Tulip is right about cross-pollination - but I don't know how much seed is saved by farmers to replant like they used to. The way to beat the issue is that seed is saved from the center of a "pure" stand. So Marquise's father would get his sweet corn from a different field, or just buy the seeds for a few rows next to the house. I'm sure the cows and pigs appreciated a bit of sweetness in their corn.

Back when I was a kid living in France, my Mom had a housekeeper who was appalled that Mom was giving us corn-on-the-cob, as the only corn Madame knew was field corn. Mom tried to explain that no, it was sweet corn, not pig food. Mom grew up on a farm and knew the difference. Madame wasn't convinced and brought us a bunch of garden vegies the next week because if we were eating pig food, maybe we weren't doing as well financially as she thought - she got her wages, but the children eating pig food, perish the thought!