To reduce the amount of pasture that they eat in the couple of hours on pasture, try the use of a grazing muzzle.
If the donkeys are sound, also increase the exercise. If big enough, you might consider teaching them to be ridden. All donkeys can be driven regardless of size.
As far as all the supplements on the market, there is no scientific evidence that they are effective. Instead of supplements, weight loss is most affected by less/more appropriate feed and/or more exercise. Also, weight loss should be gradual. Donkeys are prone to fatty liver/hyperlipedemia if weight loss occurs too quickly. To judge how your donkeys are doing on their diets, take pictures at biweekly intervals and record a body condition score for each animal. Go to the Horse Quest Learning Lesson - How to Body Condition Score Horses
