I agree with getting her checked for selenium deficiency, because that can have very devastating consequences as I just recently met a mare who had had it. She was extremely unhealthy and unsound, and eventually had to be euthanized because of it. Not trying to alarm you, but get her checked. I also have another theory, though. My dachshunds do the same thing, and I had them chcked and tested and everything, to no avail. Then someone (a biologist) told me that there are myceliums (sp?) in the soil- a type of fungus- that they have developed a taste for, kind of like truffles. No idea if this could apply to horses, but just throwing it out there!