Either the Lite Balance or Safe Choice, reason being is our horses are easy keepers and they are on a lot smaller than what you have *we do have 20 acres*
I am sure your feeding table is fine just have to remember that over the years a horse ages and its system changes is all.
A cresty neck, laminitis incidents, only a resting blood test can detect it. Our mare passed the first one and failed the next one some months later. Removing the horse from all sugar and starches and high protein. Low NSC hay and pasture. Timothy is low in NSC, NO alfalfa or wheat type hay or rye grass.
Insulin Resistance is like type 2 diabetes in humans controlled with Diet and Exercise mainly providing the horse isn't having a laminitic episode. Some don't go laminitic but it does come with the territory. Usually caused by a pituitary gland tumor a slow growing one.
Some people use Cypro for these horses but it backfired on my mare and she was removed from it.
Yes more and more horses are coming up with this and you have to fit the diet to the horse not every horse will respond the same way.
A great place to learn if you can handle stuffy people is a Yahoo group at
http://pets.groups.yahoo.com/group/EquineCushings/
But still great information!!!