I have ponies, well, a lot of other animals too ....and always wanted donkeys. A couple of years back I acquired two donkey geldings......nice little chaps, and I was thrilled......until I found that every Autumn and Spring they eat the post and rail fencing. They have chomped their way through a lot of rails, and made a hole big enough in the gate to allow them, and two mini shetlands to go into the other field. It has cost a lot of money to replace and the land is only rented. I am terrified of them doing it again. The vet said there is nothing wrong with them, its just something donkeys or some of them do. None of my horses have ever done this. I have tried suppliments, lick, they have plenty of food, and even branches to gnaw on. But, no the fence is better. Even creosoted it does not stop them.
I have got them in my own field, which is dry stone walls, and they have not eaten the stones yet.
Apart from this problem, they are lovely. They do bray loudly for their feed, and need stabling when the weather is bad in winter. Oh, and mine eat an alarming amount of hay and straw.