From what I've been told and learned, donkeys can't handle a lot of protein. If they get to much, that is what causes the fat pockets to develop. Once they get them, it is really hard to get rid of them.
Kilroy gets 2 cups of rolled barley 2x a day + 2# of good grass 2x a day. I do give him a small handful of alfalfa once in a while.....just to keep him happy. He does work though, drives and breeds, so he works any fat he might get off.
I have a friend that is feeding her donks alfalfa and so far they don't show fat.
The farm I bought Kilroy from was feeding good grass hay, oats and a cup of sweet feed as a treat. They all looked really good.