Even access to a salt block may not be enough. Ask your vet the same questions you asked here, and also have your pasture checked by the local agent. Smartpak has many different sorts of supplements and experienced personnel to help you decide if your horse needs one or more. TC also sells a mineral supplement you could check at your local feed store. As for the hay: what sort of “grass”? There are probably millions of different grasses, some more nutritious than others. Alfalfa is awfully rich, and not all horses can handle it. Generally, I prefer a good timothy/clover mix, but if I cannot get it, I feed timothy hay cubes once daily, amount determined on the individual horse (mare is fat as a tick, gelding looks like a toothpick, etc.). There are a lot of hoof care products that may help, and elsewhere on this forum are several discussions about the merits of these – you might find them interesting and helpful.
If you did not plant your pasture, and aren’t quite sure what is growing in it, there is no way to guess whether or not it is adequate for her needs without testing.
Keep us posted on your progress!
It is never the horse's fault