Natural out-crossing rates were evaluated for conventional soybeans
(Glycine max (L.) Merr.) cultivated adjacent to genetically modified (GM)
glyphosate-tolerant soybeans under field conditions during a four-year
period in Japan. A total of 107 846 progeny of 2772 plants harvested from
conventional varieties were screened for glyphosate herbicide tolerance. The
highest out-crossing rates, 0.19% in 2001 and 0.16% in 2002, were
observed in adjacent rows 0.7 m from the pollen source. The highest rate in
2004 was 0.052%, which was observed at 2.1 m. No out-crossing was
observed in the rows 10.5 m from the pollen source over the four-year
period. The farthest distances between receptor and pollen source at which
out-crossing was observed were 7 m in 2001, 2.8 m in 2002, and 3.5 m in
2004. The greatest airborne pollen density during the flowering period,
determined by Durham pollen samplers located between the rows of each
variety, was 0.368 grains.cm-2.day-1, with the average value at 0.18
grains.cm-2.day-1, indicating that the possibility of out-crossing by wind
is minimal. Thrips species and predatory Hemiptera visited the soybean
flowers more frequently during the four-year period than any other common
pollinators, such as bees.