Figure (solely for the sake of this calculation, so make up any odds you want) the odds of pollen from the tassels in one field landing on the silk of a corn plant in another field.
There is one silk per kernel, about 150,000 kernels per pound of corn, 56 pounds of corn per acre, the average corn yield in the US is something in the range of 150 bushels per acre and in my part of the country the average corn field is maybe 80 acres.
It is not possible for all of the kernels in one field to have been pollinated by pollen from the tassels in a neighboring or more distant field.
When seed corn is raised, the tassels of the plants sought to be pollinated are removed (or otherwise made not to produce pollen) and the pollen from the variety the corn is to be crossed with is artificially dusted onto the silk. It's a labor intensive, expensive proposition.