Could the loss of pigmentation around a horse's eyes be dietary related either through grain mixture, hay or plants in the pasture?

Loss of pigmentation (termed vitiligo or leukoderma) can occur in horses especially around hairless or thinly haired areas. In most of these conditions, the skin is normal other than the loss of pigmentation. I assume that the skin is normal in your horse, or your veterinarians would have diagnosed and treated the problem. Check with your veterinarian about any possible new treatments for this condition. Some horse owners have reported an improvement in the condition with copper supplementation in the diet, and your veterinarian should be able to test the horse’s blood copper levels to see if they are within a normal range. If the horse is deficient in copper and you decide to try supplementation, there are many commercial supplements available that contain copper (including trace mineralized salt) that you can feed to your horse according to the manufacturer’s directions. Depigmentation is more common in some breeds and families of horses, suggesting there may be a genetic basis to the condition. Arabians, in particular, have a juvenile form of depigmentation commonly referred to as “pinky condition” because of the pink skin around their face. However, in most cases of the condition, a specific cause cannot be pinpointed. The good news is if the condition is simply depigmentation, it is harmless to the horse. However, if you notice any changes in your horse’s skin or general health, make sure you consult your veterinarian immediately.