This question reminds me of an Extension presentation on 
apple pollination research.  The obvious cross pollination
we think of occurs from bees going from flower to
flower on one trip.  The nonobvious is pollen mixing in
the hive.  Apparently, foragers do not leave the hive
perfectly clean.

If the seed grower is normally content with a 1/4 mile
buffer, then the in-hive pollen transfer does not
present a significant problem on this crop and he just
needs to consider the likelihood of foragers working
both sides of the buffer on one trip.

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