Well, it's been almost a year since I initially built the hay ring so I thought that I'd update...
As you can see it's survived with not one repair, I'm happy to report. I did belt-sand the whole structure, then double-coat painted the top rail in a pleasing colour. The rest of the wood I linseed oiled as a preservative to the elements:
If I've got a smaller round of hay then I only use 5 panels, a 5' round uses all 6 panels.
I've kept using the rope to secure the whole thing together (after I use the drop pins to position it) as the horses have never been able to knock it down with the rope binding it. There have been some interesting geometric shapes left from them straining to get that last bit of hay, but it's always stood up.
I have had to re-bend some of the tin hinges using a rubber mallet from time to time, but none of the hinges have broken.