Statistical analysis of outcrossing between adjacent maize grain production fields

The presence of transgenes in conventional maize, Zea mays L., crops is a serious concern when the genetic purity affects the value of the harvested product (i.e., specialty markets, organic products, crops with value-added traits, and hybrid seed production). Gene flow from a central transgenic source plot into a conventional grain production field was quantified using a combination of three marker genes: the y1 seed color gene, the Bt-Cry1Ab gene derived from the soil bacterium Bacillus thuringiensis (Bt) and the Roundup Ready® (RR) gene. Two fields of approximately 36 ha were planted with a nontransgenic, white-seeded corn hybrid in Ankeny, IA, in 2003 and 2004. In the center of each field, a 1 ha plot of yellow-seeded, Bt/RR hybrid corn was planted as an adventitious pollen source. Detailed measurements of flowering dynamics confirmed the white- and yellow-seeded hybrids flowered synchronously both years. Grain samples were collected at 1, 10, 35, 100, 150, 200, and 250 m from the transgenic pollen source along eight transects (north [N], northeast [NE], east [E], southeast [SE], south [S], southwest [SW], west [W], and northwest [NW]) and were analyzed for number of Bt-/RR-/y1- kernels. The statistical model describes the proportion of outcrossed kernels to decrease exponentially with distance from the yellow pollen source and linearly with the wind speed and direction during silking of the white hybrid. On average, outcrossing at 35 m was 0.4% in both years. At 100 m and beyond, the average level of outcrossing decreased to 0.05% or less. A few Bt-/RR-/y1- kernels, however, were detected in the white corn field even at 250 m from the source plot. A single empirical model captured the field-scale patterns of outcrossing from the source plot for both years. These results indicate gene flow from a transgenic pollen source follows a fairly predictable pattern. The results also suggest that extent of outcrossing can be reduced by surrounding the transgenic pollen source with nontransgenic corn producing a high density of local pollen.