Evaluation of Metabolizable

Energy for Poultry

By Pran Vohra
Department of Avian Sciences, University of
California, Davis, California 95616

M ANY authors have reviewed the subject of energy concepts for

poultry nutrition (Titus, 1961; Lockhart et al., 1963; Vohra,
1966; Kurnick, 1967; Kohler and Kuzmicky, 1970). Currently,
metabolizable energy values are popular among the consulting
nutritionists for use in computer calculations for least cost formulations.
No doubt, metabolizable energy (ME) is easier to determine and a
more practical measure than net energy or Fraps’ productive energy
which are measured by the energy stored up as fat and protein in a
growing and fattening animal. We are really interested in the efficiency
of utilization of metabolizable energy by the animal. In an ideal
measure the energy of the diet should equal the sum of the energy
values of its constituents. Gross energy is the only such measure but
has no practical applicability because not all of this energy is available
to the animal. The partition of gross energy (GE) into its various
components is outlined in Figure 1.
As feces and urine are voided together by poultry, digestible energy
(DE) cannot be measured without surgically altering the digestive
tract. For the same reason, the determination of digestibility of
proteins is equally difficult but has been done by a number of workers
(Fraps, 1944; Eckman et al., 1949; Titus, 1957; Rolton, 1957; Kabota
and Morimoto, 1965; Vogt and Stute, 1970), who have used any
of the following methods for this purpose:
(a) Total collection and separation of various urinary constituents.
(b) Separate collection of feces and urine by surgically altering the
animals.
(c) Insert indicator i n the diet and determination of urinary
constituents.
By its very concept, metabolizable energy is “the energy available
for anabolism (the building of body substance, milk or eggs) and for
katabolimi (the heat production of animals) and requires no correction
for nitrogen balance” (Kleiber, 1961). Thus:
ME-GE, = GE,
where GE, - Gross energy of the dietary intake
GEe = Gross energy of the equivalent excreta
-
If the metabolizable energy value is corrected for nitrogen balance,
then it should be termed as ME,, (NRC, 1966). The term nitrogen
204
 205
FlGURE 1
The partition of gross energy of food
Food intake
(Gross Energy GEI)
I
(1)
I
Digestible energy
I
(2) Fecal energy
(W ( FE)
(a) Food origin
(b) Metabolic origin
I
I I
( I ) Urinary energy (2) Metabolizable energy
WE) (ME) or (MEn) if corrected
(a) Food origin for nitrogen balance)
(b) Endogenous origin

I
(1) Net energy (2) Heat increment
(NEm t P) (HI)

(1) Mamtenake energy (2) ProductiAn energq

(N Ern) WEP)
(a) Basal metabolism (BM) (A) Energy of work (NE,+)
(b) Voluntary activity (VA) (B) Energy of storage
(c) Heat to keep body warm (HBW) (a) Growth
(d) Heat to keep body cool (HBC) (b) Fattening
(c) Eggs
(d) Feathers
balance (NB) is measured by subtracting the nitrogen in excreta (N,)
from nitrogen in feed intake (N ).
ME* = Me - NB
GE, - GE,&(N,-N,)
Again: M E =: HP+NE,=(Heat production) (Net energy of +
production)
Correction for Nitrogen Balance
It is doubtful the ME, is a better measure than ME because a
correction should also be made for the loss of nitrogen in the daily
shedding of scales and feathers, and for deposition of nitrogen in the
eggs of laying birds (Foster, 1968). The nitrogen retention is very
high during the early life but is comparatively low in adult birds
(Lockhart et nl., 1963). The correction for nitrogen balance varies
with age, the genetic make up of the birds, species as well as breeds
(Sibbald and Slinger, 1963b).
The use of correction for nitrogen balance has been questioned by
many investigators (Baldini, 1961 ; Sibbald and Slinger, 1962; Foster,
1968). In many cases, the nitrogen corrected metabolizable energy
value is directly proportional to non-corrected value (Proudman et al.,
1970). In general, the energy contribution due to nitrogen balance
is less than 5 per cent (Shannon and Brown, 1970) and nitrogen correc-
tion tends to underestimate the ME value of protein-rich feedstuffs and
over-estimate the energy-rich sources (Hartfiel et al., 1970).
206

If metabolizable energy of the diets after necessary correction for

nitrogen balance were equal to the sum of the ME, values of the
ingredients, there would have been some justification for carrying out
this correction. Unfortunately, this relationship holds true only when
a limited number of ingredients are used for formulating diets. These
include corn (maize), soybean meal, and fish meal. Under these con-
ditions the same relationship holds true for ME without nitrogen
correction.
If all the nitrogen were excreted as uric acid which has a gross
energy of 8.22 kcal per gram nitrogen, this value should be used for
correction for nitrogen balance (Hill and Anderson, 1958). A more
accurate value appears to be 8.73 kcal/gm. N, the gross energy of an
average sample of urinary nitrogen (Titus et al., 1959). Actually, the
diet has an influence on the distribution of nitrogenous products in the
urine and the following values have been reported as percentage of
total urinary nitrogen: Uric acid N, 60-81; allantoin N, 3.8-(?);
other purines, 9-20; ammonia N, 5-6-17.3; Urea N, 4.5-10.4; creatine-
creatinine N, 0.2-8; amino acid N, 1.7-10; other N, 1.2-2.8 (O’Dell
et al., 1960).
Determination of Metabolizable Energy
Before the metabolizable energy of a diet can be determined some
method is needed to quantitatively relate the excreta to the food
consumed. This can be done by:
(1) Determination of total food intake and quantitative collection
of excreta.
(2) Use of an inert indicator in the diet which should not be
absorbed or altered during its passage through the gut thus eliminating
the need for total collection of excrement.
In the total collection method, the diet is fed to a group of five to
ten birds comfortably housed in battery cages for a preliminary period
of three days followed by a test period of four days during which the
excrement is collected twice a day and dried in a suitable oven. The
feed wastage is reduced to a minimum and any scattered food is collected
and weighed to determine the exact food intake.
The excreta samples are mixed and ground. The difficulties of
drying of excreta to prevent any loss of nitrogen and energy have also
been studied (Manoukas et al., 1964; Shannon and Brown, 1969).
From the gross energy of the diet and the excreta, metabolizable energy
of the diet is calculated (Fraps et al., 1940; Shannon and Brown, 1969,
1970).
A number of indicators have been employed to determine the ratio
between the feed and the equivalent excrement from it. These include
Cr203 (Edin, 1918); ferric oxide (Bergheim, 1926); barium sulfate
(Whitson et al., 1943); silica (Gallup, 1929); lignin (Kane et al., 1950);
and crude fiber (Almquist and Halloran, 1971). The most commonly
used indicator is CrzO3. Neither Cr203 nor Bas04 are completely
inert indicators (Vohra and Kratzer, 1967).
 207

Problem with BaSQ4: Barium sulfate is a good indicator from

the standpoint of having no electrostatic properties and mixing very
easily with the diet for a uniform distribution. Its chemical deter-
mination is difficult, however, because it involves fusion and extraction
(Whitson et al., 1943). Recent reports describe a method to overcome
this difficulty and needs further investigation (Dick, 1967 ; Figueroa
et al., 1968).
Problems with cr203: Chromic oxide presents the problem of a
uniform distribution in the diet because of its electrostatic properties.
To partially ovcrcoine this difficulty, it is mixed with wheat flour and
water to form a dough containing 30 per cent Cr203. The dough is
dried in thin layers and ground to a fine powder (Kane et al., 1950)
but the procedure does not overcome the electrostatic properties
entirely.
The next difficulty is in the chemical determination of Cr203.
Any error in this determination is further reflected in the ME calcu-
lations. Before an assay can be carried out, CnO i must be brought
into solution.
One of the earlier methods is the fusion of the ashed sample with
sodium peroxide (Schurch et al., 1950). This has been replaced by
subjecting the sample to wet digestion with a mixture containing 10 gm.
sodium molybdate in 150 ml. distilled water to which 150 ml. conc.
H2S04 has been added slowly while cooling followed by 200 ml.
perchloric acid (70-72 per cent) and has proved very valuable (Bolin
et a/., 1952). The Cr203 concentration is then determined colori-
metrically (Bolin et al., 1952; Mill and Anderson, 1958). The details of
the digestion have been further modified (Czarnocki et a/., 1961) to
prevent a reversal of yellow to green color.
The main difficulty in this procedure is to get a stable uniform
color for reading at wavelengths of 430 mp (Hill and Anderson, 1958)
or 444 mp (Czarnocki et al., 1961).
The standard curve is prepared by digesting known quantities of
Crz03 but different color readings are obtained if Cr203 was initially
mixed with a protein or a carbohydrate source (Vohra, unpublished
data). It is very difficult to reproduce the results from one laboratory
to another. The accuracy may possibly be improved by determining
the Cr content of the solution using an atomic absorption method
because the color of the solution will have no effect. The difficulty of
Cr203 analysis is illustrated in Table 1 where the same materials were
analyzed in three different laboratories and the results were highly
variable (Halloran, 1971). Since the rest of the calculations depend
upon the Cr203 analysis, the reliability of ME values from this sort
of data is questioned.
Another difficulty is encountered in grinding of samples to prevent
separation of Cr203 from excreta or from high sugar reference diets.
The latter can be mixed with dry ice to prevent overheating of the
grinding equipment. The diets tend to become sticky otherwise.
208
TABLE 1
The values for Cr203 in the same samples in three different laboratories.*

Lab #I 1 Lab82 I Lab #3

Sample % % %
_- ~

Diets
DA . . . . . .
DB . . . . . .
DC . . . . . .
... 1

'.'
...
I
I
i
0.53
0.46
0.46
1 0.53
0.53
0.53
0.37
0.31
0.38
I
DD . . . . . . ... 0.47 0.53 0.31
I

DE . . . . . . ... 0.54 I 0.54 0.38

1
DF . . . . . . ... 0.45
I
0.54 0.36
Excreta I
1

EA . . . . . . ... ,
~

1.30 2.05 1.32

I
EB . . . . . . ... 0.80 1.35 0.89
i
EC . . . . . . ... I 0.80 I 1.30 0.86
ED . . . . . . ... 1 0.8 1 ~ 1.35 I 0.88
I
EE . . . . . . ". i 0.75 1.35 0.86
I
EF ...... ... 1 0.91 1.55 0.98
* Halloran, 1971.
The Choice of Reference Diets
The metabolizable energy value of an ingredient should be indepen-
dent of the other dietary components. A number of investigators
have found that the utilization of the energy components of the diet
varied with the nature of the test diet, namely a semipurified diet (Hill
and Anderson, 1958) or a practical diet (Sibbald and Slinger, 1963a)
The composition of these two diets is given in Table 2.
In general, the chicks are raised on the reference diet to about
two weeks of age and then divided into groups on the basis of body
weights, equalizing both mean and weight distribution among the
groups. The reference diet and the test diets contain 0.3 per cent
Cr203. Glucose or the whole diet is substituted by the test substances
at a level of 20 per cent and these test diets are fed to duplicate groups
for a period of two to four weeks. Excrements over the last consecutive
four days are collected twice daily, dried, mixed together and ground.
The diets and excreta are analyzed for gross energy and Cr203 and the
metabolizable energy of the diets is calculated as follows:
ME (kcal/gm.yof feed=Gross energy/gm. feed-
Cr203 gm. feed
x gross energy (kcal/gm.) excreta
Cr2Os/gm. excreta
 209
TABLE 2
Composition of the diets used in ME determination.
1 Reference diet
(per 100 gm.) ~(Sibb!%&ddi&ger,
(Hill and Anderson, 1 1963a)
1958) I
I
~

Glucose. . . . . . . . . . . . 44.1 I -
I
~

Ground wheat ... ...I 9.0 35.0

Soybean oil meal (44 per cent) I 17.5 -

-
Stablized fat . . . . . . . . . 1 2.5 5.0
Ground limestone ...... I 2.0 -
Dicalciumphosphate . . . . . . j 1 .o -
Salt iodized . . . . . . . . . 0.5 -
Mineral mixture . . . . . . ~
i-3

1 The mineral mixture supplied in mg./100 gin diet: K2HP04, 220; MgS04, 120;
MnS04, 30; FeS04 .7H20, 30; CuSO4 .5I-I20,0+3.
2 The vitamins mg./100 gm. diet: thiamin, 0.3; riboflavin, 0.4; calcium panto-
thenate, 1.0; pyridoxine, 0.5; niacin, 2.6 folacin, 0.07; mendaione, 0.09; biotin,
0.01; vitamin 8 1 2 , 0.001; choline chloride, 130; vitamin A, 1,000 U.S.P.;
vitamin D3, 100 I.C.U.; vitamin E, 2.2.
3 Mg./100 gm. diet: CaCO3, 1,000; Ca2HP04, 1,040; NaCJ, 240; MnS04, H20,12;
ZnC03, 6.5; Cr203, 300.
4 Per 100 gm. diet (in mg.): riboflavin, 0.22; calcium pantothenate, 0.44; niacin, 1.11;
menadione, 0.23; vitamin B12, 0.001 ; choline chloride, 22.77; vitamin A, 500 1.U.;
vitamin D3, 80 I.C.U.; DL-niethionine, 50; 3-nitro-4-hydroxyarsinilic acid, 5.

Logically, a carbohydrate test material should be substituted for

glucose and a protein for some protein source in the reference diet.
Substituting the test substance for a portion of the total test diet in
order to keep the levels of minerals and vitamins constant also appears
to be more desirable (Sibbald and Slinger, 1963a).
However, the ME values obtained by the use of these two reference
diets may differ by as much as 600 kcal/kg. as in the case of rape seed
meal where average metabolizable energy values of 1,320 kcal and
1,698 kcal/kg. were obtained (Rao and Clandinin, 1970). The accuracy
of existing M E values for fats based on substitution of fats for glucose
has also been questioned (Cullen et al,. 1962); (Sibbald and Slinger,
1963b).
Levels of Test Substance
What should be the level of substitution of test substance in the
reference diets during ME determination? Substances like alfalfa meal
are seldom added to poultry diets at levels of 20 per cent, the level
210

commonly used in ME determination. The rationale is that a sub-

sitution level of 5 per cent gives highly unreliable results. Actually,
the level of inclusion makes a lot difference on the determined values.
For alfalfa, values of 1.08, 1.88, 1.06 and 0.23 kcal/gm. dry matter
metabolizable energy were obtained at inclusion levels of 10, 20, 30
and 40 per cent, respectively (Vohra and Kratzer, 1970). When rape
seed was substituted for glucose in the reference diet at levels of 10,
20 and 30 per cent, the ME values were 1.63, 1.53 and 1.00 kcal/gm.,
respectively. The corresponding values on a practical type of diet
were 1-84, 1.68 and 1.59 kcal/gm. (Rao and Clandinin, 1970).

Effect of Age, Breeds and Species on ME

It has been suggested that ME, value of an ingredient is not
influenced by the age of the birds within a single strain (Sibbald et al.,
1960; Hill, 1965). However, a number of exceptions to this generali-
zation are known. The following values for ME,, (kcal/gm.) have been
reported for chicks and laying hens, respectively: fish meal, 3.06, 3.57:
alfalfa meal 1.0, 1.49 (Hoshii et al., 1970); overheated soybean meal,
2.02, 2.55 (Hill and Renner, 1963); rape seed meal, 1.20, 1.78 (Lodhi
et a/., 1969). An increase in ME, value from 1.13 to 1.32 lccal/gm.
was observed for rape seed meal when the test period was extended
from 14 to 42 days (Rao and Clandinin, 1970).
The ME values may also be influenced by the breeds of chickens.
U p to an age of four weeks, White Leghorn chicks derived about 2.5 per
cent more ME per gram food than White Rock chicks, but no differences
due to sex were observed (Sibbald and Slinger, 1963b). Again,
White Leghorn chicks metabolized significantly more energy from a
high energy diet than a fast growing India River x White Rock chicken
(Slinger et al., 1964). In another report, the ME value of a diet was
found to be lower for Brown Leghorn or Rhode Island Red breeds
than for Light Sussex or White Leghorn breeds (Foster, 1968).
A few reports on the effects of species on the ME determination
are also available. It appears that chickens metabolized significantly
more energy per unit feed than the turkeys from a high energy diet
while the turkeys had the advantage from a low energy diet (Slinger
et a]., 1964). The ME, values of a number of ingredients for chickens
and Coturnix, respectively, were reported to be as follows (kcal/gm.) :
yellow corn, 3.48, 3.31 ; soybean meal (44 per cent), 2.28, 1.91 ; fish meal
(63 per cent), 3.06, 2.71 petroleum yeast, 3.27, 3.02; wheat bran, 2.25,
1.73; defiiited rice bran, 1.71, 1.11 (Hoshii, et al., 1970).

Indirect Methods for ME Determination

A number of attempts have been made to derive the metabolizable
energy value of a feedstuff from its proximate composition utilizing the
published information on the digestability of the nutrients and the gross
energy of the nutrient. The information on the metabolizable energy
of digestible nutrients which has been most widely used is giben in
 21 1

Table 3 (Titus, 1957). The metabolizable energy can be then calcu-

lated easily as follows:
ME=( % Protein x digestibility coeff. x ME/gm. of digestable pro-
tein)
-( % of ether extract x digestibility coeff. x ME/gm. of
digestible ether extract)+( % NFE x digestibility coeff. x
ME/gm. of digestible NFE)&( "/,Fiber x digestibility coeff. x
ME/gm. of digestible fiber).
Other relationships which have been used for calculating metaboli-
zable energy are as follows:
( I ) M E (kcal/kg. diet at 10% moisture)=53+38X where X=
crudeprotein+2*25x% ether extract+ 1.1 x % starch+% sugar.
This expression of Carpenter and Clegg (1956) gives reliable data
for glucose, corn, milo, oats, rye, sesame meal, soybean meal and wheat,
and is in agreement with the values determined biologically (Vohra
1966). It could be used for guidance purposes.
(2) (Sibbald et al., 1963):
ME (kcal/gm. dry matter)=0.059+3.8 X where X=(l.l x
starch+sugarf-crude protein-12.25 x ether extract)/dry matter

TABLE 3

Metabolizable energy of digestable nutrients (kcal/gm. digestible nutrient).

Class of feedstuff Nitrogen-free extracl

Grain and most other seeds ... ... .. ... 4.2

Legume seeds and rice products ... ... ... 4.0
Meat and fish byproducts ... ... ... ... 3.9
Legume leaves and stems ... ... ... ... 3.8
Milk byproducts ... ... ... ... ... 3.1
Ether extract
Animal fats ... ... ... .. . 9.49
Meat and fish byproducts ... ... ... .. , 9.33
Corn oil ... ... ... ... .._ 9.28
Milk byproducts ... ... ... ... ... 9.25
Grains and other seeds .. . ... ... ... 9.11
Animal products Criirle protein
Milk . .. ... . .. ... ... _.. 4.40
Casein, eggs .. . ... ... ... ... 4.35
Fish, meat ... ... ... ... ... 4.25
Gelatin ... ... ... ... ... ... 3.30
Cereals
Corn, sorghum ... .. . ... ... ... 4.40
Wheat bean ... ... ... ... ... 4.20
Rice . .. ... . .. ... ... ... 4.10
Barley, oats, millet, rye, wheat ... ... 4.00
Legumes
Beans . .. ... .. . ... ... ... 4.30
Soybeans ... ... .. . ... ... ... 3.90
Alfalfa . .. .. . ... ... ... ... 365
Peanuts . .. .. . . .. ... ... ... 3.60
Others . . ... . .. . .. . .. ... ._. 3.40
212

(3) (Kabota and Morimoto, 1965):

ME=3.23 TDN+57.1
=9.48 DE+ 127.6
where TDN=total digestible nutrient
(4) Bolton (1967):
M E (kcal/kg. diet)=40.81 [0.87 (crude proteinS2.25 x ether
extract +available carbohydrate)+ K]
where K=4.9 for adult birds and 2.5 for young chicks.
A lot of information has been furnished on the ME value of
ingredients using this formula in Bolton’s book.
(5) Fraps et al., (1940):
ME-4.2 (% digestible protein + % digestible fat x 2.25 $- %
digestible NFE)
Other relationships are also suggested by other authors (Janssen
and Terpstra, 1971).
Any factors which influence the digestibility of the nutrients have
an eldect on subsequent ME determination.

Digestibility Data
As feces and urine are voided together it is very difficult to get
accurate information on the digestibility of nutriznts especially proteins
for poultry. Various attempts have been made to determine digestib-
ility directly using total collection techniques (Pryor and Connor,
1966), inert indicator method (Olsen and Kihlen, 1948; Eckman et al.,
1949) or by separate collection of urine and feces by surgically altering
the animals (Laerdal et al., 1957; Newberne et al., 1957; Q’Dell et a].,
1960; Richardson et al., 1968).
This type of information is accurate only if the ingredients do not
contain tannins (Kratzer et a]., 1967) certain types of complex poly-
saccharides (Bornstein et al., 1965; Kratzer et al., 1967), gossypol like
substances (Hill and Totsuka, 1964), antitrypsin (Brambila ~t al.,
1961) and other interfering substances (Lodhi et al., 1970). These
substances influence the digestibility of the rest of the nutrients of the
diets and lead to erroneous results for metabolizable energy.

CONCLUSIONS
The direct method of metabolizable energy determination is as
useful and accurate a measure as the direct determination of metaboli-
zable energy for formulation of poultry diets. The indirect methods of
calculation are handy in correlating the metabolizable energy to the
proximate analysis of the feed ingredient.
Metabolizable energy values of dietary ingredients are strictly
for guidance purposes and are not biological constants because the
metabolizable energy values of the mixed diets is equal to the sum of the
metabolizable energy values of its constituents only for a limited number
of ingredients used in diet formulations.
 213

ResumC
EVALUATION DE L’ENERGIE METABOLISABLE POUR
LES VOLAILLES
Pran Vohra
La mtthode indirecte de dktermination de l’bnergie mttabolisable
est aussi utile et aussi exacte que sa ditermination directe dans la
formulation des rations pour volailles. Les mCthodes indirectes de
calcul sont commodes en mettant en corrtlation 1’Cnergie mitabolisable
avec I’analyse approximative des composants de l’aliment.
Les valeurs d’Cnergie mttabolisable des composants des rations ont
un but strictement indicatif et ne sont pas des constantes biologiques,
car 1’Cnergie mktabolisable d’une ration “melangte” n’est Cgale B la
soinme des valeurs biologiques de ses constituants que pour un nombre
limit6 d’ingrkdients utilisCs dans la formulation des rations.
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