Maize is a wind-pollinating crop and is able to cross-pollinate in neighbouring fields. Research on maize pollen dispersal and cross-pollination has been carried out for many years with the goal of improving seed purity (Salamov, 1940, Jones and Brooks, 1950 and Burris, 2001). Recently, with the introduction of genetically modified (GM) maize and the definition of GM tolerance thresholds and labelling requirements in some countries, there is legislation to establish and guarantee coexistence of both ways of agriculture. The interest in long distance pollen transport was fueled by the discovery of transgenic sequences in maize landraces in Mexico, where the cultivation of transgenic maize was forbidden to protect biodiversity and as a result pollen dispersal from the USA across the border was suspected (Quist and Chapela, 2001). Bellon and Berthaud (2004) on the other hand, concluded that these transgenic sequences were due to the custom of small-scale Mexican farmers to incorporate improved commercial varieties into their open-pollinated landraces.