Corn pollen is spherical and much larger than the pollen produced by most grasses (Burris, 2002; Gray, 2003). Corn pollen is among the largest particles found in the air. Although it is readily dispersed by wind and gravity, it drifts to the earth quickly (about 1 foot/second) and normally travels relatively short distances compared to the pollen produced by other members of the grass family. Pollen may remain viable from a few hours to several days. Pollen can survive up to nine days when stored in refrigerated conditions. However, under ambient field conditions, pollen is viable for only 1 to 2 hours. High temperatures and low humidity reduce viability. Elevated temperatures have a greater negative impact on pollen viability than humidity, with viability greatly reduced at temperatures above 100 degrees F. At flowering, 60% of pollen fresh weight consists of water; pollen longevity diminishes rapidly if the water content drops below 40%. Corn plants typically shed pollen for 5 to 6 days, whereas a whole field may take 10 to 14 days to complete pollen shed, due to the natural variation in growth and development among plants (Nielsen, 2003b). Peak pollen shed generally occurs 2 to 3 days after 50% of the plants have shed pollen. Individual corn plants produce one half million or more pollen grains, although variation exists among hybrids and plant densities (Abendroth et al., 2011). Therefore, even if only a small percentage of the total pollen shed by a field of corn drifts into a neighboring field, there is considerable potential for contamination through cross pollination.
Many studies have been conducted to determine how far pollen will travel—some have evaluated the density of pollen at varying distances from a corn source, whereas others have measured pollen drift by measuring outcrossing in neighboring corn. This latter approach is probably more meaningful when it comes to assessing the impact of pollen drift from GMO corn fields.
Once released from the anthers into the atmosphere, pollen grains can travel as far as ½ mile with a 15 mph wind in a couple of minutes (Nielsen, 2003b). However, most of a corn field’s pollen is deposited within a short distance of the field.
Past studies have shown that at a distance of 200 feet from a source of pollen, the concentration of pollen averaged only 1% compared with the pollen samples collected about 3 feet from the pollen source (Burris, 2002). The number of outcrosses is reduced in half at a distance of 12 feet from a pollen source, and at a distance of 40 to 50 feet, the number of outcrosses is reduced by 99%. Other research has indicated that cross-pollination between corn fields could be limited to 1% or less on a whole field basis by a separation distance of 660 ft., and limited to 0.5% or less on a whole field basis by a separation distance of 984 ft. However, cross-pollination could not be limited to 0.1% consistently even with isolation distances of 1640 ft.
Several studies have been performed evaluating the impact of pollen drift from GMO fields on neighboring non-GMO fields. A Colorado study (Byrne et al. 2003) tracked the drift of pollen from blue corn and GMO Roundup Ready corn into adjacent conventional corn. Corn with marker traits (blue kernels or Roundup herbicide tolerance) was planted adjacent to corn without those traits. Cross pollination was greatest at the closest sampling site—up to 46% outcrossing about 3 ft. from the edge of the test plots containing blue corn. Cross pollination dropped off rapidly with only 0.23% cross pollinated kernels near the blue corn plot at 150 ft. Only 0.75% of the corn showed cross-pollination with the Roundup Ready plot at 150 ft. The farthest distance any cross pollination was detected was 600 ft. These results suggest that 150 ft. may be a reasonable buffer between GMO and non-GMO corn to prevent significant cross pollination due to pollen drifting from one field to another.
http://ohioline.osu.edu/factsheet/agf-153