The stump of the corn plant is going to be the last think they eat.  Nitrates in grazing mature corn is one of the last health issues I'd worry about, unless you put 200 units of N on it and it grew good for a while then droughted out.  You can overanalyze any situation like this, but if it were strip grazed, it would be very easy to control as you could move the fence before they have to start grazing the stumps hard.  Grazing corn is no different than grazing harvested stalks, except the grain has been taken off the ears.  Out of all the stalks I've grazed I've never lost a cow to nitrates, and I've grazed stalks on some very droughty years.  Also graze some standing corn almost every year in timber patches that are hard to get to with harvest equipment and never had a problem.  Good luck, keep a very close eye on them every day and adjust if problems arise, good observation is the most important thing you can do and don't force them to lick the field clean.