One approach to ensuring coexistence of genetically modified (GM) and conventional maize (Zea mays L.) is reducing pollen-mediated gene flow.  Field experiments were conducted in 2005 at four sites in Germany to compare a tall sunflower crop (Helianthus annuus L.) vs. a short clover?grass crop (Trifolium pratense L. and Lolium spp.) with regard to their ability to reduce outcrossing when grown as buffer between pollen donor and recipient maize plots.  Three different maize test systems were used: (i) quantification of a donor transgene via real-time polymerase chain reaction (rt PCR), (ii) a nontransgenic test system based on a dominant kernel color trait, and (iii) a molecular marker test system based on rt PCR quantification of a cultivar-specific nontransgenic DNA sequence.  We found that the three test systems yielded comparable results concerning buffer-crop effectiveness and edge effects.  There was no difference in outcrossing rates when comparing the sunflower vs. clover?grass buffer crop.  Outcrossing rates downwind beyond 12 m sunflower as buffer crop within adjacent 12-m-wide recipient maize were 4.2, 11.7, and 3.8% for the GM maize, the kernel color, and the molecular marker test system compared with clover?grass with 4.3, 9.6, and 3.6%.  Pronounced edge effects were detected at the edges of recipient maize fields.  Based on the present study, growing sunflower as a tall crop between GM and non-GM maize cannot be recommended as an appropriate coexistence measure.