I live in a notoriously low selenium area in the Great Lakes. My neighbor milked around 120 head of Holsteins for decades and several years ago, suddenly started having calf after calf born weak that only survived for a day or so. He finally grabbed one right after birth, put it in the cab of his pick up and drove it straight to Michigan State. It died shortly after arrival, but the diagnosis was WMD. Had his custom supplement mix tested, that was supposed to have selenium in it, and voila, no selenium of measurable usefulness. Different custom mix company and no more problems.
I also have had vets who routinely give a selenium/vit E injection to the foal as part of the foal check after birth. May be something to discuss with your vet.
Dawn Wagstaff and Tipperary
Saline, MI 2003