Ensuring the purity of conventional crops grown in the vicinity of
genetically modified (GM) crops depends on understanding both short and long
distance pollen flows. New research shows that current guidelines on the
safe isolation distances for GM maize may not adequately prevent cross
pollination of conventional crops.


Contamination of conventional crops can occur where GM pollen
cross-fertilises non-GM maize. The proportion of cross-contaminated seeds in
the conventional field is the 'impurity rate' for that crop. Under European
Union rules1, if the accidental proportion of GM to non-GM seeds exceeds 0.9
per cent then the crop must be reclassified and labelled as GM. Existing
safe distances were largely established using 'paired field' comparisons,
where contamination from a GM field is measured in a specific nearby field.
The distances between the two 'paired' fields can then be adjusted to
determine a 'safe' distance between fields. However, on a landscape level,
other GM or non-GM maize crops in the vicinity may have an effect on pollen
flow.

French researchers modelled the spread of pollen in a landscape containing a
patchwork of GM and non-GM maize fields, as well as other non-maize fields.
By taking into account the pattern of both short and long distance dispersal
of GM pollen, the study explored the additional impact of more distant GM
maize fields (i.e. not the closest GM field) on the impurity rate of the
non-GM maize. For comparison, the impurity rates in a conventional field
were also calculated using only the distance to the closest GM field.

Overall, the study showed that pollen from GM fields closest to conventional
fields and the size of the conventionally planted fields have the greatest
impact on the degree of contamination. However, as the proportion of GM
maize to non-GM maize increases within the landscape, the impurity rate of
conventional fields also increases. This increase was caused by long
distance pollination from GM fields further from the conventional fields and
suggests that if GM maize becomes more widely adopted by farmers, then
existing models will underestimate the 'safe' distance between GM and non-GM
crops. Importantly, the level of underestimation increased as more GM maize
was included in the modelled landscape and when the isolation distance
between GM and non-GM fields increased.

The researchers therefore suggest that, as long-distance dispersal of GM
pollen can contaminate fields of non-GM crops and potentially raise the
impurity rate above 0.9 per cent, pollen from all GM fields in the landscape
needs be considered when setting isolation distances between fields of GM
and non-GM crops. Further research is required to determine how to model
these effects at the landscape level.
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