Acti-flex is hands-down the best, IMO. I give the powder to my mini who has upward fixation of the patella (along with many other issues -__- ). It has the most garuntee of the most important ingrediants in each ration. I don't know how much you'd have to give a full-sized horse--my mini gets a full tsp with his gain 2x a day, and it works Miracles! Though make sure that you don't get the senior Acti-Flex because it is more comparable to an all-around supplement--they add more vitamins etc that the geriatric horse needs. :3
Hope that helped!
~The Golden Filly!~
It has been proven (by MANY studies done by vet schools and clinics) that feed through joint supplements just don't work. Firstly, the aprovel is easy to get. It is a very lax system on getting aprovel to sell a supplement. That means that exactly how much of what is in that tub isn't held to a high standard, making it easy to just put whatever in. Also, many of the things put into joint supplements (feed through) don't even get digested. That or they are completly digested in the stomach and not absorbed into the blood stream. If you are looking for a joint supplement, go with an IM injection, those are held to a VERY high standard, and you get exactly what they tell you
I personally have had a lot of success with the 4 flex supplement, and I do recommend it, but that being said, I have never had ultra sounds, or radiographs, or anything else like that done on my horse before and/or after starting a joint supplement, so I have no scientific proof of how well it works or doesn't work.
Have you talked to your vet? That's the best way to go, they can do flex tests to see if he needs a supplement, and then go from there. IM injections can only be distributed by a vet also, so if you do use one, you have to get it through your vet