My poor horse has high and low ring bone, bone spavin and pedal osteitis in all 4 feet and the vet recommended Synequin. I noticed a difference in a week.
Its an odourless white power which you mare shouldnt even notice. It is expensive, but its worth it.
Could also try blue chip dynamic, my boy is on this as well until he eats it all.
I like the smell of NAF Superflex! I think they do a liquid version which she may take? I think its the best joint supplement and even my 2 picky eaters love it.
My old man is on global herbs bute-x and global herbs movefree liquid. He has been on both for about a month now and I have really noticed a difference. When the liquid is gone I will take him off the bute-x and put him on movefree plus powder. This is on recommendation from global herbs rep.
My vet recommends cortaflex but it didnt do anything for my horse. He is on superflex which seems to be the only one that has done anything - he isnt bothered by the taste and he can smell medicine in his food at 10 paces.
I have recently but him on pernamax which according to my vet is the best joint supplement - it smells really bad so not sude if your mare will like it. I have to crush the tablets up as he wont eat them even though they are meant to be "spearmint" flavour. Apparently horses get used to the taste (neither of mine like it though). It is expensive and I am trying it out for the next couple of weeks to see how it goes. The other one you could try is newmarket joint supplement. Cellie on here recommended this to me as she uses it with her TB who has had bone spavin and she says it is really good. It is also on my list of things to try.
I had my horse on Synequin at the vet's recommendation. I've since changed to Riaflex, which actually has a slightly better spec and works out cheaper overall. It's an odourless white powder too and never had any problems with getting my boy to eat it.