Yes, donkeys can eat alfalfa hay. Ideally horses and donkeys need 1.5-2.0% of their body weight in roughage daily. A minimum of 1.0% of roughage daily is needed to maintain gut health. They can most certainly eat it, just don't over feed it (as with anything else). Keep in mind that while alfalfa is an acceptable source of roughage, your donkeys would be just fine eating grass hay. Very few horses (high level performance horses that are expending a large amount of calories) need the protein and energy level provided by alfalfa hay. Below is a link to an excellent feeding guide by Colorado State Extension.