The standard term for them is/was PSP (Pierced Steel Plank) during WWII. Sometimes called portable runway. They make great cattle pens. Don;t need a post in the middle, just the ends.
Don;t see why they wouldn;t work for hay. One advantage would be you don;t have to move them out of the way, just drive right on them.
Have similar up here we call rig matting. For oil rigs working in soft condtions. Could have miles of matting going into a location, and then the location itself will be matted as well