During the 1980's the University of Illinois tests, there was no yield advantage with 30" spacing.

Today with preplant 30" spacing and side dress 30" spacing, the soil profile has a reasonable amount of nitrogen at tassel plus organic nitrogen which

Test tool bar shown here with Dietrich N'Clozrs™ on 60" spacing was painted with epoxy to detect any high stress points during testing. Actual product is Hagie yellow.
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becomes more available with warmer soil. Therefore the extra nitrogen placed at tassel is not the only nitrogen available to the plant.

Now the question of 30" versus 60" is, if the U of I could not get a yield

increase with 100% nitrogen on 30" versus 60" spacing, how we can we expect a yield increase on 30" versus 60" with approximately 75% on 30" spacing and approximately 25% on 60" centers?