How do you determine what supplements to give your horse?

How much do you know about horse nutrition? I know that I often feel as though I am a bit lost when trying to figure out what to feed my horse. The different grains and supplements have lists of their benefits, and I of course ask my trainer and other riders what they feed, but I still sometimes wonder if what I'm giving my horse is correct. How do you determine what to feed your horse, and when? Is there a resource for horse owners that lists the benefits and detriments of each supplement? Have found a way that you can determine which are working for your horse? 

Gosh Kara, I spend skads of time reading SmartPackEquine catalogs and on their website. I like their comparison charts and the detail they give on each supplement as well as how they catogorize their products. My schedule for Logan includes a combo joint supplement with Hyluronic Acid (currently Reitsport HA) because he's a big boy, a hoof supplement (Farrier Magic), something for the coat (Blackenall) and in summer, an electrolyte (Apple-A-Day) to keep him hydrated. I'm sure there are other vendors available as well.

What do you feed Josh?