Coexistence in Maize: Effect on Pollen-Mediated Gene Flow by Conventional Maize Border Rows Edging Genetically Modified Maize Fields

In addition to or as substitution for a regulated isolation distance, conventional maize (Zea mays L.) border rows at the genetically modified (GM) maize field edge are considered as feasible measure to ensure coexistence between GM and conventional or organic maize. Therefore, we examined the effectiveness of 9-m- and 18-m-wide non-GM maize borders at the GM maize field edge on outcrossing rates into a neighboring non-GM maize field. One field experiment each was conducted in 2008 at three sites in Germany using a field orientation representing a worst-case scenario concerning wind direction. In each case, the distance between GM and non-GM maize fields was 51 m. At two sites, sizes of GM and non-GM maize fields were 0.8 ha, respectively, and at the third site 0.5 ha (GM) and 0.3 ha (non-GM). The GM percentages of individual samples taken at recipient field depths between 0 and 90 m were quantified by real-time polymerase chain reaction. Overall, no pollen-mediated gene flow reducing effect of border rows was observed in the present study, although synchrony of anthesis between GM maize and border row maize was given at each site. Calculated GM contents of the total non-GM fields harvest were always below the European Union labeling threshold of 0.9%. In consequence, planting 9-m- or 18-m-wide conventional maize border rows at the GM field edge is no reliable coexistence measure, at least if combined with an isolation distance.