The positive risk assessments of GM grain crops have been based on the premise that, because they are adapted for wind pollination and largely self-pollinating, there is little concern about gene-flow from them into other crops.
Grain crop have a flower structure which aids wind-pollination, and their pollen is not well adapted for carriage by insects. Tests have shown that the amounts of pollen carried by wind decrease exponentially with distance from the crop, reaching zero within a few meters. Also, pollen is short-lived. The risk of gene pollution arising from GM grain crops has, therefore, been considered effectively zero.
However, a recently published study has challenged this view.