If, as it seems, you are saying that she has this all over her body and not just in the girth area, then it would be a good idea to have your vet determine exactly what is causing it. Might not be the girth at all. If you are bathing her monthly, that might be causing dry skin. Horses really don’t need a bath that often. Grooming, yes – daily; but a complete bath too often can do more harm than good. It could also be caused by some dietary deficiency. In fact, the possible causes are legion, which is why a vet check should be the first thing to do. No sense in randomly guessing, and spending money for things that won’t address the problem.
It is never the horse's fault