Flax seed produces a great coat, and a nice mane, tail, and hooves as well. I've seen horses on it that look wet! :) It also prevents bleaching. I don't think it helps with weight, though. Make sure you get the kind that is ground, or gring it up yourself, or they cannot digest it. I buy it at the grocery store in a powder form.
I've never used boss, so I can't compare the two... but that's my opinion on flax! :)
Every great dream begins with a dreamer. Always remember, you have within you the strength, the patience, and the passion to reach for the stars to change the world. ~Harriet Tubman