Water founder in a mule is so rare as to be notable when it does occur. Mules have fewer feeding problems than horses do. Many farmers keep their draft and work mules together in pens with feed available at all times, yet the mules rarely overeat to the point of colic or founder. Mules from pony mares, however, may grass or grain or road founder, so the idea that a mule never founders is not true. People who buy cheaper weedy hay find that their mules clean out the weeds first.
Mules eat less than horses do.
Mules that are not working usually don't need grain at all. Good pasture or clean hay is the usual maintenance ration, unless extra fat is required for show purposes. Many a man has complained that his mules won't fatten because they won't eat enough, requiring the owner to spend extra money buying richer food to put the fat on. Of course, a mule must be fed enough for its size, its metabolism, and the work it is doing. Mules rarely have hoof problems.