For horse and people health alike I would say step number one is making sure the diet contains all the required nutrients. And add a comprehensive vitamin mineral supplement. This can be a powder in a mash or pellets or a ration balancer or a full feeding of a well fortified feed, with the caveat that full servings of such feed are often huge.
Your horse is getting old, and both Cushings and DSLD are chronic, permanent and progressive. Cushings makes healing from injuries slower too. So you are looking at symptom management more than a cure.
DSLD is the deterioration of muscles and ligaments. It’s not joints per se. So you don’t necessarily want or need joint supplements, if indeed they do much.
Prascend can also reduce the appetite. And you have her on a painkiller NSAID. Cushings is connected to insulin problems so you don’t want to put a lot of high starch bagged feed into her.
I would suggest starting with a palatable ration balancer that will give you concentrated nutrition covering everything required for soft tissue health in a pound a day serving. Or a VMS in a beet pulp mash if she likes mash. Mash is a very good way to hide powdered supplements.
DSLD is an emerging health topic that wasn’t widely recognized until quite recently. People used to just think their horses were getting injured in turnout etc. I think there may be online discussion groups that could give up to date information.
