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Jennifer, your description of the selenium-deficient areas is just a little
off. :-)  West of the Rockies can be highly variable, anything from very
deficient to toxic levels.  The Pacific Northwest from N Calif and up tends
to be very deficient, most of the SW region is "adequate" and a few toxic
areas here and there (you won't see hay being grown there).

Yes, there are specific areas throughout the Midwest that can range from
adequate to toxic.  Local extension agents will have a handle on those,
toxic areas are really an issue for range cattle, not for hay growing areas.
It's hard to generalize about selenium content in the Midwest regions, as
one field can contain a toxic level of Se, while the field right next door
is barely adequate or even deficient.

Other notoriously deficient areas are the Great Lakes regions, the NE, along
much of the eastern seaboard (variable) and yes, Howard, Florida too.
Florida is basically six inches of sand on top of a coral reef, so
supplementing horses in that region is a good idea.

As Angie commented, selenium is definitely not something you should
supplement willy-nilly, and yes, it absolutely can cause toxicity issues.
However, an individual horse's requirements vary according to the amount of
exercise, other oxidation issues present (rancid feeds or fats, for
example), individual variation and so on.  There are alot of distance horses
that need to be supplemented with 5-10 times the 1-mg-per-day dose
recommended in some of the older literature.  If you have a cumulative
deficiency brewing, you'll usually see it as inappropriate muscle soreness
even when the horse is being ridden within his conditioning capacity,
possibly other signs of muscle pathology such as dark-colored urine.  Keep
in mind that some of those early symptoms can easily be missed with a horse
with more ambition than is good for him.

Regarding toxic levels, you really have to feed a horse a pretty high level
of selenium (say, 20 mg a day) for a long time to cause a toxicity problem,
and assuming the problem is corrected before the feet fall off, the
condition is reversible.  Injectable selenium is much more likely to cause
toxic problems (it can occasionally cause fatal anaphylactic reactions), so
leave the injectables to your vet---not something to try at home on a whim.
Most commercial horse feeds do contain selenium, but at a fairly low level
formulated primarily for horses at maintenance---not endurance horses
working a lot of miles.

Howard, my suggestion to you is to check your current level of selenium your
working horses are getting in their ration, and consider getting the serum
levels checked.  It's a reasonably inexpensive test to run.  Muscle damage
*is* something that can sneak up on you without realizing there's a problem
brewing, and even a recent BC isn't a sure-fire guarantee you don't need to
add some additional Se to correct a deficiency.

JMO. :-)

Susan G



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