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OCR for page 1
NUTRITIONAL ENERGETICS
OF DOMESTIC ANIMALS
AN D G LOSSARY
OF ENERGY TERMS
I ntrocluction
The quantitative nutrient requirements of domestic animals are
complex and change depending upon sex, physiological state, and
a variety of environmental factors. The energetic economy of the
animal is sustained by the catabolism of fuels. The total intake of
food by animals feeding aa' libitum is related to their energy needs
and to the concentration of available fuels in the diet. Many as-
sumptions are required to condense the total energetics of an ani-
mal into the relatively simple tabulations used in practice to quan-
tify the dietary energy requirements of domestic animals and man.
The primary objective of this publication is to outline the most
commonly used systems for the description of energy requirements
in terms of an idealized flow of energy through an animal. To this
end, some classical measures of energy metabolism are defined in a
system of abbreviations. These have been useful to describe the
flow of energy and with suitable modification can be used to de-
scribe the flow of any element through an animal.
It is beyond the scope of this publication to present in detail
methods used to measure metabolic transfers in animal systems.
Rather, the objective is to define energy metabolism terminology
within a general biological framework applicable to all animal
OCR for page 2
2
species. A brief account of conventional schemes for the descrip-
tion of energy metabolism is presented along with the historical
basis for each. Finally, the energy systems in common use within
the United States and Canada for various species of domestic ani-
mals are outlined. The appendix contains a complete list of ab-
breviations and symbols commonly used to describe energy trans-
actions in domestic animals.
Units of Measurement
Energy is an abstraction that can be measured only in its transfor-
mation from one form to another. Thus all of the defined units
to measure energy are equally absolute. The joule has been adopted
by Le Systeme International d'Unites (SI; International System of
Units) and the National Bureau of Standards (U.S.A.) as the pre-
ferred unit for expressing electrical, mechanical, and chemical en-
ergy. The joule is defined in mechanical terms (i.e., the force
needed to accelerate a mass), but can be converted to ergs, watt-
seconds, and calories. The converse is also true.
The joule has replaced the calorie as the unit for energy in nu-
tritional work in some countries. One reason for replacing the
calorie is the acceptance of the joule as the metric measure of
energy by SI (Moore, 1977~. Another reason for replacing the
calorie was some variation in the fourth figure regarding its
exact relationship to the joule, a factor of greater importance to
the physicist than to the nutritionist. The conversion of the calorie
to the joule has now been arbitrarily standardized as ~ cat (calorie) =
4.184 ~ (joule). Nutritional investigators generally standardize their
bomb calorimeters using a thermochemical standard, usually spe-
cially purified benzoic acid whose heat of combustion has been
determined in electrical units and computed in terms of joules/
gram mole.
Joule Alp. The joule is ~ 07 ergs, where 1 erg is the amount of energy
expended in accelerating a mass of ~ g (gram) by ~ cm/s (centi-
meter per second). The international joule is defined as the en-
OCR for page 3
3
ergy liberated by one international ampere flowing through a
resistance of one international ohm in ~ s.
Calorie (cal). The calorie is defined as 4. ~ 84 J. This amount of en-
ergy raises the temperature of ~ g of water from 16.5° to 17.5°C.
In practice, both the joule and the calorie are so small that
nutritionists work with multiple units:
Kilojoule (kJ) and Kilocalorie (kcal) are ~ 03 times greater than the
joule and the calorie, respectively, and
Megajoule (MI) and Megacalorie (Meal) are lo6 times greater than
the joule and the calorie, respectively.
Gross Energy (E) is the energy released as heat when an organic
substance is completely oxidized to carbon dioxide and water.
It is often referred to as "heat of combustion" and generally
measured in an oxygen bomb calorimeter.
Ace tab olic Body Size (W'7 5 ~ is the body weight in kilograms of an
animal raised to the three-fourths power. It is useful in compar-
ing metabolic rates of mature animals of different body sizes.
The exponent .75 is generally used, but other exponents hav
merit and may be more appropriate in some situations.
Biological Basis of Energy Partition
Lavoisier (as cited by Blaxter, 1956) during the eighteenth century
first enunciated the principles of combustion both outside and
within the body. In ~ 894, M. Rubner (as cited by Blaxter, ~ 962)
working with dogs first demonstrated that the fundamental laws
of thermodynamics apply to an intact live animal system. The flow
of energy through an animal as outlined in Figure ~ is an attempt
to reconcile traditional methods of describing energy transactions
in an animal with present knowledge of intermediate steps in the
utilization of dietary nutrients.
OCR for page 4
4
Intake of Energy
in Food (IE)
Total Heat
Production
(HE) I
a. Basal Metabolism (HeE)
b. Voluntary Activity (HjE)
c. Product Formation (HrE) l
| d. Digestion and Absorption (HdE) I
I e. Thermal Regulation (HCE) l
I f. Heat of Fermentation (HfE) I
| 9. Waste Formation and I
Excretion (HWE)
Energy (DE) ~ Fecal Energy
\] Metabolizable
Energy (ME)
1 ~
_ .
l ,
Recovered
Energy (RE)
(useful product)
Gaseous Energy.
(GE)
Waste Energy
a. Urine (UK)
b. Gill (ZE)
c. Surface (SE)
r
a. Tissue (TE)
b. Lactation ( LE)
c. Ovum (Egg) (OK)
d Conceptus (YE)
e. Wool, Hair, Feathers (VE)
Under some circumstances the energy contained could be considered to be a useful product for fuel.
FIGURE 1 The idealized flow of energy through an animal.
1
_ ~
Figure 1 shows dietary energy (IE) passing through two stages,
digestible energy (DE) and metabolizable energy (ME), enroute
from food energy (IE) to retention as some useful product (RE).
Energy is lost in forms other than useful products, such as fecal
energy (FE), gaseous energy (GE), urine (UE), gill excretion (ZE),
and surface or skin secretions (SE), and as heat (HE). The first law
of thermodynamics, or the law of conservation of energy, requires
that lE = FE + GE + UE + ZE ~ SE + HE + RE. Within this frame-
work, each energy fraction can be partitioned on the basis of ori-
gin, metabolic pathway, and other criteria. For example, energy
yielding components in feces are, in part, of food origin (FiE) and,
in part, of metabolic origin (Fm E). The sum of these fractions is
gross energy contained in the feces or FE = FiE + Fin E.
OCR for page 5
5
The methods used for measurement and pitfalls in interpreting
energy exchange in an animal are beyond the scope of this publi-
cation. However' the assumption inherent in all measurements of
energy exchange is steady state equilibrium. In quantitative nu-
trition an animal seldom, if ever, truly reaches a steady equilib-
rium state. Thus, if the time scale of the fluctuations of energy
balance about the equilibrium is greater than the period of obser-
vation, the measurements will be in error. For example, measure-
ment of heat exchange for a period of a few minutes can give a
very precise measure of heat emission at that period in time, but
is not representative of the average rate over 24 h (hour). Simi-
larly, the feces produced voluntarily by an animal during a given
period can be measured very accurately, but feces are a result of
food ingested and metabolic processes that occurred at some time
prior to their excretion. Therefore the flow of energy through the
animal as diagramed in Figure ~ represents a system that can only
be measured approximately over any particular time interval.
Knowledge and understanding of a biological system are required
to determine the time constants and appropriate methods to use
in obtaining the best measurements of energy flux for any par-
ticular application.
Definition of Terms
A number of abbreviations have been used in the past to describe
energy fractions in an animal system. The system of abbreviations
used in Figure ~ to describe the flow of energy has application to
other nutrients as well. The first measurement in a nutritional eval-
uation of energy exchange is defined as gross energy (E). The nu-
tritional fractions typically measured in an animal system are ab-
breviated by a series of uppercase letters as shown in Figure I.
Therefore total intake of food energy is lE, where ~ is the amount
of food consumed and E is the gross energy per unit weight of
food. Similarly, total energy contained in feces is FE, where F is
the amount of feces voided and E is the gross energy per unit
weight of the feces.
OCR for page 6
6
Intake of Food Energy (IE) is the gross energy in the food con-
sumed. {E is the weight of food consumed times the gross
energy of a unit weight of food.
Fecal Energy (FE) is the gross energy in the feces. FE is the weight
of feces times the gross energy of a unit weight of feces. FE can
be partitioned into energy from undigestecl food (FiE) and en-
ergy from compounds of metabolic origin (Fm E).
Apparently Digested Energy (DE)
energy in feces: DE = lE - FE.
is energy in food consumed less
True Digested Energy (TDE) is the intake of energy minus fecal
energy of food origin (FiE = FE - FeE - Fm E) minus heat of
fermentation and digestive gaseous losses: TDE - lE - FE +
Fe E ~ Fm E - HfE - GE.
Gaseous Products of Digestion (GE) includes combustible gases
produced in the digestive tract incident to fermentation of food
by microorganisms. Methane makes up the major proportion of
combustible gas normally produced in both ruminant and non-
~uminant species. Hydrogen, carbon monoxide, acetone, ethane,
and hydrogen sulfide are produced in trace amounts and can
reach significant levels under certain dietary conditions. Present
knowledge indicates that energy lost as methane in ruminants
and nonruminant herbivores is quantitatively the most signifi-
cant GE loss.
Urinary Energy (UK) is the total gross energy in urine. It includes
energy from nonutilized absorbed compounds from the food
(UiE), end products of metabolic processes (Um E), and end
products of endogenous origin (UeE).
Metabolizablle Energy (ME) is the energy in the food less energy
lost in feces, urine, and combustible gas: ME = lE - (FE +
UE + GE).
OCR for page 7
7
N-Corrected MetaboZizable Energy (Mn E) is ME adjusted for total
nitrogen retained or lost from body tissue: Mn E = ME - (k X TN).
For birds or monogastric mammals, gaseous energy is usually not
considered. The correction for mammals is generally k = 7.45 kcal
per grain of nitrogen retained in body tissue (TN). The factor
of 8.22 kcal per gram of TN is used for birds representing the
energy equivalent of uric acid per gram of nitrogen. A number
of different values for k have been suggested and used (see
species sections).
True Metabolizable Energy (TME) is the intake of true digestible
energy minus urine energy of food origin: TME = TDE -
UE + UeE.
Total Heat Production (HE) is the energy lost from an animal sys-
tem in a form other than as a combustible compound. Heat
production may be measured by either direct or indirect calo-
rimetry. In direct calorimetry, heat production is measured
directly by physical methods, whereas indirect calorimetry
involves some indirect measure of heat such as the measurement
of oxygen uptake and carbon dioxide production using the ther-
mal equivalent of oxygen based upon respiratory quotient (RQ)
and theoretical considerations. The commonly accepted equa-
tion for indirect computation of heat production from respira-
tory exchange is HE (kcal) = 3.866 (liters O2 ~ ~ ~ .200 (liters
CO2) - I.431 (g UN) - 0.518 (liters CH4) (Brouwer, 19651.
Heat production may also be measured by difference from the
determination of total carbon and nitrogen balance or from a
comparative slaughter experiment. These methods arrive at total
heat production by different calculations and are subject to
systematic errors of measurement.
Basal Metabolism (He E) reflects the need to sustain the life pro-
cesses of an animal in the fasting arid resting state. This energy
is used to maintain vital cellular activity, respiration, and blood
circulation and is referred to as the basal metabolic rate (BMR).
OCR for page 8
8
For the measurement of BMR the animal must be in a thermo-
neutral environment; a postabsorptive state; resting, but con-
scious; in quiescence; and in sexual repose. It is difficult to
determine when ruminants reach the postabsorptive state, but
a common criterion is the absence of methane production. The
length of the fasting period should be specified. A common
benchmark of fasting metabolism is when the respiratory
quotient becomes equivalent to the catabolism of fat or near
0.7. This has been achieved experimentally in 48 to 144 h after
the last meal.
Heat of Activity (HjE) is the heat production resulting from mus-
cular activity required in, for example, getting up, standing,
moving about to obtain food, grazing, drinking, and lying down.
Heat of Digestion and Absorption (H<3 E) is the heat produced as a
result of the action of digestive enzymes on the food within the
digestive tract and the heat produced by the digestive tract in
moving digesta through the tract as well as in moving absorbed
nutrients through the wall of the digestive tract.
Heat of Fermentation (HfE) is the heat produced in the digestive
tract as a result of microbial action. In ruminants, HfE is a ma-
jor component often included in the heat of digestion (H~ E).
Heat of Product Formation (HrE) is the heat produced in associa-
tion with the metabolic processes of product formation from
absorbed metabolites. in its simplest form, HrE is the heat
produced by a biosynthetic pathway.
Heat of ThermalRegulation (HcE) is the additional heat needed to
maintain body temperature when environmental temperature
drops below the zone of thermal neutrality, or it is the addi-
tional heat produced as the result of an animal's efforts to
maintain body temperature when environmental temperature
goes above the zone of thermal neutrality.
OCR for page 9
9
Heat of Waste Formation and Excretion (Hw E) is the additional
heat production associated with the synthesis and excretion of
waste products. For example, synthesis of urea from ammonia is
an energy costly process in mammalian species and results in a
measurable increase in total heat production.
Heat Ir~cremer~t (HiE) is the increase in heat production following
consumption of food by an animal in a thermoneutral environ-
ment. Included in HiE are heat of fermentation (HfE) and en-
ergy expenditure in the digestive process (Hd E) as well as heat
produced as a result of nutrient metabolism (Hr E + Hw E). Heat
increment is usually considered to be a nonusefu] energy loss,
but under conditions of cold stress HiE helps to maintain body
temperature.
Recoverer! Energy (RE), commonly called Energy Balance, is that
portion of the feed energy retained as part of the body or voided
as a useful product. In animals raised for meat, RE = TE, whereas
in a lactating animal, RE is the sum of tissue energy, lactation
energy, and energy in products of conception: RE = TE ~ LE +
YE.
Conventional Scheme
The law of conservation of energy and the law of initial and final
states are the fundamental principles that form the basis of bio-
energetics. Thus, if an increased amount of energy is found in one
place (an animal body), an equal quantity of energy has been re-
moved from another place (the food that has been consumed).
Also, the amount of energy transformed in an isolated system
(the breakdown and synthesis of chemical substances in the animal.
for example) as a result of a change in the system depends only on
the initial and final states of the system. That is, the amount of
heat produced or absorbed during a chemical transformation is
independent of the number and kind of intermediate steps in-
volved or the rate at which the transformation occurs. Inherent
OCR for page 10
10
in the above principles is the concept that all known forms of en-
ergy (chemical, electrical, magnetic, and gravitational) can be con-
verted quantitatively to heat.
The basis of bioenergetics as defined by the two laws and appli-
cation to whole animal nutritional energetics may be stated by
using the terminology defined earlier (Figure ~ ):
TE= FE+ GE+ UK+ ZE+ SE+ HE+ RE.
This simple identity partitions the energy consumed by an ani-
mal into the major components associated with animal energetics.
It can be expanded to include a few or many of the intermediate
steps involved, and each individual component can be subdivided
into several constituent parts, but the expression will still remain
compatible with the two laws. That is, the use or failure to use
information obtained in detailed studies of the energetics of inter-
mediate transformations does not prejudice the balance of the
equation.
All energy balance techniques and all systems used to describe
the relationship between an animal's requirement for energy and
the ability of a foodstuff to supply this energy are related to this
classical energy balance identity. In general use, each term is a rate
with the basis of time an interval of 24 h. Shorter or longer periods
can be used.
The terms of the balance equation have been defined earlier.
The components TE, FE, UK, GE, ZE, SE, and RE are heats of
combustion determined in a bomb calorimeter and represent the
total energy released during the oxidation of that component to
carbon dioxide and water. Other terms used to describe the heat
of combustion are gross energy or' simply, energy value. The gross
energy (E) of a substance is the sum of the E value of its constit-
uents and is thus related to chemical composition. For example,
the E values of foods can be estimated by using average factors to
convert quantities of protein' fat, and carbohydrate to amounts
of energy. This estimate will be less precise than values obtained
by bomb calorimetry.
OCR for page 11
11
Gross energy intake (lE), or the total energy contained in a
feedstuff, is of little value in assessing the worth of a particular
diet or dietary component as a source of energy for the animal.
Gross energy expressed as kilocalories per unit of dry weight can
indicate in a relative way the potential of a substance to furnish
energy. Many foodstuffs are composed of carbohydrates that have
an E of approximately 4.2 kcal/g; a higher E value could indicate
the presence of protein and/or lipids, whereas a lower value might
be explained by the presence of large amounts of inorganic sub-
stances. In either case, the gross energy value does not provide any
clue as to how available the energy is to the animal.
Digestible energy (DE) does provide some clue as to availability
of energy. Similar terms are apparent absorbed energy or energy of
apparently digested food. The word "apparent" is sometimes used
in conjunction with digestible energy to recognize the fact that not
all the energy in feces (FE) is derived from food residue. As was
mentioned previously, FE has two major components, fecal energy
of food origin (FiE) and fecal energy of metabolic origin (Fm E). Ac-
tually, there is a third component, fecal energy of microbial origin.
Because the energy of the microbes originated either with the feed
or with the metabolic products, it need not be considered sepa-
rately. It should be recognized that there are additional losses of
energy associated with the digestion of a food or feedstuff that in
the conventional determination of DE are not subtracted from lE.
Gaseous products of digestion (GE), for example, are actually
losses associated with the digestive process.
Metabolizable energy (ME) is an estimate of the dietary energy
that becomes available for metabolism by the tissues of the animal.
Metabolizable energy is defined by the relationship:
ME= {E-(FE+UE+GE)
or
ME = lE - (FE + UE + GE + ZE) for fish.
OCR for page 34
34
For lactation the NE requirement is predicted from the energy
value of milk:
LE (Meal/kg) = 0.009464 BE + 0.004900 SNF - 0.0564,
where LE is energy in ~ kg of milk, BE is grams of butterfat in
~ kg of milk, and SNF is grams of solids-not-fat in ~ kg of milk,
respectively. Thus ME required for milk is
MET = I,E/k~.
In Ministry of Agriculture, Fisheries and Food (1975), kit is
assumed to be 0.62, and
MET= 1.613 LE.
The coefficient was increased to 1.694 to include a safety mar-
gin. Adjustments are made for the energy value of live weight
change: ~ kg live weight loss adds 6.69 Mcal to ME available for
maintenance and milk production; ~ kg live weight gain requires
an additional 8. ~ 3 Mcal of ME from the diet.
It must be recognized that certain compromises are involved
with each of the estimates needed in these systems. As more pre-
cise data become available on the efficiency of various digestive
and metabolic functions in dairy cows and other lactating rumi-
nants, adjustments will be made in the systems discussed here.
Application to Nonruminant Herbivores,
Especially Horses and Rabbits
Energy metabolism studies with horses are limited in number' but
considerable data on digestibility are available. For this reason the
system used to describe energy requirements of the horse is based
on DE (NRC, 197Sb). Because of a lack of enough data to do
otherwise and because of the work component of total energy
balance, body weight and its maintenance play a significant role
OCR for page 35
35
in the evaluation of feeds and energy requirements. When TON
data are available, they are converted to DE by
DE (Meal) = 4.4 TDN (kg).
Maintenance DE, defined as zero weight change plus norms ac-
tivity in the nonworking horse, is described by the equation:
DEm (kcal/day) = ~ 5 5 W 7 5,
where W is the body weight in kilograms.
With regard to growth the DE need above maintenance is esti-
mated from
Y = 3.8 + 12.3 X - 6.6 x2,
where Y is kilocalories of DE per gram of gain and X is the frac-
tion of adult weight.
When applied to the nursing foal, the utilization of DE is as-
sumed to be 10 percent greater than that in the mature horse. The
kf for mature horses is approximately 0.84 (Kane et al., ~ 978~.
The requirement of DE for pregnancy is considered only during
the last 90 days of gestation. Early estimates (NRC, 1973) suggest
that the DE need for pregnancy is 6 percent greater than for main-
tenance. Recent estimates (Ott, 1971; Brewer, 1975) suggest that
12 percent of maintenance is needed. A general formula would be
DEy = 0.12 DEm.
Assumptions involved irt the above are as follows:
I. The products of conception contain ~ .040 Mcal/kg.
2. The products of conception constitute ~ O percent of body
weight for animals of 450 kg or more and 12 percent for weight
of less than 450 kg.
3. The detectable deposition occurs during the last 90 days of
gestation.
OCR for page 36
36
4. The efficiency of use of DE for fetal growth and associated
tissues is 30 percent.
Requirements of DE for lactation are based on the following:
1. The LE value of mare's milk is 475 kcal/kg.
2. The partial efficiency of use of DE for LE is 0.60.
3. Milk production (percent of body weight) for horses is 3 per-
cent during weeks 1 through ~ 2 and 2 percent dunng weeks ~ 3
through 24, whereas for ponies it is 4 percent during weeks 1
through ~ 2 and 3 percent during weeks ~ 3 through 24 of lactation.
Thus the DEN is
DEN (kcal/day) = (475/0.60~(W)(F),
where W is the body weight in kilograms and F is the production
rate (fraction of W).
Although the work output of horses is of major importance,
quantitative relationships between level of work and DE require-
ment for work (DEj) have not been made. A large number of
factors (intensity and duration of work, environmental conditions,
and degree of fatigue) influence the energy requirement associated
with work. The NRC (197Sb) has reported some guidelines for
DEj, added to DEm, based on body weight and work intensity:
Activity
Walking
Slow trotting, some cantering
Strenuous, full speed
DEj /hr/kg
_
0.5
5.0
39.0
Recent data (Willard et al., 1978) suggest that DEj increases for
a given distance traveled, as the speed of travel increases. Thus the
influence of work on total daily metabolism is in need of study.
Little information is available on the utilization of energy by the
rabbit. The recent NRC publications (NRC, 1966, 1977) have used
OCR for page 37
37
TDN and data from other species. Conversion from TDN to DE is
assumed to be 4.4 kcal of DE per gram of TDN. When TDN data
are not available, the DE value of forage (kcal/kg DM) is estimated
from crude fiber content by using the following equations:
legumes (DE) = 4340 - 68 (percent crude fiber),
grasses (DE) = 4340 - 79 (percent crude fiber).
The fasting metabolic rate is computed from the formula of
Kleiber (19613: HeE (kcal/clay) = 70 W 75, where W is the body
weight in kilograms. The results of Heliberg ~ ~ 949) suggest that
a coefficient of 77 may be more appropriate.
The influence of level of intake on digestion has been ir~vesti-
gated (Heliberg, ~ 949), and DE was found to decline with increas-
ing intake at a rate similar to that for ruminants. The net utiliza-
tion of ME for gain in rabbits is about 70 percent (Heliberg, ~ 949),
and the DE allowance for gain is 9.5 kcal/g (NRC, 19771. The ca-
loric density (DE, kcal/kg) of the diet should be 2100 for adults
compared to 2500 to 2900 for young rabbits.
Application to Swine
Total digestible nutrients (TDN), starch equivalents, Scandinavian
feed units, oat units, DE, ME, MEn, and NE are energy units that
have been used in swine nutrition. Digestible energy defined as
food intake of energy minus the fecal energy (DE = lE - FE) has
been used by the Agricultural Research Council (1967) and NRC
(1979) to define energy requirements and energy contents of diets
for swine. The loss of energy as combustible gas from the digestive
tract is usually small (less than ~ percent of {E) and is normally
ignored. If diets high in structural carbohydrate and/or protein
that escapes digestion in the small intestine are fed to pigs, a fer-
mentation can develop in the digestive tract. Many of the tabular
values (NRC, 1971a) for DE of feed ingredients for pigs have been
calculated from tabular TDN values by using ~ kg TDN = 4400
kcal DE.
OCR for page 38
- ~
38
Metabolizable energy (ME) defined as DE - UE has been used
by NRC (1979) to define energy requirements and energy value
of diets for swine. In the formulation of diets in the United States,
this measure of ME is generally used. Many of the ME values re-
ported by N R C ( 1 97 1 b) have been calculated by converting TDN
to DE as noted above and then calculating ME by using the fol-
lowing relationship:
_ 0.202 X percent of crude protein \
ME= DE 96 - 100 .
Experimentally derived values are primarily from the work of
Diggs et al. ~ ~ 9651.
Metabolizable energy corrected to nitrogen equilibrium (MnE)
has been reported, but is not commonly used in diet formulation.
While the correction to nitrogen equilibrium may be valid for ma-
ture animals, nitrogen retention is normal in growing animals, and
the correction probably is not necessary. The correction is made
by the following formula:
Mn E = [E - FE - UE + kRN.
The constant k has been estimated from the urinary energy per
gram of urinary nitrogen. A value of 7.45 kcal/g of nitrogen (Rub-
ner, ~ 885) determined with dogs has been used most commonly.
A number of other values have been reported from work done with
swine: 6.77 kcal/g (Diggs et al., ~ 9S9), 9. ~ 7 kcal/g (Morgan et al.,
1975), 7.83 kcal/g (Wu and Ewan, 1979), and 7.0 kcal/g (NRC,
979~.
Net energy defined as ME - HiE has been used to describe energy
requirements and energy values of feeds. In contrast to cattle and
sheep, the pig can utilize ME as efficiently for growth as for main-
tenance. Therefore the net energy requirements and net energy
values of feeds can be expressed as a single value similar to lac-
tating ruminants (Nehring and Haeniein, 1973; Just-Nielsen, 1975;
Ewan, 19761. Nehring and Haeniein (1973) reported the evolution
OCR for page 39
39
of the East German net energy system reported in detail by Schi~e-
mann et al. ~ ~ 97 ~ ). In this system the net energy values of feeds
are expressed in terms of the ability to promote fat deposition
(NEf) in mature animals. Both the requirement for maintenance
and that for growth are expressed in terms of NEf.
Studies of the utilization of ME for growth by comparative
slaughter techniques have been reported by Just-Nielsen (1975)
and Ewan (19761. Just-Nielsen (1975) concluded that a system
based on energy gain of growing animals was comparable with
the German system based on NEf.
Nehring and Haeniein (1973) concluded that the net energy sys-
tem is necessary because performance cannot be predicted from
the metabolizable energy value of the feed. The efficiency of uti-
lization of ME for energy gain (NEg) In growing pigs has been re-
ported to vary from 27 percent for wheat middlings to 75 percent
for soybean oil (Ewan, 19761. Kromann et al. (1976) by feeding
wheat and barley at different ratios also observed different partial
efficiencies of utilization of ME for gain for these two cereal grains
At present, experimentally determined net energy values of feed
ingredients and the requirements for maintenance and growth ex-
pressed in terms of net energy are limited. Therefore energy re-
quirements of swine are expressed in terms of DE or ME, but the
development of a net energy system may provide a more accurate
method of ration formulation for swine.
Application to Poultry
For many years the poultry industry relied on productive energy
values to define energy requirements and to describe the available
energy in feedstuffs. Productive energy is a form of net energy and
is determined by measuring the energy stored as fat and protein in
growing or fattening birds (Fraps, ~ 946~. This assay is relatively
difficult because it involves measuring weight change, feed intake,
and change in carcass composition. It also involves several assump-
tions of questionable validity. Productive energy values are not
always additive, and there are data showing them to be unreliable
(Davidson et al, ~ 95 7; Hill and Anderson, ~ 95 S).
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Attempts have been made to measure digestible energy values
with birds, but these are complicated by the excretion of feces and
urine via a common cloaca. Surgical techniques have been used to
permit the separation of feces and urine, but there can be no proof
that a modified bird behaves in the same manner as a normal bird.
Chemical procedures have been used to measure the amount of
urine in excrete, but the techniques are not wholly satisfactory.
Metabolizable energy values have been measured with poultry
for many years, but it was not until about ~ 960 that they became
widely accepted. In the measurement of metabolizable energy the
gaseous products of digestion are ignored, but correction is usually
made for nitrogen gained or lost during the assay.
Mn E = IE - (FE + UE - kRN)
Two constants have been widely used: 8.22 kcal/g of nitrogen,
which was derived from the gross energy value of uric acid, and
8.73 kcal/g, which was calculated from the gross energy values of
the various nitrogenous compounds in chicken urine (Titus et al.,
1959~.
Recently, it was shown that ME values vary with feed intake
because the metabolic fecal (Fm E) and endogenous urinary (UeE)
energy losses are charged against the feed (Sibbald, 1975~. This is
of importance when feedstuffs of low palatability are being as-
sayed because it is normal to maximize the level of the test mate-
rial in the assay diet. A bioassay for true metabolizable energy is
now available (SibbaId, ~ 976, ~ 9801:
TME = {E - (FE - Em E) - (UE - UeE)
or
TME = IE - (FiE + UiE).
The assay is faster, less complex, and more accurate than those
for Mn E. More important, the data are less affected by variation in
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feed intake and species of bird used, while the TME values are ad-
ditive. When the assay uses adult male birds, the deviation from
nitrogen balance is small and can be neglected.
Application to Aquatic Animals
Much interest has recently been shown in the nutritional energetics
of aquatic animals. Most of the work has been confined to trout,
salmon, catfish, carp, and a few other finfish species. Development
of large-scale commercial production of fish has emphasized the
lack of adequate information regarding the ability of the different
species of fish to utilize energy from the diet.
Several unique problems are associated with the study of energy
utilization in fish. Usually, the animals are small, less than 500 g
each. This requires microtechniques for analysis of fecal and urine
samples or the use of groups of animals. The waste products are
difficult to separate from the aquatic environment, and leaching
and dilution must be considered. Care must be taken to avoid mix-
ing uneaten food with waste products. Most aquatic organisms
excrete waste nitrogen from protein catabolism as ammonia
through the gills. The gill excretions (ZE) must be collected to
account for all energy and nitrogen loss. Body temperature and
its relationship to metabolic rate must also be considered (Smith
et al., ~ 978a). Body temperatures of most fish are very near the
temperature of the water and can vary over a wide range with no
ill effects. Within species, adaptations can be made to compensate
for up to 20°C change; between species, adaptation is even greater.
Some species live and grow in arctic or antarctic seas at tempera-
tures below the freezing point of fresh water, and others inhabit
hot springs at temperatures above 37°C. It must be remembered
that there is as much difference between species in fish as in mam-
mals or birds—there are herbivorous, carnivorous, and omnivorous
species of fish.
Several methods have been developed to determine DE and ME
values of fish foods and dietary ingredients. A metabolism chamber
has been developed in which individual fish can be held for total
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collection of feces, urine, and gill excretions (Smith, ~ 97 ~ ). Other
have used an indigestible marker and partial fecal collection. Cho
et al. (1976) use a column from which all settleable material can
be removed. Windell et al. (1978) use a suction technique to re-
move the feces from the lower intestine. Others have used various
fecal collection methods such as removing fecal matter from the
aquarium with a fine mesh net, filtration of aquarium water, and
centrifugation. Each of these methods has its advantages and dis-
advantages. The fish in the metabolism chambers are undoubtedly
under some stress because they must be closely confined and tube
fed. The metabolism chamber permits collection of urine and gill
excretions, which makes possible the calculation of ME. Methods
that depend on separation of the feces from the aquarium water
presume that fecal loss by leaching is negligible. If ME is calcu-
lated, it must be assumed that all insoluble material is fecal waste
anti that the soluble material is of urinary or gill origin. Smith et
al. (1980) have shown that considerable loss occurs in the first
hour that the fecal matter is in contact with the water. There is
handling loss when fecal matter is netted or siphoned from an
aquarium. The suction method assumes that digestion and absorp-
tion is complete when the material is removed from the lower in-
testine. Use of an indigestible marker raises the question "was the
amount of marker in the analyzed sample representative of the
amount excreted?" Methods using an indigestible marker permit
studies with groups of small fish that need not be force-fed.
Most ME values reported for fish have not been corrected for
nitrogen balance. However, most trials have been done near nitro-
gen equilibrium. There is no evidence that the constant 7.45 kcal/g
of nitrogen, obtained with dogs, is applicable to fish. It is tenuous
to apply to fish constants that were obtained with mammals or
birds. The heat equivalent of oxygen is different for animals excret
ing ammonia than for those excreting urea or uric acid. The study
of energy metabolism in fish is much the same as with other ani-
mals when the unique problems of fish are considered (Smith and
Rumsey, ~ 9761. The gaseous products of digestion can be ignored,
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but the energy loss in the gait excretions (ZE) must be considered.
The formula for metabolizable energy then becomes
ME = IE - (FE + UE + ZE).
Care must be taken in estimating energy values from proximate
analysis. Carnivorous fish utilize raw starch poorly, and fiber has
very little value. Cooking increases the digestibility of starch. Re-
cent work indicates that protein has a higher net energy value for
fish than it does for mammals and birds because less energy is ex-
pended by fish to excrete the waste nitrogen (Smith et al., 1978b).
There are not enough published data to determine if digestibil-
ity values alone are sufficient to evaluate feed materials for fish or
if the additional work required to determine ME is justified.
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Representative terms from entire chapter:
heat production
