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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
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-
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.
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.
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.
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).
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).
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.
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
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.
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.
12 The equations and the energy balance identity indicate that me- tabolizable energy can appear in only two formseither as heat (HE) or in the energy of products formed (RE). Thus IRE = RE + HE. For the overall assessment of energy balance in an animal, me- tabolizable energy does not need to be corrected for the metabolic (Fm E) and endogenous (UeE) sources of energy. However, in some investigations, particularly when feedstuffs are being compared as sources of ME, it may be advantageous to correct for Em E and UeE to obtain a true metabolizable energy value (TME). The true metabolizable energy content of a feedstuff will be higher than the ME content. Metabolizable energy has sometimes been adjusted to the basis of nitrogen equilibrium (Mn E). The goal of this computation for feed evaluation was to remove from the estimate of ME any bias associated with the particular conditions (i.e., physiological state of the animal or feeding management) that happened to prevail during a particular experiment. The justification for this "correc- tion" is controversial. The adjustment is neither necessary nor justified for a particular energy balance trial. Tissue energy retained or lost from the animal's body plus en- ergy recovered as useful animal products (RE) is commonly known as energy balance. In keeping with the general energy balance iden- tity, RE is the heat of combustion of all animal products (TE, LE, YE, OK, etc.) that may be produced or lost with a given energy in- take (lE) not accounted for in any other category. RE may be a positive or a negative quantity. RE= 13E-(FE+ UE+GE+HE) or RE= ME- HE. Total heat production (HE) is the amount of energy that is transferred from the animal to the environment in a form other
13 than combustible energy. The total heat production consists of many components: fasting metabolism in animals or basal me- tabolism in humans (He E), heat associated with voluntary activity (Hj E), heat of product formation (Hr E), heat for thermal regula- tion (HCE), heat of digestion (Hd E), heat of waste formation and excretion (Hw E), and heat of fermentation (HfE). In many apply cations of these relationships the components HrE ~ He E + Hw E ~ HfE are combined and considered to be the heat increment or HiE. There are several equations that help to visualize the com- ponents of HE. HE = HeE + HjE + H:E + Hc E + Hd E + Hw E + HfE HiE= HrE~ HaE+HWE+HfE HE = HeE + Hj E + Hc E ~ HiE in nonstressful environments where Hc E would be zero or negli- gible, the components of HE consist of the heat produced under fasting conditions (He E), the heat produced as a result of activity (HjE), and the heat increment of feeding (HiE): HE = HeE + HjE + HiE. A rearrangement of a previous equation (RE = ME - HE) gives HE = ME- RE. It is evident that RE will be a negative quantity if ME is less than HE. That is, an animal will be using energy from body tissues any time total heat production exceeds metabolizable energy intake. Relationship of Environmental Temperature to Energy Metabolism The relationship between the climatic environment and the parti- tion of dietary and body energy into various components is related
14 to body size (surface), amount of insulation (surface covering), level of feeding, ration balance, and type of production. Climatic conditions may influence rate of loss of energy in feces, urine, gases, and heat, thereby playing a significant role in energetic ef- ficiency. Extensive attention has been given to this by Kleiber ~ ~ 96 ~ ~ and Brody ~ ~ 945) and in an N R C publication on the sub- ject of environmental effects on animal production (in prepara- tion). The relationship between animals and their thermal environ- ment begins with the thermoneutral zone (TNZ), which is some- times referred to as the thermal comfort zone or the zone of thermal indifference. The thermoneutral zone is defined as the effective ambient temperature (EAT) where heat production at the thermoneutral rate is offset by net heat loss to the environ- ment without aid of special heat-conserving or heat-dissipating mechanisms. Below the TNZ lies the coo} zone, where animals invoke mechanisms that conserve metabolic heat. These are mainly postural adjustments and changes in hair or feathers and in periph- eral blood vessels that affect cover insulation and tissue insulation, respectively. As EAT decreases within this zone, metabolic rate of the fed animal remains at the thermoneutral level. The various insulative and behavioral responses to cold stress are at maximal effectiveness at the lower limit of the coo! zone, a point called the lower critical temperature (LCT). Below this point lies the cold zone, where the only way an animal can maintain homeotherm is to increase its rate of metabolic heat production. As effective ambient temperature rises above the upper limit of the thermo- neutral zone, the animal is in the warm zone, where thermoregu- latory reactions are mainly limited to passive facilitation of heat loss. Decreasing tissue insulation by vasodilation and increasing effective surface area by changing posture are major mechanisms used to facilitate rate of heat loss. As the environment rises above the warm zone, the homeothermic animals must call into play active heat-dissipating mechanisms employing evaporative heat loss, such as sweating and panting. Then the animal is in the hot
, 15 zone. The EAT at which the animal passes from the warm zone to heat stress is called the upper critical temperature (UCT). It is important to recognize that the relationship between cold critical temperatures, TNZ, and heat remains consistent. Cold temperatures always refer to those temperatures below the TNZ even though the TNZ may change. Factors that alter critical tem- peratures or cause a shift in the TNZ are insulation, plane of nutri- tion, activity, solar radiation, or any other factor altering rate of energy exchange between the animal and its environment. For ex- ample, lower critical temperature may change from 0°C in a sheep with fleece to 20°C in shorn sheep, other factors remaining con- stant. In other words, ambient conditions are of little value in pre- dicting the effect on energy needs unless one also knows the ther- moneutral zone of the animal involved. Other factors remaining constant, it is magnitude of cold (degrees below LCT) or magnitude of heat (degrees above UCT) that is important in relating animals to their thermal environment. Some studies (Young, ~ 976) have indicated lower energy digesti- bility during cold. Often, increased intake is credited with lower digestibility during cold, but covariate analysis has shown that de- creased dry matter digestibility is due to temperature alone. Al- though all available information relating energy digestibility to heat stress does not agree, most data tend to support the hypothe- sis that digestibility increases during heat stress. This may be a result, in part, of decreased voluntary intake rather than a direct effect of increased temperature. Feed consumption increases the rate of heat production. This additional amount of heat is referred to as heat increment (HiE) and can be identified in Figure ~ as HrE, Hd E, HW E, and HfE. The sum of these sources of heat is important in describing ani- mals' TNZ, since additional heat production lowers the TNZ. For example, thermoneutral temperature is relatively low for animals with high levels of production compared with similar animals on a maintenance diet largely because of relatively high total heat pro- duction. It is important to understand the difference in effect of
Insulation = 16 heat increment on animals exposed to cold as compared with ani mats exposed to heat. During cold, heat increment is useful in off- setting increased rate of heat loss. Conversely, heat increment aggravates the problem at high temperatures by adding more heat to an already heat-stressed system. Consequently, the relationship of heat increment to energetic efficiency is positive during cold but negative during heat. An animal exposed to temperatures below its critical tempera- ture must compensate for increased energy loss by additional energy expenditure. Two major characteristics of the animal de- termine the rate of heat loss on exposure to cold: (~) thermal gradient between core temperature and ambient temperature and (2) amount of insulation provided by hair coat or fleece, tissue, and air interface. In general, the thermal gradient and animal insu- lation can be used to predict the rate of heat loss by the following linear relationship: rate of sensible heat loss _ temperature gradient AK. By estimating the amount of total insulation, the increased en- ergy requirement per degree of cold stress may be determined by the following equation: Mm E = aW 75 + b(AT/AE), where Mm E= metabolizable energy for maintenance (kcal/day), ~ magnitude of difference between lower critical temperature and effective ambient temperature, = total insulation of animal (kcal/T/M2 /day), = coefficient of maintenance requirement for animal in TNZ (kcal ME/W 7s/day), b = surface area of animal (M2 ), and W as = metabolic body size (kg). HE a
17 The relationship between heat production and environmental temperature cannot be extrapolated for temperatures above the thermoneutral zone. instead, the energy requirement increases nonlinearly during heat stress. This increase is attributed to the need for thermal regulatory mechanisms such as sweating and panting. It has also been reported (Kleiber, 196 ~ ~ that the Qua O effect of increased core temperature (increases that are within the range expected for homeothermy) may contribute significantly to an increased rate of metabolic heat production during exposure to heat stress. While nonlinear increases in metabolic heat pro- duction have been reported, more world is needed to better define relationships between magnitude of heat stress and increased rate of heat production. Above the upper critical temperature, core body temperature and heat production increase, the latter according to Van't Hoff's law (Kleiber, ~ 96 ~ ): HOE = HoE Q1O where HOE = metabolic rate at temperature T°C, Ho E = metabolic rate at 0°C, and Qua 0 = Van't Hoff~s quotient (approximately 21. The above process becomes fatal if the cellular processes remain uncontrolled. An example is taken from Kleiber (1961) for illus- trative purposes. The metabolic rate (kcal/day/M2 ~ of some human subjects was 890 at 24°C (known to be within the zone of thermal neutrality) and ~ 5 ~ 5 at 2°C. The latter was assumed to be the thermostatic heat requirement at 2°C. The thermostatic heat requirement is directly proportional to the difference between body temperature (37°C) and environmental temperature (2° C). The formula is i\HE/AT= Q(Ti - Te)'
18 where Ti T le AHE/AT = rate of heat production needed to supply thermostatic heat requirement (kcal/day/M2 ), Q = effective heat transmissivity (kcal/M2 /day/°C), = body temperature (°C), and = effective environmental temperature (°C). From the above, Q AHE/AT lSlS Ti_Te 37-2 = = 3 5 = 43 kcal/M2 /day/° C. The lower critical temperature (Tc) can be defined as the point at which the thermostatic heat requirement (heat to maintain body temperature) is equal to defined metabolic rate. It is the tempera- ture below which the animal can no longer regulate E to keep HE = 0, and metabolic rate must increase in proportion to the need for heat. Thus AHE/AT = or 43~37 ~ Tc ~ = 890 Tc= 16.3°C. One expects the rate of heat production to be (ideally) indepen- dent of environmental temperature in the TNZ. Above the upper critical temperature it increases independently of animal control. Below the lower critical temperature it increases (controlled) to a maximum (summit metabolism) as temperature declines. When the rate of heat loss exceeds metabolic capacity, body temperature falls and death results from hypothermia. However, in poikilo- thermic animals a reduction in body temperature causes only a reduction in metabolic rate. Level of feed (energy) intake does not influence the thermo-
19 static heat requirement directly. It does, however, influence the upper and lower critical temperatures. As metabolic rate increases with increased feeding within the zone of thermal neutrality, the lower critical temperature decreases. For example, at a metabolic rate of 1275 at 24°C, Tc = 7.4°C. Although not precisely defined, the upper critical temperature would be expected to drop with increased intake, but in practice the animal reduces metabolic rate by reducing food consumption at high temperatures. The influence of environmental temperature on animal produc- tion is important. At high environmental temperatures, voluntary feed intake is usually reduced, as one means of helping the animal regulate body temperature and heat load. At decreasing tempera- tures in productive animals, HiE becomes a useful product used to meet the thermostatic heat requirement. Other physiological mech- anisms can come to bear that improve the heat conservation of the animal. Systems Used to Express Feed Energy Values and an Animal's Requirement for Energy The historical aspects of feed evaluation systems have been re- viewed by Blaxter (1955, 1956) and Reid (19621. As pointed out by Blaxter, the advances in feed evaluation have been in three ma- jor overlapping steps. The beginning was the "hay value" of Thaer published in ~ 809 and revised at intervals through most of the nine- teenth century. The second major advance came from the studies of Henneberg and Stohmann during the period ~ 850 to ~ 880 that produced the proximate analysis scheme and the conventional di- gestion trial. The third major step, also started by Henneberg and Stohmann, who recognized the deficiencies of a feeding system based on "digestible nutrients," was the almost simultaneous for- mulation of the net energy concept by Armsby in the United States and Keliner in Ge~any during the early part of the twentieth century. The fundamental principles of Armsby and Keliner are equivalent. Keliner's starch equivalent and the net calorie of Armsby are interconvertible, each expressing the value
20 of the feed in terms of the energy retention (RE) that it promotes. The three steps outlined above in a historical context provide the basis for the feeding systems in current use. Digestible Energy Since the toss of energy in feces (FE) associated with the cor~sump- tion of particular foods or diets accounts for or is related to the varying abilities of these foods or diets to fulfill an animal's re- quirement for energy, it is possible to evaluate feeds and assess animal requirements in terms of DE. For some feeding situations, particularly the formulation of diets where only a few feeds are involved and the animal's requirements in terms of DE have pre- viously been determined by using similar feeds and levels of ani- mal production, satisfactory results can be obtained. The major weakness of DE as a basis for a ruminant feeding system is that it overeva;tuates high-fiber feedstuffs (hays, straws) in relation to low- fiber products (grains). In rations for nontuminants the DE con- centrations of feed ingredients are similar and the shortcomings of DE as a system for evaluating feeds are minimized. Feed ingre- dients used in diets for ruminants vary widely in DE content and the weakness of DE is greatest. Me-tabolizable Energy Metabolizable energy is superior to DE for use as a measure to ex- press feed values and energy requirements because it considers losses of energy in the urine (UK) and combustible gases (GE). It has been used to express the feed value and energy requirements of poultry for many years and is the general basis for the physiolog- ical fuel values used to describe the energy values of foods and the energy requirements of humans. It is an alternate system for stating the energy requirements of several species and is currently used by several subcommittees of the NRC. As is indicated by the relation- ship relating ME to RE and HE, it has major significance as a ref- erence unit and as the starting point for nearly all systems that are based on the net energy concept. However, ME has many of the
21 same deficiencies as DE. The reason for this is UE and GE are pre- dictable from DE, and therefore the correlation between DE and ME is very high. True metabolizable energy (ME + Em E + UeE may be a sufficiently precise measure of the energy value of feed- stuffs for those species (like poultry) whose diets are highly diges- tible and less variable in composition than diets fed to ruminants and nonruminant herbivores. For ruminants in particular, and for nonruminant herbivores, ME is of most value as an integrant in the development of a feeding system based on the net energy principle. Total Digestible Nutrients f TDNJ The total digestible nutrient system of feed evaluation developed in the United States is a direct result of the German work on "di- gestible nutrients." The major feature added was the multiplica- tion of digested fat by 2.25 to account for the higher energy concentration of lipids. Thus TDN is the sum of the digestible protein, digestible carbohydrates, and 2.25 times the digestible fat. Maynard (1953) reviewed the TDN concept, pointing out the inconsistency of a "caloric correction" for digestible fat and the lack of a similar correction for protein. The failure to use an "en- ergy correction" factor for protein has the effect of accounting for some urinary energy losses and therefore makes TDN a hybrid mea- sure (not precisely DE or ME). The failure to make a correction for the energy in protein was not particularly detrimental (in fact may have been helpful) in a practical sense, since feed values and feed- ing standards based on TDN were estimated on the basis of the fat correction only. Nevertheless, TDN does not measure digestible nutrients as the name implies; it does not provide a more useful basis for a feeding system than DE, and, unlike ME, it is not a measure that has direct relevance to net energy or energy metab- olism in general. Physiological Fuel Values (PFVJ The assessment of the energy value of food for humans was re- viewed by Widdowson (19551. These energy values (PFV) are ex-
22 pressed in kilocalories and usually are calculated from the amounts of protein, fat, and carbohydrate present as determined by chem- ical methods. The bases for these calculations were derived from work in Germany by Rubner from ISS0 to 1901 and by At- water (once a pupil of Rubner) during the period ~ 895 to ~ 906 in the United States. Physiological fuel values are essentially ME values derived by using average heats of combustion and average digestion coefficients for protein, fat, and carbohydrate. Calories from protein are corrected for energy loss in the urine based on the determination that 7.9 kcal of energy were lost for each gram of nitrogen in the urine of human subjects consuming mixed diets. If 1 g of nitrogen in the urine is assumed to represent the break- down of 6.25 g of digestible protein, there would be 7.9/6.25 = 1.26 kcal/g of digestible protein. The values calculated according to the general findings of Rubner and Atwater have varied some- what depending upon the factors used to convert nitrogen to pro- tein, the assumed digestion coefficients for protein, carbohydrate, and fat as well as the chemical procedure used to determine car- bohydrate. The physiological fuel values commonly quoted (4, 4, and 9 kcalig of protein, carbohydrate, and fat) may give reason- able estimates of metabolizable energy that can be used as a basis to formulate diets and approximate the requirements of humans for different functions. Net Energy Concepts The net energy (NE) value of a food or diet in the classical sense can be illustrated by the simple identity: NE= ~RE/~IE. The ability of food energy to promote energy retention is mea- sured by determining RE at two levels of energy intake. In this simplified expression, RE can be a positive or negative quantity. Since it is more convenient to have NE expressed as the concen- tration of energy (kcal/g, for example) the denominator of this
23 expression is generally the change in quantity of food consumed rather than the heat of combustion of the food. Net energy values determined by the "difference trial," as indi- cated by this classical expression, assume that the relationship be- tween food intake and RE is rectilinear. Actually, the relationship between RE and food intake over the complete range of intake from zero to aa' libitum is curvilinear (Lof~een and Garrett, ~ 968~. The convention adopted to accommodate this fact is an approxi- matiort with two straight lines (Figure 21. The point common to each line is where RE = 0. Thus energy intakes that result in nega- tive RE represent one segment of the cube (zero to maintenance level of feeding), and energy intake that results in positive RE - LU _ 1+) is LL UJ z ~ UJ cr Energy Equilibrium RE = 0 \ Fasting Energy Loss FOOD I NTAKE ( 1 ) FIGURE 2 Representation of the relationship between RE and NE. The dashed line shows the curvilinearity between RE and food intake; the solid lines are linear approximations.
24 (maintenance to ad Zibitum levels of feeding) represents the other segment of the curve. To express the relationship between ME, RE, HE, and NE, it is necessary to consider the following expressions: NEmr = NEm + NEr' where and NEm = ~ Oe RE - Rm E r ~ _ I but Rm E= 0 by definition and -ReE = HeE or fasting heat produc- tion. Therefore NEm = HeE/Im ~ and NE = RE -r I-Im ~- These expressions are the result of the convention that describes the relationship between RE and lE (or RE and food intake, if pre- ferred) as two straight lines intersecting at zero RE (see Figure 21. Since NEm is the difference in RE between levels of intake from nil to where RE becomes zero rather than negative, it represents the quantity of energy that the animal would have to use from tis- sue fat and protein to remain alive. In domestic animals this quan- tity is equivalent to the fasting metabolism (He E). In a similar manner, NEr is the difference in RE from an intake above mainte- nance to some higher level (up to the appetite of the animal), and by this definition Rg E = NEr.
25 The relationship ME = RE + HE can be written in terms of NEm and NEr: ME = NEr + NEm + HjE + HiE In the application to practical feeding situations, HjE, the heat associated with activities incident to obtaining food, may be in- cludecl with HeE. In this event, NEm= HeE + HjE and the expres- . . . SlOn IS SlMp Y ME= NE' +NE~ +H E. 111 1 The term NET (RE in an animal above maintenance) as used in these expressions does not make a distinction between RE as a re- sult of growth (as fat and protein deposition), milk production, fetal development, or egg production. Thus NEr could consist of one or several components. In a preg- nant lactating dairy heifer, for example, NEr might have three ma- jor components, the energy in the milk, the energy being stored in the products of conception, and the energy stored as a result of growth (protein and fat deposition in the heifer's tissues). In this example, RE = LE + YE + TE. or NED = NE1 + NEy + NEg. Since the end products of digestion (the constituents that make up the energy of ME) are used at different levels of efficiency for maintenance or conversion to the various productive functions, it is not possible to assign a sinJe NE value to each feedstuff. For the net energy system to have practical application, each feed has to be assigned multiple NE values. Alternate procedures are to adjust the ME value of feeds according to the quantities of energy that are
26 necessary for maintenance and the particular production functions or to scale the animal's energy requirements for the various pro- ductive functions to a single NE value. The species sections below will illustrate how some of this information has been adopted for use in practical feeding systems. Efficiency of MetabolizabZe Energy Utilization As was mentioned in the previous section, it is possible to adjust or convert ME values to NE values if the efficiency of ME use (k) for a particular function is known. For example, the km or efficiency of ME use for maintenance (NEm ~ can be expressed: km = HeE/MEm thus MEm = He E/km In a similar manner, kg represents the efficiency with which ME is used to promote weight gain (protein and fat deposition) in an animal. The expression to calculate kit is C3 NE k = g g ME - MEm It is immediately apparent that km and kg have a term in com- mon. Thus NEg - kg (ME - HeE/km Sir~ce HeE is the fasting heat production and is usually expressed as awn, the equation becomes NEg = kg (ME - aWn /km
27 or NEg/Wn = kg (ME/Wn) - akg/km . This equation has the form of a straight line with slope kg and an intercept C (which is akg/km ). This is a convenient relationship for estimating kg and km if "a" is known or estimated from fasting heat production (He E) provided NEg, ME' and the size of the ani- mal have been determined. The metabolizable energy for mainte- nance and km can also be estimated from this regression of NEg/ wn against ME/Wn. If NEg/Wn is set to zero, then ME becomes MEm and kg MEm = (akg/km Own = CWn or MEm = aWn /km = CWn /kg. It is important to recognize that the values obtained for km and kg vary depending upon the source of the ME. In general, the ef- ficiency of ME use for growth is greater as the concentration of ME (kcal/g) in the diet increases. Application to Growing Ruminants From a historical basis the measure of the energy value of feed- stuffs for growing ruminants most used in the United States has been TDN. More recently, requirements have been stated in terms of TDN, DE, ME, and NE for sheep (N RC, ~ 975) and dairy cattle (NRC, 197Sa) and TDN, ME, and NE for beef cattle (NRC, 19761. We requirements for growth of cattle expressed as TDN, DE, and ME have been calculated from net energy determinations. In Eu- rope the starch equivalent and the Scandinavian feed unit have been the systems used most extensively. Most of Europe has now adopted a net energy system that uses experimentally derived relationships
28 between ME and NE; i.e., ME is adjusted depending upon the com- puted efficiency of its utilization for maintenance (km ~ or growth (kg). This system and the procedures currently in use to adapt it to practical use are found in various publications (Agricultural Research Council, 1965; Harkins et al., 1974; Ministry of Agricul- ture, Fisheries and Food, ~ 975; Van der Honing et al., ~ 977; Bicke} and Landis, 1978; Institut National de la Recherche Agro- nomique, 19781. In East Germany a net energy system (NEf) has been adopted. This system evaluates feeds on the basis of their ability to promote fattening and then expresses the requirements for maintenance and growth in terms of this unit (NEf) (Nehring and Haeniein, 1973~. In the U.S.S.R.,preparationsare being made to replace the oat unit system by a system based on ME (Van Es, 19761. Details regarding this system are not currently available. It is evident that systems based on the net energy concept are becoming the method of choice for expressing the energy require- ments of growing ruminants. The procedures used to apply the concepts to practical conditions have been more variable. The committees of the NRC have adopted the system of Lof- green and Garrett (1968) for use with growing ruminants. This system assigns to each feedstuff two net energy values. One value, net energy for maintenance (NEm ), is based on that quantity of feed necessary to prevent tissue loss from the animal's body (RE = 0 and NEm = km EME] Mcal/kg DM). The second value, net energy for gain (NEg), is the amount of energy stored in the animal's body as a result of a given amount of a feedstuff being consumed above that necessary for maintenance (RE is positive and NEg = kg [ME] Mcal/kg DM). By using this procedure the animal's total energy requirement is factored into a maintenance requirement (requirement for NEm ~ and a growth requirement (requirement for NEg). The net energy required for growth (equivalent to RgE) can be factored into the major components of energy required for fat deposition and energy required for protein deposition; i.e., RgE = RfE + RpE. Since estimates of the efficiency of ME use for RgE indicate that kf and kp are not identical (Kielanowski,
29 1976; Puliar and Webster, 1977; Thorbek, 1 977) and that kf ~ kp, it is apparent that kg and therefore NEg are not constant but vary depending upon the composition of the gain. The com- parative slaughter procedure used by Lofgreen and Garrett (1968) assigns an average NEg value to a feedstuff as determined under practical feeding conditions. These NEg values are used in con- junction with concurrently determiner! relationships that trans- late RE into a weight unit (k~Iogram of gain). The relationship between RE and weight gain is necessarily variable depending upon species, breed, body size, rate of growth, and sex. The convention using only NEm and NEg to state feed values and energy requirements for sheep does not give a separate al- lowance for woo] growth. The energy retained in the fleece (VE) is thus added to the energy retained ire the animal's body tissue. In this instance, RgE = RfE + RpE + RVE. The separate efficiency of metabolizable energy use for wool growth (kv) has not been estimated. Rattray et al. ~ ~ 973) reported that the relationship to convert RE to a weight basis is unchanged by including woo] energy in RE provided the weight of the woo} is included in the weight gain. The gross efficiency of ME use for conceptus development (ky has been estimated to range between ~ O and ~ 6 percent for cattle and sheep (Garrett et al., 19761. The procedure in use to account for the gestation requirement under the NEm and NEg convention (NRC, 1976) is to add an equivalent amount of energy to the preg- nant animal's maintenance requirement. That is, the NEm require- ment is scaled upward depending upon the estimated energy re- tention in the conceptus. The efficiency of conversion of ME to NEm and NEg deter- mined by comparative slaughter techniques ranges from about 58 to 70 percent and 25 to 50 percent, respectively, as ME concentra- tion in the dry matter of feeds increases from 2. ~ to 3.2 kcal/g. In the system in use in Europe, km and kg have been estimated from experiments conducted in respiration calorimeters. In this instance, km has varied from 66 to 75 percent and kg from 33 to 64 percent as ME concentration increases from 2. ~ to 3.2 kcal/g.
30 The difference between the two techniques (comparative slaughter and respiration calorimetry) may be caused by systematic errors, probably in both techniques, that result in a slight overestimation of energy retention by respiration calorimetry and slight under- estimation of energy conversion to animal tissue in comparative slaughter trials. For example, hair and surface cell loss over the several months of a comparative slaughter trial would not be in- cluded in energy retained, but might be included in short-term balance studies conducted with a respiration calorimetry tech- n~que. There are other differences between the European and the U.S. systems. Perhaps the most controversial is that the European sys- tem uses a level of feeding correction factor to account for a de- pression in the metabolizability of feeds or diets fed at levels above maintenance. This type of correction has not been applied to the data obtained in comparative slaughter trials. Some correction for level of feed is actually inherent in the comparative slaughter meth- ods used because one level of feeding employed during the trials to estimate NEg is ad libitum consumption. The use of the net energy concept as the basis for estimating feed values and requirements for growing ruminants is likely to continue. The general relationships now in use will be improved and eventually become more specific with regard to applications under practical conditions. The refinements of any general system to the point where it can be translated to fit with precision into all, or at least most, practical feeding situations adds to its com- plexity. However, the availability of inexpensive computers and programmable calculators will make it possible to use increasingly sophisticated methods to determine comparative feed values, spe- cific animal requirements, diet formulations, and the prediction of the response of an animal to a particular diet. It is quite likely that feeds will eventually be evaluated on the basis of how they supply energy and nutrients for some specific animal production response in comparison to other feeds avail- able at the same time. Feeds will be evaluated, diets formulated, and animal response predicted by relatively complex computer
31 programs that will use a detailed chemical and physical descrip- tion of each feed and a knowledge of the biochemical, physio- logical, and physical processes involved in animal metabolism. The information necessary for this application has not yet been accumulated. Application to Lactating Ruminants There has been a steady progression in refinement of the energy evaluation of feeds and in the understanding of energy exchanges for lactating ruminants. Requirements and feed values have been stated in all of the following energy terms: TDN, DE, ME, ENE (Morrison, 1956), NEm, NEg, and NET. While they have been criticized as measures of useful energy (Moore et al., ~ 953), TDN and DE values are still widely used, in part because a significant bank of data is available for those terms. As ME and NE data be- come available, they will replace the less precise TDN and DE values. Although ENE values are used and are readily available (Morrison, ~ 956), the specific origin of these values is unclear. Although it is routine to compute such values in feed analysis measurements, ENE is based on prediction from chemical com- position rather than from precise energetic measurements. The assignment of NE values to feeds depends on the physio- logical functions in progress. For example, dairy animals can have the functions of maintenance, growth, gestation, and lactation going on simultaneously, and the NE value of the feed may vary considerably. Although not fully understood, the reasons for variation in NE value include the composition of the product formed, the composition of the diet fed, and the energy cost of maintaining tissue in a static mass (Bull et al., ~ 9761. if animals are lactating, however, the partial efficiencies of ME use for maintenance and fattening are similar to lactation (Moe and Flatt, ~ 969), whereas partial efficiencies differ markedly be- tween maintenance and fattening in the noniactating ruminant (Armstrong et al., 1964; Flatt et al., 1965; Lofgreen and Garrett, 19681.
32 A system for applying the NE concept to lactating dairy cattle has been described by Moe et al. (1972) and takes advantage of the similarity in partial efficiencies of ME use for maintenance and lactation in lactating animals. A single energy value (NE~) is used to define all requirements for the lactating cow and to describe the energy value of feeds. In the development of the NET system, actual respiration calorimetry energy balance data were used, and various physiological functions were described in milk energy equivalents in order to arrive at a specific relationship between ME input and milk energy (NE~) output. For example, an adjustment is made to account for cases where tissue energy balance (TE) is not zero (Moe et al., 19711. Additional adjust- ments included are for the energy cost of consuming nitrogen in excess of that required by the animal (Tyrrell et al., 1970) and for the energy cost of pregnancy (Moe and Tyrrell, 19721. The results of the adjustments produce a situation in which the total response of the animal to a change in energy intake is re- covered as a change in milk energy output. The adjusted data were used to relate milk energy yield to the metabolizable energy intake. Two equations (Moe et al., 1972) are LE (kcal/kg 75) = 0.608 ME (kcal/kg 7 5 ~ - 67.7, and ME (kcal/kg 75) = 1.547 LE(kcal/kg7s)+ 122.1. The maintenance requirement for energy expressed as LE was determined from these equations by setting ME- 0. The two esti- mates of-67.7 and -78.9 average to -73.3, which is a value vir- tually identical to estimates of fasting metabolic rate. This also means that kit is virtually the same as km, so that net energy for maintenance and net energy for lactation can be defined as a single value in the lactating ruminant, or NEm = NET. In practice, the NET required for maintenance is increased by 10 percent to account for environmental conditions different from those of a respiration chamber (NRC, 197Sa). Having defined maintenance requirement
33 in units of NET, the NET value of total mixed rations can be computed. The following relationships have been developed to relate dif- ferent measures of feed energy to NET (Meal/kg DM): NE1 = 0.677 DE (Meal/kg DM) - 0.36, NE1 = 0.702 ME (Meal/kg DM) - 0. 19. NE1 = 0.0266 TDN (% of DM) - 0.12. These relationships are all based on energy values actually mea- sured rather than tabulated values. Thus the influence of level of intake on digestive efficiency is removed from these equations, but must be included when DE, ME, or TDN measured at mainte- nance is to be used to predict NET. In practice, a correction for level of intake has been used in the development of the NET value of feeds by assuming an average decline in digestibility of 4 per- cent per multiple of maintenance and an average intake of three times maintenance. The relationship between NET (Meal/kg DM) and TDN thus becomes (N R C, ~ 97Sa) NE1 = 0.0245 TDN Coo of DM) - 0. 12. The use of ME to describe the energy requirement of lactating ruminants is common in some areas of Europe, particularly in the United Kingdom (Ministry of Agriculture, Fisheries and Food, 1975~. The system is based on NE for the animal requirements, converted to ME by using the partial efficiency of use of ME for maintenance, lactation, and body weight change as follows: MEm = NEm /km Here NEm is assumed to be 94.6 kcal/W 73 and km is assumed to be 0.72. Thus MEm=131.4kcal/W73.
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
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.
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
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.
- ~ 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
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).
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
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 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 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
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,
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.
