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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

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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-

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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 17.5C. 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.

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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.

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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.

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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).

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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).

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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.

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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

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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.

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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.

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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

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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.

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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

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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.

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- ~ 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

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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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40 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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41 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 20C 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 37C. It must be remembered that there is as much difference between species in fish as in mam- mals or birdsthere 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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42 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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43 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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