diversity of wild sea beet populations. Molecular Ecology, 8, 1733-1741.