If you have your cylinder,concave settings proper only cobs go over the walkers in one whole piece and and all you have to deal with is shelled corn over the top sieve. Set your concave clearance a little less than the diameter of a shelled off cob and adjust your cylinder speed to just roll the kernals off of the cob. This works very well on conventional combines, I can't speak for rotaries. If you have different corn variaties in the same rows this can be difficult as some varieties have different moistures and different actual cob sizes, you nhave to keep a happy medium.