If you plant types that come ripe days to weeks apart, you can get way with little to no separation.
I have planted field corn and sweet corn, often five to eight varieties with separation of only three feet in eight by eight foot blocks.
I plant plant spacing on average of six to eight inches between seeds and average of 28 inch row spacing.
With such a dense planting you may still get slight crossing on the outside row but I have never had problems with sweet corn being ruined by field corn crossing.
If you plan on seed saving purity then three hundred to seven hundred feet is best.
