Selinium requirements

[Jim Mitchell <navion@lightspeed.net>]

I can't seem to find agreement on the amount of selinium a horse needs and what is 
toxic. Below I have summarized information from posts to this group and some books.
   I have calculated the required and toxic amounts below for a 900 lb horse eating 20 
lb.s of food per day.

Heidi Smith DVM says 8 to 10 mg/day is correct and 60 to 90 mg/day will cause chronic 
toxicity

Susan Garlinghouse says 2mg/kg of diet which is 16 mg/day, almost twice Heidi's for my 
horse.  Susan says toxic is 3.3mg/kg of body weight which is over 1200mg/day

Duncan Fletcher quotes NRC which matches Susan's recommended amount.

Courtney Hart's new book says .11mg/100 lb of body weight or 1 mg/day recommended
   Courtney states .22 can be low level toxic or 2mg/day can be cause chronic toxicity. 
This value is 1/5 to 1/10 what Susan & Heidi recommend

Horse Owner's Vet Handbook (Giffin & Gore) pg373 states .1mg/kg of diet or .83mg/day 
recommended for my horse. 5mg/kg is toxic or 41 mg/day is toxic

The UCD Book of Horses states .3ppm in the diet is recommended which calcs out to 2.5 
mg/day for my horse. It says "relatively small excesses may be toxic"

   Anyway you can see that what some people recommend is considered toxic by others. The 
UCD book is new and I have heard the NRC values may be outdated?  Any comments?

Jim Mitchell