I know this sounds weird. But try bute-less. I have my TB on it and he is a little stiff. It has helped wonders. Its on smartpak(i think).
My trainer wrote this artical on it; http://www.horsenation.com/2017/04/18/product-review-absorbine-bute-less-performance/
It is an anti-inflammatory which works pretty well and gives secondary effects on some horses. Unfortunately, it doesn't come under the feed, that's why we can't give you a precise answer on the subject.