THE RELATIONSHIP BETWEEN BODY WEIGHT
AND ENERGY REQUIREMENTS IN HORSES
J.D. Pagan and H.F. H i n t z
Cornel1 University, Ithaca, New York 14853
Introduction
The National Research Council (NRC 1978) defines a horse's maintenance
energy requirement t o be Digestible Energy (kcal/day) = 155 W . 7 5 where W
equals weight i n kilograms. T h i s equation incorporates W * " or "metabolic
body size" t o describe the relationship between body weight and energy
requirements i n horses. The use of the three-fourths power of body weight i s
based on studies done by Brody and Procter (1932) and Kleiber (1932) in which
each independently evaluated the relationship between fasting heat production
and body weight i n a large number of species ranging i n s i z e from a mouse t o
a cow. Both researchers reported t h a t the mass exponent which best described
the interspecies heat production-body weight relationship was close t o .75.
Their experiments, however, did not determine the relationship between energy
requirements and body weight within a given species. Despite this, the
three-fourths power o f weight has been adopted as an intraspecies metabolic
body size. Blaxter (1972) and Thonney e t a1 (1976) showed t h a t while W a 7 '
adequately describes the heat production-body weight re1 ationship between a
number of species, i t is an inappropriate mass exponent w i t h i n some species.
Blaxter (1972) stated that between mature individuals within a species,
metabolism varies w i t h a higher power of weight than .75; possibly .90.
Materials and Methods
In order t o investigate the relationship between energy requirements and
body weight i n mature equids, we conducted a s e r i e s of experiments i n which
the resting maintenance energy requirements of equids varying greatly in body
s i z e were measured. Four male animals weighing 125 k g . , 206 kg., 500 kg.,
and 856 kg. were used. Each animal was a t l e a s t ten years o l d . The horses
were f e d three d i f f e r e n t levels of intake of a ration and the net amount of
energy retained a t each level was determined. The i n d i v i d u a l ' s resting
maintenance requirement was then determined by regression analysis of energy
intake against energy balance. The level of energy intake t h a t would r e s u l t
in zero energy balance was considered t o be the animal's maintenance energy
requirement. The animals were fed a pelleted ration t h a t consisted of 75%
a l f a l f a meal and 25% oats. The horses were fed each level o f intake for a
two week adjustment period followed by a f i v e day digestion t r i a l i n which
total feces and urine were collected. After each digestion t r i a l , the daily
heat production of the animals was measured indirectly. The animals were fed
a t twelve hour intervals i n the morning and the evening. Thirty minutes
a f t e r the morning meal, oxygen consumption and carbon dioxide production were
measured f o r a t h i r t y minute period. Thirty minute measurements were taken
each hour f o r the next eleven hours so t h a t a total of twelve t h i r t y minute
measurements were taken between meals. Oxygen consumption and carbon dioxide
production were measured u s i n g a face mask open circuit respiration
calorimetry system. Total daily oxygen consumed and carbon dioxide produced
were calculated a s four times the amount consumed o r produced d u r i n g the six
t o t a l hours o f measurement. Values from the calorimetry measurements and
urinary nitrogen excretions were used t o calculate heat production (HP) u s i n g
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methods described by Brody (1945). Gross energy of feed, feces, and urine
was measured by combusting samples i n a bomb calorimeter. Fermentative gas
losses from the g u t were estimated with Wolin fermentation balances (Wolin
1960) u s i n g cecal VFA r a t i o s measured in three cecal f i s t u l a t e d ponies fed
the experimental ration. Fermentation substrate was assumed t o be neutral
detergent f i b e r and this f i b e r was further assumed t o consist of hexose units.
Methane losses averaged about 4.6% of digestible energy intake. Net energy
retention was determined by deducting energy losses in feces, urine, methane,
and total heat production from gross energy intake.
Results and Discussion
The amount of net energy retained a t each level of metabolizable energy
i n t a k e for the f o u r animals i s shown i n figure 1 . Energy retention increased
l i n e a r l y w i t h increased metabolizable energy intake i n each o f the animals,
so t h a t the amount of metabolizable energy needed by each animal t o m a i n t a i n
zero energy balance can be calculated w i t h l i n e a r regression. A common
method used t o present energy balance data i n animals of the same species i s
t o a d j u s t the data t o a "metabolic body size" basis by dividing t h e energy
retention and energy intake values by W . 7 5 . The data from figure 1 are presented i n t h i s fashion i n figure 2. A t any level of metabolizable energy
intake the smaller animals appear t o retain more energy than the larger
animals. There are e s s e n t i a l l y four parallel s e t s of data points p o s i t i o n e d
according to body weight. Linear regression o f these data r e s u l t s i n the
equation Y = -41.94 + .4&(X) w i t h an R 2 value o f .71. When the data are
expressed on a body weight basis rather than on a metabolic body size basis
(figure 3 ) , a higher correlation between energy intake and energy retention
is found. Linear regression o f the data expressed on a body weight basis
results i n the equation Y = -10.48 + .505(X) with an R 2 value of .93. The
best relationship between energy intake and energy balance i n these four
animals of greatly d i f f e r e n t body weights occurs when the data are divided by
body weight raised t o the power .88 (figure 4 ) . Linear regression of these
data gives the equation Y = -26.46 t .60(X) w i t h an R2 value o f .97. The
slope of this l i n e indicates t h a t the efficiency of u t i l i z a t i o n o f the
metabolizable energy from this d i e t f o r g a i n i s 60%. A similar regression of
energy balance on digesTfble energy intake shows t h a t the efficiency of
u t i l i z a t i o n o f digestible energy for gain i s 55%.
While weight raised t o the power .88 r e s u l t s i n the best relationship
between energy intake and energy balance i n these animals w i t h greatly
d i f f e r e n t body weights, the amount o f energy required t o maintain zero energy
balance (resting energy requirement) can be d i r e c t l y related t o body weight.
Figure 5 shows the relationship between resting maintenance DE and ME intakes
and body weight i n the four animals. Both regressions have high R2 values,
.999 and .997 respectively. Benedict (1938) reported data from Ritzman
(figure 6 ) which showed a similar relationship between fasting heat product i o n and body weight i n horses of greatly different body size.
The r e s u l t s of this study suggest t h a t equid's maintenance energy
requirements vary l i n e a r l y w i t h body weight and not w i t h W * 7 5 . The
maintenance energy requirements found i n this study, however, a r e somewhat
theoretical in nature. Maintenance requirements under practical management
conditions will almost certainly be higher because of increased a c t i v i t y and
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different environmental conditions. Further studies are needed to evaluate
the effect of these factors on the maintenance energy requirement of the
horse so that a practical set o f requirements can be formulated. Furthermore,
these feeding standards should be expressed on a body weight basis rather
than as a function of metabolic body size.
Literature Cited
Benedict, F.G.
No. 503.
1938.
Vital Energetics. Carnegie Institute Washington Pub.
Blaxter, K.L. 1972. Fasting Metabolism and the Energy Required by Animals
for Maintenance. In: Festsdrift til Knut Breirem. Mariendals Boktrykkeri ,
Gjovik.
Brody, S. 1945.
Bioenergetics and Growth. Reinhold Pub. Co., New York.
Brody, S‘. and Procter, R.C. 1932. Growth and Development with Special
Reference to Domestic Animals XXIII. Relation Between Basal Metabolism
and Mature Body Weight in Different Species of Mammals and Birds. Univ.
of Missouri Agr. Exp. Sta. Res. Bull. 166, pp. 89-101.
K1 e ber,
M.
1932.
Body Size and Metabolism. Hilgardia 6:315.
NRC. 1978. Nutrient Requirements of Horses. National Academy of Sciences.
Fourth revision. Washington, DC.
Thonney, M.L., Touchberry, R.W., Goodrich, R.D. and Meiske, J.C. 1976.
Intraspecies Relationship Between Fasting Heat Production and Body
Weight: A Reevaluation o f W O ’ ~ . J . Anim. Sci. Vol. 43, No. 3, pp. 692-704.
Wolin, M.J.
40: 1452.
1960.
A Theoretical Rumen Fermentation Balance. J. Dairy Sci.
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a
a
b
Q
c
W
-
I
0
M o t m b o l l r ~ b l oEnergy I n t a k e ( K o m l l d a y )
Figure 1. Relationship between net energy retention and
metabolizable energy intake in f o u r horses.
a--
e
30--
20
--
10
--
+
Y= -41.94 .48 X
FI2=.71
0
e 1 2 5 Kg
A 206 Kg
0500Kg
0856 K g
lo--
Figure 2 .
Energy balances expressed on a "metabolic body size"
basis ( W * 7 5 ) .
-1 7 1-
-
*
a
0
0
9
-a
s
2
u
Y
Y
0
0
c
Y= -10.48+
0
R2= .93
a
a
rn
n
0
c
0
c
.So5X
@12S K q
A 206 Kg
OWKg
0 856 Kg
W
t
-*-.
F i g u r e 3.
I
I
I1
1
I
I
I
r
Energy balances expressed on a body w e i g h t b a s i s
(W1.Oo).
21
14
Y=-26.48+.6OX
7
RQ .97
0
a 1 2 5 Kg
A 208 K g
O500Kg
0858 Kg
-7
30
I
40
I
I
50
6t
60
M e t a b o 1 I z a b I e En e r g y I n t a k e ( K c a I I k g.'*
F i g u r e 4.
Energy balances expressed on a
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(We")
I
I
70
00
I day 1
basis.
0
.
I
a
a'
0
c
Ly
I
B o d y W o l g h t (kg)
Figure 5. Linear regressions of maintenance digestible energy (DE)
and metabolizable energy (ME) on body weight [Bl) in
four horses.
c
a
0
t
Figure 6.
Body Weight (kg)
Linear regression of fasting heat production on body
weight in four horses (Adapted from Benedict, 1938)
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