It is possible with some of the new enhanced varieties to grow high density corn. But it requires excessive amounts of N and water. Even some modern varieties of Sweet corn can be forced under the same conditions. However most of these varieties can be bought only by professional growers who agree to a list of conditions. Remember traditional op field corn was designed to be planted at 42 inch spacing in 42 inch rows (Check planting so you could cultivate in 4 directions)

You sure the fields you pass by that have such high density planting are sweet corn and not field corn? Very different methods of planting and harvesting.

Dave

Silage corn is planted very densely because they want the whole leave and stem biomass for silage.