If you are running the soil finisher in the spring you are likely loosing CoC population due to hairpinning residue into the seed zone. This type hairpinned residue can not be removed by row cleaners without digging a trench with the row cleaners.

"Minimum till" corn on corn works best when you do your tillage in the fall and nothing in the spring other than row cleaners on the planter (or row cleaners on a spring strip till machine).  Leaving a lot of tough bt residue on the surface over the winter seems like a good idea for "conservation" tillage.  But then come in with a shallow working (often just about seed depth) finisher type tool in the spring a hairpin all that over wintered on the surface residue right into the seed zone. 

Even following with row cleaners on the planter can't remove residue sliced and mixed in with the soil just below the surface. A certain percentage of the seed planted is then in contact with this soil finished incororprated residue and the alleleopathic reaction causes a reduction in the stand.

Many folks complain about "yield drag" of CoC but are using spring tillage practices which may make the soil surface look "good" in the spring but hairpin a lot of the surface residue into the soil at seed depth.

If you are going to use a VT tool, use it in the FALL not the spring in CoC. 

Most of the finisher type tools are really for planting beans into corn or corn into beans, not corn into corn.

From many years of experience.

Jim at Dawn