Diet for Horse with Cushing's Disease

At this time there is not a cure for Cushing's disease, so treating them is  is a lifelong effort and diet is a big part of the treatment. Their diet can be very beneficial.
 Avoid feed that has a high level of sugar and starch in them The diet should maintain less than 20% Non structured Carbohydrates.
Since most horses are old that develop Cushings, you might think senior food is a good option, but most senior feed is actually quite high in non structured carbohydrates and is not a good choice. Stay away from anything that has molasses in it, including treats. Alfalfa and grain should be eliminated from the diet too. Sweet grass is also high in nonstuctured carbohydrates and should be limited.
We feed our Cushing's mare Senior Glo by Alliance. it is a pellet feed  and it is low in non structured carbohydrates, but I am sure there are other companies, such as Purina etc. that make a feed that would be suitable.
It is also very important to keep weight on a horse with Cushing's disease, because they tend to experience muscle wasting. It can be hard to keep weight on them, especially because you are cutting down on the  sugars, and starches. They should do well with a fiber-rich grass hay or grass hay pellets. Avoid alfalfa hay totally! If symptoms are not severe, they might do well with a feed containing soy or beet pulp to keep weight on and to add a higher-fat supplement containing oil or stabilized rice bran. I believe the Senior Glo has all those things in it..
I hope this information has been helpful. With a good diet and medication, your horse with Cushing's disease can live a long and a  good quality life.
Hopefully they will find a cure for this disease in the near future!