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Hiya,
Every time I teach an Outreach class, I get this same question, and I
think people saying you shouldn't give a hot horse water comes from alot
of people just remembering the "rule" without understanding the
physiology behind it.
When cool/cold water hits a horse's stomach, that water has to be warmed
up to body temperature by circulating blood. If a horse is working
hard, as in during an endurance or conditioning ride, then there's alot
of demand for blood going on all throughout the body---the gut needs it
to maintain motility, the muscles need it for work output, the skin
needs it for cooling, the lungs need it for gas exchange. In other
words, virtually every system in the body is working in high gear, and
all those systems are competing for blood. If a gallon or so of cold
water hits the stomach, then an additional demand for blood has been
added.
However, it doesn't take more than few minutes for the water to be
warmed up, and I don't have to go into exhaustive detail about what
beneficial things water is going to do for a horse that's hot, sweating,
dehydrated, etc. If the horse wants water, it's because he needs water
and the worst thing you can do is deprive him at any time except under
very special circumstances. However, what you need to do is be aware of
how cold water hitting the stomach is going to affect the body (shunt
blood to the stomach away from other parts of the body) and manage it
accordingly with just a minimal amount of common sense.
When the different body parts have to compete for blood, there's a sort
of priority system of who gets cut out of the loop first.
Unfortunately, low man on the totem pole is almost always the gut, which
is why horses that are being overworked and whose muscles are hogging
all the blood tend to start getting colicky---blood is being shunted
away from the gut and it starts to shut down, and voila, colic. So if
you have a hot horse and all of a sudden a new demand for blood is
thrown into the equation (blood is needed to warm up cold water in the
belly), then blood is being shunted away from somewhere else---almost
certainly the gut, possibly muscles, etc to one extent or another.
So the trick is to keep your horse hydrated, but to do everything you
can to keep blood moving everywhere as much as possible, so that no one
part of the system gets deprived and starts getting cranky and causing
problems (read "sick horse").
Keep in mind that especially in fit horses, the heart rate drops very
quickly after cessation of exercise---there are dozens and dozens of
people on this list whose horses heart rates will be at 60 bpm before
they ever get to the water trough and below 50 within a minute or three
after that. OK, that's fine for hitting P & R's quickly, but keep in
mind that for every 1 bpm drop in heart rate, that's almost a liter of
blood LESS PER MINUTE that's being ejected by the heart ventricles to do
its thing throughout the body.
For example, let's say you have two average 900 pound, well-conditioned
horses that both just came into a lunch check, hit criteria promptly,
went and had a nice long drink of cool/coldish water and now have 45
minutes before they go out again. One horse gets tied to a trailer and
stands so that his HR drops to 45 bpm, the other one is kept walking
around so that his HR is maintained at 65 bpm (these are just arbitrary
figures, of course). All other things being equal, the horse that is
still walking around has roughly 20 more liters of blood (about 5.3
gallons, or about 62% of total blood volume) being circulated throughout
the body PER MINUTE than the horse that is just standing. Which means
that not only is that cold water in the tummy being warmed up more
quickly so that it can be absorbed and utilized, in addition there's
much more blood keeping the gut moving (that's why you walk a colicky
horse), more blood moving waste products away from the muscles (and btw,
the still-contracting muscles are also acting as a pump to remove waste
products from the muscle cells faster) and more blood moving through the
skin surface capillaries to remove heat from the body and reduce the
core temperature, which is what you really need to be concerned about.
Yes, it is true that it's a little harder for horses to eat while
they're walking around---but most endurance horses figure out pretty
quickly that this is going to be a long day, and are willing to munch
hay out of your hand while they're being led around. Or you can stop
and let them slurp up a few mouthfuls of mash for a minute or two while
you're sponging him down, then get them moving again. Although it might
seem that logically letting a horse just stand will let him recover
faster, in reality the entire body will stay looser, cool faster and the
gut stay more motile if he keeps exercising lightly during the rest
period. This has been pretty well substantiated through numerous
studies that looked at various physiological functions after strenuous
exercise both in horses and humans (anyone that ever ran track, remember
how your coach always made you keep walking around for a few minutes
after the race instead of collapsing on the ground? Felt better after
walking, didn't you?)
Anyway, back to water. The whole point is to let horses have just as
much water as they want at every opportunity but DON'T let them just
stand there afterwards---keep them moving so the blood can warm that
water up. If he's really hot and the water really cold, then try not to
let too much cold water hit the belly all at once, try to let him have a
few good swallows, walk around for a minute or two, then a few more
swallows. And if you're in the middle of a ride and your horse just
tanked up, reduce your exercise intensity (as in walk) for a few minutes
afterwards while the blood is busy warming that water up---in other
words, reduce the muscle's demands for blood for a few minutes while the
blood is busy off doing other things. Once the tummy has warmed up the
water, then fine, off you go and the horse is all the better for having
rehydrated himself.
As usual, a long tirade for a simple concept. Hope this answers your
question.
Happy trails,
Susan Evans Garlinghouse
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