Concentrated food / hard feed for your donkey? Skinny donkey?
Donkeys only need concentrate in exceptional cases, such as when a nursing mare loses too much weight, a donkey is / is ill or when a donkey has to work a lot. An additional source of energy can be a little good vegetable oil.
In short, it is better to feed a lean donkey slowly with good roughage and a fiber-rich supplementary feed than quickly with grain.