Since I have always lived in the dry parts of the
NW my experience is probably different than other's.
I've found that an unconditioned/young/unfit horse
will sweat foamy, salty, thick and sticky sweat. It drys crusty and salty
on the skin/hair.
As the horse gets in better shape the sweat begins
to get cleaner and more watery. A really fit horse's sweat hardly
tastes salty at all, and leaves almost no residue on the hair, just
wet.
Perhaps it's just my horses, or the weather here,
or something, but I think, after many years horseback, that some
horse's systems do adapt to conserve some e-lytes.
Perhaps it's in some families of horses to be more
adaptable to hard work.
And yes I believe most hard working horses should
have some e-lyte supplementation. I also know that many horses have
been ridden hard day after day in hot weather for many seasons without anything
except free choice salt without any ill effects.
I've done some 100s years ago without any
e-lytes.
Probably wouldn't do it now, they are too easy to
use.
(And no, I'm not planning on any hundreds
anymore, and the horse I'm riding now doesn't seem to need much in the way
of e-lytes either, but we're not asking much of her.)
Gayle Ecker, who has been studing electrolyte loss in endurance
horses since the late 80's and probably has looked at over 5000 endurance
horse's blood told me one time that even in training horses can loose
significant electrolytes and they don't acclimate to that loss - that is they
don't get better at not losing them.