I'll pass on what I was taught in equine nutrition classes regarding
feeding cider vinegar and/or oranges to horses to change the gut pH.
Please note that I'm PASSING ON the info, this isn't my own opinion, so
if anyone has heard/feels differently, fine by me.
Cider vinegar is an extremely dilute form of acetic acid, and oranges
contain an even weaker citric acid. Off the top of my head, I can't
remember the exact pH, but it is a very weak acid, probably around maybe
pH of 4.0. So you add a cup of this to the horse's food and that cup of
weak acid is mixed into the stomach contents, along with a gallon or so
of other foodstuffs, water, etc., diluting it even more. In the
stomach, part of normal digestion is the addition of hydrochloric acid,
a far stronger acid than oranges or cider vinegar, which helps break
down food particles and make nutrients available for absorption. The pH
in the stomach is around 2, which is 1000 times more acidic than is the
cider vinegar you just fed. Adding a bit of weak acid to an already
strongly acidic environment is like throwing a bucket of salt water into
the ocean and saying you just made the ocean salty. It just isn't going
to change that environment very much.
On top of that, after food is churned around in the stomach for awhile,
it all dumps into the small intestine, still strongly acidic. If it
stayed that acidic, the walls of the gut would be irritated, so the
liver excretes bile (containing among other things, bicarbonate) which
is alkaline and neutralizes all that acid back to a pH of more-or-less
neutral 7. That tiny bit of acid you just fed your horse is easily
neutralized along with the hydrochloric acid of the stomach and by the
time that load of ingesta makes it back to the hindgut where all those
enteroliths are, the pH is not very different than it ever is. Maybe a
little, but not much. In order to start dissolving the enteroliths, the
pH in the gut has to drop below 6.5 and one cup of vinegar or an
orange or two isn't going to do that.
A third factor is the capacity of the horse's digestive tract. The
stomach is relatively small and holds about a gallon at a time. But by
the time you've gotten back to the GI tract, the total capacity of the
small intestince, cecum and hindgut is around 45 gallons or so. Simply
out of sheer capacity, how much of a change in pH are you getting by
adding a few oranges or a cup or two of a weak (and now
pretty much neutralized) acid?
So, the bottom line from what I was taught was that feeding a cup of
vinegar (or a few oranges) isn't going to help very much with dissolving
enterolith formations. HOWEVER, there is some evidence, not yet
quantified or really substantiated but there nevertheless, that feeding
2 or 3 cups may have some effect if fed on a daily basis, or for three
weeks on, two weeks off, etc.
A much more effective management tool is to discourage their formation
in the first place by not over-feeding alfalfa and high protein feeds.
(the nitrogen contributes to ammonia, a constiuent of enteroliths)(plus
California-grown alfalfa is seven times higher in magnesium than in
other parts of the country, and there are more instances of enteroliths
in California than anywhere else), and not over-feeding high phosphorus
feeds such as wheat or rice bran. If you can also avoid hard water,
that's great, but hard to do sometimes if that's what you have, and hard
water doesn't contribute as much to alkalinity as alfalfa does, anyway.
OK, here's the good news. Most endurance horses are not fed straight
alfalfa anymore and many get a pretty good amount of grain a fair
amount of the time. Switching from alfalfa to grass hays (at least in
part) and increasing the grain-to-hay ratio decreases the pH of the gut
(makes it more acidic) all by itself and therefore may contribute to
dissolving the enteroliths. Nifty, huh? So, instead of feeding oranges
and/or vinegar, probably the best thing you can do for your horse to
avoid/dissolve enteroliths is get him off of an all-alfalfa diet and
increase the gran-to-hay ratio whenever possible (ie if you have a hard
worker that needs more calories, provide those calories in the form of
grain, not more hay). If you suspect your horse may already have
enteroliths, then feeding vinegar may help at least some, but in that
case, feed 2 or 3 cups on a daily basis.
Hope this helps.
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