Re: Selenium requirements

[Susan Evans Garlinghouse <suendavid@worldnet.att.net>]

Jim Mitchell wrote:
> 
> Hi Susan,
>   If Duncan's post is correct, that NRC recommends .9mg/day for a supplement and not to
> exceed 16mg/day then I think everybody is in pretty close agreement except maybe for
> Heidi. (See Duncan's table) 

I think it's important to differentiate between "supplement" and
"requirements".  Duncan's numbers of .9 mg/day are based actually on an
NRC recommendation of .1 mg/kg of total ration.  So .9 mg/day would be
fine for a 800 pound horse eating 19.8 pounds of feed per day (or 2.5%
of his body weight on a daily basis, which is fairly normal).  For a
1200 pound horse, it's a little light, but still probably okay.  For a
mini horse (granted, not common in endurance), it's about two and a
quarter times higher than is required, which may or may not cause
problems.  Also, feeding 16 mg/day as an absolute value would be
excessive for an 800 pound horse, but okay for a 1500 pound draft
horse.  Again, don't calculate by absolute values, calculate by the NRC
recommended amount PER KG OF RATION.  As I discussed in my previous
post, people strongly tend to overfeed mineral supplements and alot of
owners will go with "if .9 mg per day is good, two or three times that
must be better", and that's when you get into trouble (and not just with
selenium).  So I much strongly prefer not to talk in absolute values,
but to recommend amounts based on either body weight, or mg per kg of
feed, which then allows for the wide variations between horses.  Common
mistake.

Back to the difference between "supplement" and "requirement".  It's
important that nobody confuse these numbers.  The NRC recommendations of
.1 mg/kg of total ration is the TOTAL requirement of Se per day.  That's
not the amount that needs to be SUPPLEMENTED on top of whatever Se is
contained in the feed.  That's why I recommend that owners should be at
least reasonably aware of what part of the country their hay is coming
from---hay from the NW may need some minor supplementation.  Here in the
SW, it's unlikely that any horse needs extra Se, unless specifically on
the advice of their vet for a very specific reason.



I also appreciated Duncan's explanation of LD50 which puts
> your high dosage in better perspective. 

Yes, I did state 3.3 mg/kg of body weight as being toxic, rather than as
being only 50% lethal according to the LD50 data.  And yes, in a group
of horses fed at this level, 50% of those horses will probably survive. 
Personally, I'm not willing to play those odds, are you?  As far as I'm
concerned, only a 50% chance of survival IS toxic and/or lethal for all
intents and purposes outside the laboratory. Especially in light of
significant research that has indicated that supplementation OVER .1
mg/kg of diet does not provide additional benefits. LD50 trials
generally mean the animal dies in a fairly rapid period of time---it
doesn't take into account toxic affects that may happen two or three
months down the road, which would be likely for the hoof deformities
common in Se toxicity.  So yes, an explanation of LD50 levels are
enlightening, but it's more important to apply some practical common
sense to the application of that data.  No flames here, by the way.


So as I mentioned above, the
> recommended values are all very close, basically between 1 and 2.5 mg/day.  I was aware
> of the interrelationship between Se and other minerals and feeds. I also know that
> different horses and different exercise schedules should require different amounts being
> used and discarded by the horse.

Something else to keep in mind is that all of the above numbers are
based on ORAL selenium.  Selenium is absorbed differently when fed as
opposed to injectable, which is sometimes available through catalogs or
feed stores.  A horse owner that happened to have a bottle of injectable
E-Se and injected a nice big whack of it could be causing toxicity
problems.  The only person who should EVER provide Se in an injectable
form should be a DVM.

One more point.  Ruminants (cows and sheep) don't utilize Se as well, so
feeds designed for them can sometimes have much higher levels than are
recommended for horses.  Pigs require 10 mg/kg of feed (10 times that of
horses), so you also have to be careful about feeding rations formulated
for pigs to horses.  There is also a relationship between copper and
selenium, so a horse that for some reason had high serum copper levels
is even more likely to have problems with selenium.


Susan Garlinghouse