Roy, Re-reading your original question, I would say whether there is a yield drag or booster or no effect of the cattle grazing the corn depends on what you are comparing it to.

Leaving them on the corn if it turns muddy might hurt following corn yields the first year or two again depending on the tillage system - or lack of.

However on my clay hillsides, based on what I've seen so far I don't think grazing it after frost goes out will have a significant yield effect after all of the residue hoofed in to the top few inches in the past few years.  Certainly no worse than trying to plant into combine or grain cart tracks and probably a lot easier.

I think on balance the grazing is a very large plus for the ground and overall costs. I have soil sampled and not needed to add P or K for a couple years now. Just need some N and S.  I am getting a bit borderline so I may spread some dry P & K + S & B this spring ahead of strip tilling. Then go with my 80-90 units of N as 28% banded in the strip a day ahead of planting.

As far as yield alone I think I will be very happy if and so far would guess that the cattle have had no effect on corn yield. Corn profitability however has very likely increased substantially from what I used to do. Other than seed cost it does not cost me much to grow this corn.

My reduced N has probably reduced my yield a bit from the 200 bu+ shown from fall of 08 but that is done on purpose.  

Hybrid selection is a big part of this program.   Look at the residue still between the rows in photo 2781 above.

Compaction seems to be less of an issue each year. I am thinking of leaving them on a week or two longer this spring to try to get some more weight on those steers going to the processor off of corn.

jmho.

Jim at Dawn