Hi Joyce
All the physiology books say that average resting heart rate is 44 bpm, with a
"normal" range from 23-70. However, that's all horses, all breeds, all degrees
of fitness, and all ages, including newborns. The 23 bpm is actually almost freakishly
low, I think the record belonged to Phar Lap, who at necropsy was found to have
the largest equine heart ever recorded. So much for trivia.
Since a heart's job is to (obviously) push blood, thereby delivering oxygen
(among other things) to the muscles (among other places), a lower heart rate
indicates that either the heart is larger and therefore more blood is being
pushed per heartbeat, and/or that the body is efficient enough to require
less blood, oxygen, etc. than a horse with a higher resting heart rate.
Resting heart rate is actually one of the parameters used to predict performance
potential---obviously a horse whose RESTING heart rate is 65-70 bpm is going
to have a really hard time meeting criteria at a vet check, and would be an
incredibly poor endurance prospect! I would think anything below 40 would be
acceptable in an unfit horse, although the lower the better (assuming no
cardiac problems that cause an abnormally low HR).
As your mare's fitness increases, her entire aerobic system will become more
efficient, in effect lowering her "miles per gallon". As she becomes more
"fuel efficient", she'll need less blood pumped to perform the same amount
of work as before, so yes, her heart rate will drop, though maybe not more
than 3 or 4 beats per minute. Don't be heartbroken if she only reaches 32!
Also, young horses do tend to have a higher heart rate, though I'm not sure
exactly when they reach "adult" resting heart rate. Normal heart rate for
a newborn is 70-80 bpm, and my six-month-old weanling (who is as healthy
as...well, a horse) has a HR of 52 bpm.
Anyway, physiologically, your mare is doing just fine. In my opinion (and
it's just my opinion), I would think a resting heart rate under 34 is just peachy
for a mid-condition young endurance prospect and anything you get beyond that
is just gravy. Or, just assure everyone it's entirely due to your amazing,
intuitive and even hauntingy omnipotent grasp of exercise physiology and
you modestly take full credit for all of it. What the hell, it's worth a shot
and I'll never tell:->
Good luck and happy trails...
Susan Evans
(Equine Research Center
California State Polytechnic University)