There are many aspects of donkey-keeping that depend on having an
estimate of what the animal weighs. Feeding recommendations are usually
based on a percentage of the animal's weight; if you overestimate
the weight, you'll feed too much, and the animal will tend to "expand"
to meet that "target" weight! Worming dose syringes are calibrated
by the weight of the animal---underestimating weight can lead to using
too-small doses of wormer, which aren't effective. Overestimating
the animal's weight, hence over-dosing, is less of a problem here,
as most wormers have a wide safety margin; the exception seems to
be Quest, which requires fairly careful dosing. If the vet needs to
give a tranquilizer, or other medication, he's going to do it based
on weight. How much the animal can carry (pack or rider) or pull (cart)
is also expressed as a percentage of the animal's weight.
The most accurate way to weigh an equine is on a livestock scale.
However, most of us don't have one at home, and we rarely take our
animals for an outing to the auction yard. If your vet has a scale,
great---have him weigh your animals next time they're in for something
routine. Note that, for minis, your small animal vet's dog scale might
be adequate.
Another option is to do "differential weighing". You put the animal
in his trailer, drive to a nearby truck scale (e.g. at a gravel quarry,
moving company, or feed store), weigh the loaded trailer and truck,
take the animal back home, then go back and weigh the empty trailer
and truck. (You can't weigh just the trailer's axle, as a part of
the trailer's weight, and the weight of its contents, will be transferred
onto the truck, via the hitch.) In theory, the difference between
the two weighings is the weight of the animal. In practice, most truck
scales are calibrated in fairly wide increments---if you weigh your
truck several times (with the same contents each time), there might
easily be 100 lb of difference in the weights. So, a single reading
with a donkey on board, then a single reading empty, could make for
a "worst case" error of 200 lb when you subtract the two numbers!
And, of course, there are the problems with things that can change
between two scale readings---gas you burned taking the animal home
(or that you added at the gas station), the extra person (or dog,
or groceries) that rode with you on only one trip, etc.
Small donkeys can be weighed at home, using ordinary bathroom scales.
This is done like big trucks are weighed---one "axle" at a time. I
tie Shadow up, standing on a firm, flat surface (concrete is best,
but a big sheet of plywood, or stall mat over packed gravel will do).
I then ask her to lift one front hoof; I place the scale where that
hoof was, and put the hoof down on the scale, fairly near the center.
I then ask her to lift the other front hoof, and I put it down next
to the first one, on the scale. I tell her to stand, while I read
the scale---the needle usually wiggles around as she does, so be prepared
to "average" the maximum and minimum weights that it swings to. WRITE
these numbers down, before you forget them! I then reverse the process
to help her get both hooves back to the ground safely, take the scale
around to her hind hooves, and have her let me put those hooves on
the scale. Now, add the front and back weights together.
This morning, Shadow weighed about 270 lb in front, and 220 lb behind,
for a total of 490 lb. Obviously, there's a limit on the accuracy
of this technique---my "barn" scale only goes to 260 lb, so I was
over-taxing it by putting her on it. And, there's the possibility
that the animal will lean toward or away from the scale, biasing both
measurements. So, I took our "good" bathroom scale (which has a capacity
of 300 lb) to the barn, and "weighed" Shadow again, this time with
her front hooves on the "house" scale, and her hinds on the "barn"
scale. Fortunately, my husband came out to watch, as it required two
people to read the two scales simultaneously---Shadow was shifting
her weight forward and back, wiggling slightly. This technique produced
weights of 270 lb in front and 200 lb behind, for a total of 470 lb.
Over the years, various researchers have attempted to generate formulas
that calculate approximate equine weights from measurements of animals'
bodies. The most familiar version is the horse weight tape seen in
many catalogs, in which the animal's heart girth is translated to
weights. I have a weight tape sold by Purina; it claims that Shadow
weighs between 380 and 410 lb. In some scientific trials, such tapes
have been found to be "wildly inaccurate" [ Donkey Digest
] when used on donkeys. This is not surprising, as the formulas are
developed by "fitting" their parameters to data from a particular
group of animals that have been weighed and measured, and donkeys
are built differently than horses.
There have been several formulas developed for calculating donkey
weights. All of them use a measurement of the animal's girth, called
"heart girth", which is taken just behind the withers and the elbow;
it is essentially the smallest circumference that you can find on
the donkey's chest. However, this measurement can vary somewhat; the
preferred measurement is taken when the animal is standing in a relaxed
posture, with its neck about level with the rest of the back. Morning
measurements (before breakfast) are considered to be the most representative.
The tape should be pulled up somewhat tightly---snug enough to dent
the skin a little (so you're not measuring fluffy fur), but not so
tight as to provoke a rodeo. When measured for this article, Shadow's
heart girth was 50". Various formulas use other body measurements,
which will be discussed with each formula. It is suggested that measurements
be taken on both sides of the animal, with the animal standing squarely.
The measurements will vary somewhat, depending on how the donkey is
standing, whether it bends its neck to watch what you're doing, and
how precisely you can locate the various anatomical parts.
Animal Traction
This book is a reprint of a Peace Corps manual. It gives a formula,
which they say is good for all equines, although they illustrate it
with a donkey. Basically, they're approximating the donkey as a cylinder
the size of the body, and adding some for the head. Units are in the
English system (girth and length in inches, weight in pounds):
weight = 50 + girth x girth x length / 300
The body length they use is along the animal's side, from the point
of shoulder to the point of rump. Shadow measured 47" on one side,
and 48" on the other, probably because point-of-rump can be hard to
locate exactly. This formula puts her weight at 442 or 450 lb.
Donkey Business
The first edition of this Australian book had obvious misprints in
their weight formulas (they gave both English and metric versions).
As near as I can tell, what they meant to say was:
weight = girth x girth x length / 10800 (Metric---cm, kg)
weight = girth x girth x length / 300 (English---in, lb)
However, the length measurement that they use is along the backbone
from withers to tail (point of dock). I found this measurement to
be difficult to take---one has to hold one end of the tape on the
"cross" at the withers, and feel with the other hand, to determine
where the tail actually attaches to the body (which caused Shadow
to squirm, a lot!). My measurements varied from 42" to 46", which
would give weights of 350 to 383 lb.
Donkey Business II
The second edition of this book gave only the metric formula, which
had changed slightly. (For this formula, and all that follow, I have
derived the approximately equivalent formula in the English measurement
system, for the benefit of those who are "metrically challenged".)
weight = girth x girth x length / 11000 (Metric units)
weight = girth x girth x length / 305 (English units)
However, they have also changed their definition of body length; it
is now point of shoulder to point of rump---the same measurement used
by the Animal Traction formula. Shadow's lengths of
47" or 48" produce weights of 385 or 393 lb.
Donkey Business III
The third edition of this book changed the formula again:
weight = girth x girth x length / 12000 (Metric)
weight = girth x girth x length / 333 (English)
Now, body length is defined to be from the point of shoulder to the
point of dock, i.e. going over the point of rump and wrapping part
way around the donkey's bottom, to the middle of the tail. Interesting,
I found this measurement easier to reproduce, since the middle of
the tail is easier to locate precisely than the point-of-rump spot
(especially on a "round" donkey like Shadow). Shadow's measurement
was 53" on both sides, for a calculated weight of 398 lb.
Donkey Digest
This article was a summary of an Australian scientific paper, which
apparently evaluated several formulas. Unfortunately, the summary
only mentioned the most "successful" formula, which is not an easy
one to compute. I used a scientific calculator with a button for A**B
(A raised to the power B), but the calculation can also be done with
a slide rule, or tables of logarithms.
weight = (girth ** 1.77) x length / 3000 (Metric)
weight = (girth ** 1.77) x length / 103 (English)
Here, length is measured from the point of elbow (on the back edge
of the front leg) to point of rump. That distance is 42" on Shadow,
which gives a weight of 415 lb. The article comments that this formula
predicted the weight of 62.3% of the donkeys studied within 5% of
true weight, and predicted the weight of 95% of the donkeys studied
within 12% of true weight.
The Professional Handbook of the Donkey
This book, put out by the British Donkey Sanctuary, gives two rather
complicated formulas---one for adult donkeys, and one for youngstock
(under 2 years old). The metric versions are:
adult weight = 0.000252 x ( height ** 0.240 ) x ( girth ** 2.575 )
young weight = 0.000283 x ( girth ** 2.778 )
these translate into the inch/lb system as:
adult weight = 0.00765 x ( height ** 0.240 ) x ( girth ** 2.575 )
young weight = 0.00830 x ( girth ** 2.778 )
Note that these formulas do away with the difficult-to-take body length
measurements. Instead, they use height, which is "by the stick"---taken
with the animal standing squarely, on a firm, flat surface, and leveling
across from the donkey's withers to a measuring stick, held vertically.
Shadow is 46" tall, so her calculated adult weight is 455 lb, and
young weight is 435 lb.
It's
fairly clear that none of these formulas closely predicts Shadow's
current weight. Part of the problem is that they all rely on heart-girth
measurements, which do not change much as the animal gets fatter.
In going back thru my files, I found notes from 1984-85, when I
was both weighing Shadow (the one-bathroom-scale routine), and measuring
her girths (both heart-girth and rib-girth---around her tummy at
the widest part of her ribs) 
fairly regularly. There were days, back then, on which her heart
girth was also 50", but her rib girth was only 56", and the scale
said she weighed 375 lb. Her rib girth is now 62"---that's where
she has put her fat (as well as in some chubby "cheeks" on her rump,
and a "padded" neck), and the formulas don't measure those places!
(The scale weights do show this effect---her 1984 numbers were front=245
+ hind=130; current numbers are front=270 + hind=200. She has clearly
added more mass in her ass....)
However, these formulas do provide rough estimates of donkey weights,
probably closer than one could get by guessing. The 3 formulas from
the various editions of Donkey Business came reasonably close to what
Shadow used to weigh, when she was in good condition. These formulas
would be useful to estimate how much to feed, because one way to diet
a fat donkey (or to plump up a skinny one) is to feed it as if it
weighs the amount that you want it to! These formulas are also usable
for calculating how much the donkey could carry---the load should
be under 25% of the animal's "athletic" weight. Extra blubber does
not increase the animal's ability to carry weight!
REFERENCES:
Watson, Peter. 1981. Animal Traction . Summit, NJ. Artisan
Publications. No ISBN. Page 10.
Berry, Christine, and Jinny Robinson. 1980. Donkey Business
: A Guide for Raising and Training Donkeys. Published by the authors,
Woodville, 2321, N.S.W. Australia. No ISBN. Page 57.
Berry, Christine, Jinny Robinson, and Jo-Anne Kokas. 1989. Donkey
Business II : A Guide for Raising, Training, and Showing Donkeys.
Sheaves Park Donkey Stud, Duns Creek, Maitland, N.S.W. Australia.
ISBN 0-7316-6088-9. Pages 12, 98.
Berry, Christine, and Jo-Anne Kokas. 1998. Donkey Business III
: A Guide for Raising, Training, Managing, and Showing Donkeys. Duns
Creek, N.S.W. 2320 Australia. ISBN 0-9586152-0-9. Pages 142-143.
Neale, Sue. "Predicted bodyweight of donkeys", Donkey Digest
: Magazine of the Donkey Society of Australia, April 1991 ISSN 1031-6280.
Pages 14-16.
Svendsen, Elisabeth D, editor. 1997. The Professional Handbook
of the Donkey , Third Edition. Whittet Books, London. ISBN
1-873580-37-1. Page 380.
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