Susan will probably drop in on this. It is toxic but is required as part
of a normal diet, a good book should tell you what it's for. To
demonstrate this point my father made the following illustration when I
was but a wee whipper snapper. I had asked about why too much of this
that or the other can kill you. He fill a small basin with about four
inches of water and said, "Well Nicco, do you think this water can kill
you?" I looked at it, and thought that while it might fill my stomach it
would not hurt and said so. He said well now stick your head in there
face down and keep it there. The penny dropped. By definition too much
of anything, or even a little used incorrectly, is bad for you.
Selnium content in hay in some areas of the country is high and others
too low (particularly if a piece of land has been farmed for a long
time). As an aside I read somewhere that Custer's horses suffered from
selenium poisoning because they were not used to the forage in the
region that they were persuing their target.
-- Nicco Murphy, San Diego County 'Open the bay door, please, Hal'