Trail food and fluid shifts

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

KIMBERLY PRICE wrote:
My question to add is I
> understand the horse’s gut slows considerably as blood is draw away to
> carry oxygen etc to the working muscles.  I know it is good to let the
> horse get some roughfage during the ride, to help keep the guts moving,
> but I wonder if you would have to be careful about giving him too many
> small meals during this period of work?


Forgive me for leaping into this one---this is an area that I have a
particular interest in, so I'm going to jump up and down on this one for
awhile (look out, Susan's on a roll this week <g>).  Small, frequent
meals are EXCELLENT during an endurance ride (or any stressful and
dehydrating event), and MUCH preferable to only letting the horse eat
considerably larger meals only at checks.  The reason is because when a
single, large meal is eaten (such as is the case in horses only fed
twice a day, or to a slightly lesser extent, endurance horses that go
for miles without a bite, then eat a big meal of grain/mash and hay at
the checks and lunch), there are large fluid shifts that occur that move
fluid out of the blood (which is where you want it) and into the lumen
(interior) of the digestive system.  First, you get a big influx in the
form of saliva, and the more chewing and dryness that needs to be
overcome in the feed (such as dry hay cubes), the more saliva is
produced.  In 350 lb ponies, they measured saliva production of around
8-10 LITERS in an hour (that's around four or five Pepsi bottles of
fluid being removed from the blood).  Then the feed hits the stomach and
yet more moisture is added from various gastric juices.  Pancreatic
secretions alone were measured at .5 liters/100 kg of body weight, so in
an average 900 lb endurance horse, you can estimate a total of around
22-24 liters (around 6 gallons) of fluid that's been removed directly
from plasma volume.  That's a BIG change in plasma volume here, folks. 
In the research ponies, they measured plasma volume reductions of around
15-24%---and that was in ponies that were just standing around.  Now
imagine those kinds of fluid changes occurring in endurance horses on a
hot day, sweating out yet more fluid and in all likelihood not replacing
all the lost fluids through drinking (other research has shown that
endurance horses can or will only replace through voluntary drinking 2/3
of the fluids lost during the ride, and this was in "good" drinkers). 
So we're talking some major amounts of fluids that are being removed
from the system, and either lost as sweat, or being shunted into the
digestive system---where it IS eventually recycled back into the 
plasma, but not for several hours, which can make a big difference to an
endurance horse.  Not to mention that several hours later, just as the
fluid balance is getting back to normal, that's just in time to get into
the NEXT vet check and time for another big meal so we can shift those
fluids right back out of the plasma again and back into the digestive
system.

Just so as to cover this completely (then I promise I'll shut up), there
are additional fluid shifts that go on once the food gets back into the
hindgut.  The lining of the intestine will go into two phases, first a
secretory, followed eventually by an absorptive phase.  In the secretory
phase, about .5 liters per hour will flow from the plasma volume into
the gut.  In ponies, the total fluid shift was about 6 liters, so you
could expect it to be larger in horses.  This shift takes place within
about 6-8 hours after eating the meal, so keep in mind that even though
you may be done with the race, fluid is still being removed from the
horse's system, even though temporarily.  This shift is followed by an
absorptive shift, when the fluid goes back into the plasma volume, but
except in 100-milers, this part is going to occur after the event. 
Anyway, the fluid shifts in the hindgut aren't as extreme or as
immediate as the shifts occurring in the foregut right after a big meal,
but they're still something to consider.

OK, so here's the nifty thing, now that I've got everyone in a big
lather.  Obviously, if you want to keep your horse rolling down the
trail and avoiding all the problems that go along with dehydration
(which is exactly what this is, a metabolic dehydration), all you have
to do is just not feed your horse big, episodic meals during a
competition.  Notice I didn't say don't feed your horse, I said not
"big" or "episodic".  Pretty simple, huh?  When they did all this
research on fluid shifts (and there were dozens of studies on the
various aspects), NONE of these extreme fluid shifts occurred when the
horses ate small meals every two hours or so throughout the day.  In
other words, if instead of feeding your horse two pounds of grain at
lunch, you instead packed it in a couple of baggies in your fanny pack
and handed it out here and there throughout the day, you'll still be
supplying just as many calories, electrolytes, whatever, but you WON'T
get these big fluid shifts that are going to dehydrate your horse (and
remember all this has NOTHING to do with whether or not your horse is
drinking alot at water stops---the important thing is not how much water
the horse has inside of him, it's WHERE the water is inside the horse
that makes a difference).

By the way, this is also why I'm always (and Heidi and lots of other
people) jumping up and down about feeding nice, sloppy mashes at checks,
or anywhere else you can, soaking your beet pulp and wetting down the
hay.  The more a horse can slurp and less he has to chew, the less
saliva he has to produce, and the less fluid that gets removed from the
plasma volume.  Horses should still get all the hay they can chow down,
because you also need the gut fill to maintain motility, but making a
big effort to keep the fluids where they will do the most good is going
to REALLY pay off in performance.

Anyway, this is why I think Karen's idea of feeding cookies, or anyone
else training their horse to grab a bite of d**n near anything on the
run is a TERRIFIC idea.

And as usual, I rambled on forever---as you can tell, fluid shifts are a
big deal, so I thought it appropriate given the discussion to jump up
and down on it for awhile.  Hope someone gets something useful out of
it.

Susan Garlinghouse