In general, donkeys are much less prone to laminitis than horses or ponies. As with all equids, though, risk is greatest when coupled with obesity. As  long as donkeys have plenty of space to roam, are encouraged to move, and are not supplemented with grain (you can provide them with a trace-mineralized salt), they should keep their weight down and be fine. Keep a close eye on their weight, and if they get too heavy, you may want to consider limiting their access to pasture, particularly during the seasons when it is richest (spring and fall).