I take samples from multiple areas based on soil type, topography and crop& manure history.  Take the samples when the corn reaches black layer.  Pick 10 random stalks,  measure up 6" on a stalk and cut, measure 8 more inches and cut again.  Ten 8" pieces make a sample.   Can't help you on the Ohio lab question.

FWIW I have yet to use any applied N in addition to manure, NT, and crop rotation on my corn and tests are showing sufficient at the end of the year.