Ask the Nutritionist: Dr. Getty's Forum for Equine Nutrition
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Dr. Getty
Hi Melissa,
Thank you for writing about your pony. Ponies are genetically predisposed to insulin resistance and even Cushings. So, they need a low starch/low sugar diet. It sounds like you are doing this. But, I would definitely add a magnesium/chromium supplement to his diet to reduce circulating insulin levels, known as Quiessence.
Grazing Muzzles can be very stressful for many horses and it sounds like your pony is no exception. Stress causes the release of cortisol (which is already high in the cushings horse), which further aggravates insulin resistance. And, so, you defeat your purpose, because the result of this hormonal imbalance is more fat storage. It can also lead to laminitis due to elevated insulin levels.
So, your first approach should be Quiessence, at a dose of one ounce per 250 lbs.
Since your pasture has rye (a cereal grain) be sure it does not go to seed. This will significantly increase starch levels, which are inappropriate for him.
Also, avoid any sugary treats such as carrots or apples. Skode's treats are an excellent alternative for horses that need to watch their sugar and starch intake.
I congratulate you on his exercise program. That is very important.
I hope this is helpful. If you notice any other Cushing's symptoms such as a change in his hair coat, where it is kinky and will not shed properly, consider Balance EQ, in addition to Quiessence.
All the best,
Dr. Getty 
Where are you from? Bayfield, CO