> I'd like to get other people's opinions about isolation distances
> needed for corn to maintain varietal purity.
Seed production fields are often placed surprisingly close to grain fields.
Most companies isolate yellow from yellow only 20 rods (330 feet), and
sometimes they cheat. Of course, these are big fields, and the industry
tolerates up to 5% off-types (spurious hybrids). Sometimes they cheat on
that too. Many companies including Pioneer, have more stringent standards,
and sometimes even a few tenths of a percent contamination looks like hell
in the customers field if the contaminant is tall, late, or has big tassels.
My department, production research, is studying this in a big way right now
because of the GMO contamination issue. There is not much hard data
supporting the 20 rod isolation standard, and we are finding that things
like relative position and relative size of the fields make a big difference
in whether 330 feet is enough. Obviously, the smaller your cornfield is,
the more isolation you need to maintain a given level of purity. Small, in
this context, is less than about 20 acres.
> Seed to Seed by Suzanne Ashworth states that "...corn
> pollen is light and can be carried long distances by wind.
> Isolating a variety of corn by 2 miles will ensure purity."
Actually, corn pollen is among the largest and heaviest of the angiosperms,
a little over 100 microns. Pollen size in most wind pollinated plants
ranges from 17 to 58 microns ("Pollen: Biology biochemistry and management",
R.G. Stanley and H.F. Linskens, 1974). My experience as a gardener is that
contamination falls off pretty quick by fifty feet or so. It depends on
wind speed and direction. Purity is a relative thing. If you want
approximate purity (1-10% offtype), don't worry about it (the genes you pick
up will probably be good Pioneer genes anyway). If you want to faithfully
maintain a small plot of heirloom corn in the Midwest, you *must* bag the
shoots and pollinate it by hand to get your seed. This is fun and easy, and
Susan Ashworth gives a nice recipe in "Seed to Seed."
The genetic contamination issue should not be confused with the monarch
butterfly feeding issue. Very heavy Bt pollen loads on milkweed leaves are
needed to cause deleterious effects on butterfly larvae.
This kind of thing probably only occurs right on the field margin.
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