I highly recommend getting FeedXL and inputting her current diet in to figure out what she needs.
I would say that joint supplements.. you DEFINITELY get what you pay for. I love how easy Smartpak makes it to distribute and they offer a wide variety of different types. Your mare might do good on Smartflex Senior, and it's affordable (costs the amount of a new pad per month) but if that doesn't work, just putting them on a supplement with MSM, hyaluonic acid, glucosamine, and chondroitin sulfate.
Make sure she has access to salt lick and that her feed gives her all the vitamins and minerals and energy she needs. FeedXL will help you. Even a month of use is so worth the cost. I'm telling you! I'm a year long member. I renew every time because it has been very helpful.
Hope that helped!
"Strength is the ability to use a muscle without tension"