Hello:

You are correct that the terrain can be rough on the dogs pads. Its been dry out there this summer, and the farmers have been bailing the ditchlines so it might be a good idea to bring some along if you see that it becomes a problem. Last year a fellow hunter had his springer get its belly pretty ripped up running thru some of the dry grass.
As far as shells go, you can go wrong with those #5s and #6s aren' t bad either. I shoot Remington Nitro mag 3 in # 5s and they serve me well, but people in my group use everything under the sun and are successful. I hunt with a guy that uses a Remington 1100 with a fixed modified shooting 23/4s all day and he has no problems putting more birds in the truck then I do. Depending how many days you are going to be hunting, and what type of hunting you are going to do, I would bring 4 boxes of lead, and if you are going to be hunting waterfowl protection areas, don' t forget to bring some steel as it is required in those areas. Being that close to Mitchell, you will have a Cabela' s sitting right there if you need more shells.
I will be out there the first week as well, and the hunting should be outstanding. We have a small hunting camp that we bought a couple of years ago, and I have been out there about 5 times thru the spring and summer, and have seen so many Pheasants. I think the hunting will be great. Let me know if you have any more questions, I would be happy to answer them. I have been hunting South Dakota for several years now, and have always been productive in getting birds. And the nice thing about it is that We hunt very little private, and get plenty of birds in the public hunting areas.

Good Luck hunting.

Bill(WI)