| The cows on the farm she's on have suffered from it before, apparently the soil is lacking in this important element. |
That most likely means the soil is very high in iron. Iron also depletes zinc which is another crucial element for the immune system.
The soil my area of the SE United States is very high in iron and I am more than happy to blame it as the reason for two of my horses developing metabolic issues after we moved here.
Until last year, I bought my hay only five miles away. Tests always showed it to be high in iron, low in copper and zinc
It might be worth your time and $$ to consult with an equine nutritionist at a university closest to you as my guess is your vet will want to "up the anty" on steroids.
Vets do the best they can and don't always have the finite answers to each and every issue they come upon. We often have to grab hold of those reins and take the lead
My thought is nothing will resolve itself until you get to the real root cause; if it does turn out to be the soil, then a nutritionist could help you figure out the supplements and quantities your mare needs to have to keep her healthy.
Yes, please keep updating