I use this for my mare:
Vitamin E & Selenium - Horse Antioxidant Supplements from SmartPak Equine
I mix it with grain and wet it a touch because it's a powder. It doesn't take much grain. It smells like apples; picky mare has no problem with it.
I started supplementing a couple months ago, after my mare had a near fatal azoturia episode and started dietary therapy for PSSM. We haven't had an episode since. It keeps her muscles happy.