Hi Rebecca,

Grass Advantage is over our 10% ESC+starch limit and over our 4% fat limit and Safe Starch is not actually safe for most IR/PPID horses.  You can read more about that here: https://ecir.groups.io/g/main/message/255110

I would eliminate both of those from the diet if you think there's an IR or PPID issue.  If you can source Triple Crown Timothy Balance Cubes that would be a much better option as they don't need additional minerals to be balanced, just need to have flax and vitamin E added as per the emergency diet (which you can read in detail here: https://www.ecirhorse.org/DDT+E-diet.php)

Why is she getting bute and how long has she been on it?

If you suspect that she is IR or PPID she needs to have her insulin/glucose and ACTH level checked as soon as possible.  I would change her diet over to safe feeds and see if that helps with any stomach issues you're currently seeing.  Why are you looking to add vitamin D to her diet?  Also, please note as per our guidelines https://www.ecirhorse.org/terms-of-use.php - 4.D - you need to have the owner's permission to post anything about this horse.