Hi There,
My thoroughbred mare is 26 and she has cushings and is on pergolide too. What my horse gets is a scoop of rebound,scoop of this green color grain type food..i forget what its called..then she gets a small scoop(one of those ice tea size scoops)of fat cat sprinkled on the food,and her meds sprinkled on top of that. Then she gets a squirt of vegitable oil on her food. She gets that around lunch time. Then at breakfast time and dinner time she gets her normal grain and hay. Im not sure what kind of hay it is. I don't do the am & pm feedings. I just do the lunch time feedings when im at the barn. Other times my trainer would do it on the days im not there.
A few months ago,i gave her Vintage Senior,by Blue Seal feeds. But that bag is long gone. It seemed to help a bit. I don't know if we are going to get another bag or not.
This food has helped her to much. You can't see her ribs as much.. she has a whole new coat too. She looks sooo good for her age. 
I don't know if that helped you with anything..but thats what i do for my horse.
~Laura~