If it is going to be hay field more than pasture, I'd plant it to an annual crop like oats. Harvest them for hay, or graze them, then re-work the field this fall. With the old sod, I think it will be tough to get the soil worked down and smooth if you are going to be driving hay equipment around it a few times a year. Plow it and plant the rotation crop, give a chance for the old sod to break up, then prep it for your permanent use. Orchard grass and alfalfa is a good combo in my area, but use whatever is suitable for your soil/climate.