If I look at the map, I’m in an area that is theoretically high in Se. In reality, most of the agricultural land around here has been so heavily used for so long that most of the minerals have been leached out of the soil.
I have one horse who has the same odd metabolism as msj’s horses–I have to supplement the heck out of him to keep his Se levels at anything like an acceptable level. I blood test him twice a year ($80 and three hundred phone calls to the vet’s office to get the results as they can never quite seem to get why this is so important to me) to make sure I’m not over- or under- cooking him. Since I’ve been supplementing fairly vigorously, he’s not tied up (crosses fingers, throws salt over shoulder…)