bgunzy,

First, I commend you for thinking about more diverse rotations.

Eddie is correct in that the sorghums (including sudan) are strongly allelopathic to corn.  If you remove most of the residue by baling or grazing, it might not be too bad.  However, the goal is to grow a corn crop that is healthier than average the next spring, not weaker than normal.  Corn following wheat in south-central Iowa is going to be challenging anyway, and my guess is that sorghum or sudan will make it worse no matter how it's managed.  I would look for something else, such as forage radish, or a mixture of radish + sunflower, or perhaps radish + sunflower + clover.