1
Energy

ENERGY UNITS

Energy is defined as the potential to do work and can be measured only in reference to defined, standard conditions; thus, all defined units are equally absolute. The joule is the preferred unit of expressing electrical, mechanical, and chemical energy. The joule can be converted to ergs, watt-seconds, and calories; the converse is also true. Nutritionists now standardize their combustion calorimeters using specifically purified benzoic acid, the energy content of which has been determined in electrical units and computed in terms of joules/g mole. The calorie has been standardized to equal 4.184 joules and is approximately equal to the heat required to raise the temperature of 1 g of water from 16.5° to 17.5° C. In practice the calorie is a small amount of energy; thus, the kilocalorie (1 kcal=1,000 calories) and megacalorie (1 Mcal=1,000 kcal) are more convenient for use in conjunction with animal feeding standards.

A number of abbreviations have been used to describe energy fractions in the animal system. Many of the abbreviations used throughout this text are those recommended in Nutritional Energetics of Domestic Animals and Glossary of Energy Terms (National Research Council, 1981a). Gross energy (E) or heat of combustion is the energy released as heat when an organic substance is completely oxidized to carbon dioxide and water. E is related to chemical composition, but it does not provide any information regarding availability of that energy to the animal. Thus, E is of limited use for assessing the value of a particular diet or dietary ingredient as an energy source for the animal.

Expressing Energy Values of Feeds

E of the food minus the energy lost in the feces is termed digestible energy (DE). DE as a proportion of E may vary from 0.3 for a very mature, weathered forage to nearly 0.9 for processed, high-quality cereal grains. DE has some value for feed evaluation because it reflects diet digestibility and can be measured with relative ease; however, DE fails to consider several major losses of energy associated with digestion and metabolism of food. As a result, DE overestimates the value of high-fiber feedstuffs such as hays or straws relative to low-fiber, highly digestible feedstuffs such as grains. Total digestible nutrients (TDN) is similar to DE but includes a correction for digestible protein. TDN has no particular advantages or disadvantages over DE as the unit to describe feed values or to express the energy requirements of the animal. TDN can be converted to DE by the equation

Metabolizable energy (ME) is defined as E minus fecal energy (FE), urinary energy (UE), and gaseous energy (GE) losses, or ME=DE-(UE+GE). ME is an estimate of the energy available to the animal and represents an accounting progression to assess food energy values and animal requirements. ME, however, has many of the same weaknesses as DE; and because UE and GE are highly predictable from DE, ME and DE are strongly correlated. Also, the main source of GE (the primary gas being methane) is microbial fermentation, which also results in heat production. This heat is useful in helping to maintain body temperature in cold-stressed animals but is otherwise an energy loss not accounted for by ME. For most forages and mixtures of forages and cereal grains, the ratio of ME to DE is about 0.8 but can vary considerably (Agricultural Research Council, 1980; Commonwealth Scientific and Industrial Research Organization, 1990) depending on intake, age of animal, and feed source. The definition of ME and the energy balance identity indicate ME can appear only as heat production (HE) or retained energy (RE), that is, ME=HE+RE. As indicated by this relationship, a major value of ME is used as a reference



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Nutrient Requirements of Beef Cattle: Seventh Revised Edition, 1996 1 Energy ENERGY UNITS Energy is defined as the potential to do work and can be measured only in reference to defined, standard conditions; thus, all defined units are equally absolute. The joule is the preferred unit of expressing electrical, mechanical, and chemical energy. The joule can be converted to ergs, watt-seconds, and calories; the converse is also true. Nutritionists now standardize their combustion calorimeters using specifically purified benzoic acid, the energy content of which has been determined in electrical units and computed in terms of joules/g mole. The calorie has been standardized to equal 4.184 joules and is approximately equal to the heat required to raise the temperature of 1 g of water from 16.5° to 17.5° C. In practice the calorie is a small amount of energy; thus, the kilocalorie (1 kcal=1,000 calories) and megacalorie (1 Mcal=1,000 kcal) are more convenient for use in conjunction with animal feeding standards. A number of abbreviations have been used to describe energy fractions in the animal system. Many of the abbreviations used throughout this text are those recommended in Nutritional Energetics of Domestic Animals and Glossary of Energy Terms (National Research Council, 1981a). Gross energy (E) or heat of combustion is the energy released as heat when an organic substance is completely oxidized to carbon dioxide and water. E is related to chemical composition, but it does not provide any information regarding availability of that energy to the animal. Thus, E is of limited use for assessing the value of a particular diet or dietary ingredient as an energy source for the animal. Expressing Energy Values of Feeds E of the food minus the energy lost in the feces is termed digestible energy (DE). DE as a proportion of E may vary from 0.3 for a very mature, weathered forage to nearly 0.9 for processed, high-quality cereal grains. DE has some value for feed evaluation because it reflects diet digestibility and can be measured with relative ease; however, DE fails to consider several major losses of energy associated with digestion and metabolism of food. As a result, DE overestimates the value of high-fiber feedstuffs such as hays or straws relative to low-fiber, highly digestible feedstuffs such as grains. Total digestible nutrients (TDN) is similar to DE but includes a correction for digestible protein. TDN has no particular advantages or disadvantages over DE as the unit to describe feed values or to express the energy requirements of the animal. TDN can be converted to DE by the equation Metabolizable energy (ME) is defined as E minus fecal energy (FE), urinary energy (UE), and gaseous energy (GE) losses, or ME=DE-(UE+GE). ME is an estimate of the energy available to the animal and represents an accounting progression to assess food energy values and animal requirements. ME, however, has many of the same weaknesses as DE; and because UE and GE are highly predictable from DE, ME and DE are strongly correlated. Also, the main source of GE (the primary gas being methane) is microbial fermentation, which also results in heat production. This heat is useful in helping to maintain body temperature in cold-stressed animals but is otherwise an energy loss not accounted for by ME. For most forages and mixtures of forages and cereal grains, the ratio of ME to DE is about 0.8 but can vary considerably (Agricultural Research Council, 1980; Commonwealth Scientific and Industrial Research Organization, 1990) depending on intake, age of animal, and feed source. The definition of ME and the energy balance identity indicate ME can appear only as heat production (HE) or retained energy (RE), that is, ME=HE+RE. As indicated by this relationship, a major value of ME is used as a reference

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Nutrient Requirements of Beef Cattle: Seventh Revised Edition, 1996 unit and as a starting point for most systems based on the net energy (NE) concept. The value of feed energy for the promotion of energy retention is measured by determining the RE at two or more amounts of intake energy (IE). The NE of a feed or diet has classically been illustrated by the equation: Determination of NE by this method assumes the relationship between RE and feed intake is linear. Actually the relationship is curvilinear and shows a diminishing return effect (Garrett and Johnson, 1983). The relationship is conventionally approximated by two straight lines. The intersection of the two lines is the point at which RE=0 and is defined as maintenance (M). Conversely, when RE=0, ME=HE. The relationship between feed intake and body tissue loss (negative RE) comprises one portion of the curve and the relationship between body tissue gain (positive RE) comprises a second portion of the curve. The heat production at zero feed intake (HeE) is equivalent to the animal’s NE requirement for maintenance. The ability of the food consumed to meet the NE required for maintenance is expressed as NEm and is represented by the following expression: where Im is the amount of feed consumed at RE=0. Similarly, the value of feed consumed to promote energy retention is represented by the expression NEr and is determined as where (I-Im) represents the amount of feed consumed in excess of maintenance requirements. The relationship ME=RE+HE can be rewritten in terms of NE. Thus, HE can be partitioned into HeE, HjE and HiE (heat increment of intake energy) as Because in practical situations the heat of activity associated with obtaining feed (HjE) is often included with HeE, the expression becomes The NEr used in this expression does not distinguish among different forms in which energy may be retained, such as body tissue (TE), milk, (LE) or tissues of the conceptus (YE). Thus, the former expression might be expanded such that in a pregnant lactating heifer it becomes: where NEr, NEl, and NEg are equivalent to RE, LE, YE, and TE, respectively. Thus, In this expression, a portion of the heat increment (HiE) is associated with the feed consumed for maintenance and each of the productive functions. The primary advantages of an NE system are that animal requirements stated as net energy are independent of the diet, and the energy value of feeds for different physiological functions are estimated separately—for example, NEm, NEg, NEl, NEy. This requires, however, that each feed must be assigned multiple NE values because the value varies with the function for which energy is used by the animal. Alternatively, the animal’s energy requirement for various physiological functions may be expressed in terms of a single NE value, provided the relationships among efficiencies of utilization of ME for different functions are known. Relationships for converting ME values to NEm and NEg (Mcal/kg DM) have been reported by Garrett (1980) and are The NEm and NEg values used in the derivation of these equations were based on comparative slaughter studies involving 2,766 animals fed complete, mixed diets at or near ad libitum intake for 100 to 200 days. Digestion trials were conducted on most diets fed at about 1.1 times the maintenance amount. The ME values were estimated as DE * 0.82. Data were not uniformly distributed across the range of ME concentrations encountered in practical situations (1 percent, <1.9 Mcal/kg; 22 percent, 1.9–2.6 Mcal/kg; 65 percent, 2.6–2.9 Mcal/kg; 12 percent, >2.9 Mcal/kg). Caution should be exercised in use of these equations for predicting NEm or NEg values for individual feed ingredients or for feeds outside the ranges indicated above. The relationship between DE and ME can vary considerably among feed ingredients or diets as a result of differences in intake, rate of digestion and passage, and composition (for example, fiber vs starch vs fat). In addition, conversion of ME to NEm or NEg may vary beyond that associated with variation in dietary ME in part because of differences in composition of absorbed nutrients. Available data, as discussed in subsequent sections, indicate efficiencies of ME use for lactation and maintenance are similar in beef cattle; thus, energy requirements for lactation have been expressed in NEm units. Efficiency of utilization of ME for accretion of energy in gravid uterine tissues is, likewise, discussed in a subsequent section. Some evidence is available to indicate that the efficiency of utilization of ME for maintenance (km) and pregnancy (ky) vary similarly with changes in ME concentration in the diet (Robinson et al., 1980). For convenience, estimates of

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Nutrient Requirements of Beef Cattle: Seventh Revised Edition, 1996 requirements for beef cows were converted to NEm equivalents. Conversion of requirements for lactation and pregnancy to NEm equivalents allow the energy value of feedstuffs to be adequately described by only two NE values (NEm and NEg). REQUIREMENTS FOR ENERGY Measurement of Maintenance Requirements The maintenance requirement for energy has been defined as the amount of feed energy intake that will result in no net loss or gain of energy from the tissues of the animal body. Processes or functions comprising maintenance energy requirements include body temperature regulation, essential metabolic processes, and physical activity. Energy maintenance does not necessarily equate to maintenance of body fat, body protein, or body weight. Although for many practical situations maintenance may be considered a theoretical condition, it is useful and appropriate to consider maintenance energy requirements separate from energy requirements for “production.” ME required for maintenance functions represents approximately 70 percent of the total ME required by mature, producing beef cows (Ferrell and Jenkins, 1987) and more than 90 percent of the energy required by breeding bulls. The fraction of total ME intake that growing cattle use for maintenance functions is rarely less than 0.40, even at maximum intake. Successful management of beef cattle, whether for survival and production in poor nutritive environments or for maximal production, depends on knowledge of and understanding their maintenance requirements. Basically, three methods have been used to measure maintenance energy requirements. These include the use of long-term feeding trials to determine the quantity of feed required to maintain body weight or, conversely, determine body weight maintained after feeding a predetermined amount of feed for an extended period of time (Taylor et al., 1981, 1986); calorimetric methods (Agricultural Research Council, 1965, 1980); or comparative slaughter (Lofgreen, 1965; Lofgreen and Garrett, 1968). Each approach has advantages as well as limitations. Estimates of feed required for maintenance of body weight, usually measured in long-term feeding trials, are obtainable with relative ease and can be determined with large numbers of cattle. Values obtained generally correlate well with energy maintenance in mature, nonpregnant, nonlactating cattle (Jenkins and Ferrell, 1983; Ferrell and Jenkins, 1985a; Laurenz et al., 1991; Solis et al., 1988). Changes in body composition and composition of weight change in growing, pregnant, or lactating cattle are problematic with this approach. Expression of the results in terms of ME or NE requirements depends on use of information from other approaches. The energy feeding systems of the Agricultural Research Council (ARC) (1965, 1980), Ministry of Agriculture, Fisheries, and Food (MAFF) (1976, 1984), Commonwealth Scientific and Industrial Research Organization (CSIRO) (1990), and Agricultural and Food Research Council (AFRC) (1993), and the energy requirements of dairy cows (National Research Council, 1989) are primarily based on calorimetric methods. Fasting heat production (FHP) measured by calorimetry plus urinary energy lost during the same period provide measures of fasting metabolism (FM), which by definition, equates to net energy required for maintenance (NEm). Measurement conditions are standardized such that animals are fed a specified diet at approximately maintenance for 3 weeks prior to measurement. Animals are trained to the calorimeter and kept in a thermoneutral environment. Measurements are usually made during the third and fourth day after withdrawal of feed. For practical use, FM values are adjusted for the difference between fasted weight of an animal and its liveweight when fed. In addition, recognizing that fasted animals are less physically active than fed animals, ARC (1980) adjusts FM by adding an activity allowance of 1 kcal/kg liveweight for cattle. CSIRO (1990) has incorporated additional corrections for breed, sex, proportional contribution of milk to the diet, energy intake, grazing activity, and cold stress. Because of the complexity and cost of measurements, numbers of animals that can be used is limited. With this approach, measurements are basically acute in that they are made over one or at most a few days. Practical limitations of these systems stem largely from difficulties in adjusting data obtained in well-controlled laboratory environments to the practical feeding situation. The California Net Energy System, proposed by Lofgreen and Garrett (1968) and adopted in the two preceding editions of this volume (National Research Council, 1976, 1984), is based on comparative slaughter methods. In contrast to calorimetry, in which ME intake and HE are measured and RE is determined by difference, comparative slaughter procedures measure ME and RE directly and HE by difference. RE is measured as the change in body energy content of animals fed at two or more levels of intake (one of which approximates maintenance) during a feeding period. RE equates, by definition, to NEg in a growing animal. The slope of the linear regression of RE on ME intake provides an estimate of efficiency of utilization of ME for RE and in growing animals equates to kg. The ME intake at which RE=0 provides an estimate of ME required for maintenance (MEm). By convention, the

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Nutrient Requirements of Beef Cattle: Seventh Revised Edition, 1996 intercept of the regression of log HE on ME intake is used to calculate an estimate of FHP, which equates to NEm. The efficiency of utilization of ME for maintenance (km) is calculated as the ratio of NEm to MEm. These approaches have an advantage over calorimetric methods because they allow experiments to be conducted under situations more similar to those found in the beef cattle industry. They must be conducted over extended time periods, however, to allow accurate assessment of body energy changes. Accurate assessment of body composition at the beginning and end of the feeding period is required. The NEm requirements of beef cattle have been estimated as EBW is the average empty body weight in kilograms (Lofgreen and Garrett, 1968; Garrett, 1980). This expression was derived using data from, primarily, growing steers and heifers of British ancestry that were penned in generally nonstressful environments. Effects of activity and environment are implicitly incorporated into NEm in this system. Similarly, influences of increased feed during the feeding period, altered activity, or environmental effects differing from those at maintenance are implicitly incorporated into estimates of NEg. Application to differing situations requires appropriate adjustments. Variation in Energy Requirements for Maintenance Maintenance energy expenditures vary with body weight, breed or genotype, sex, age, season, temperature, physiological state, and previous nutrition. FHP or NEm is more closely related to a fractional power of EBW than to EBW1.0 (Brody, 1945; Kleiber, 1961); the most proper power has been the subject of much debate. EBW0.75, often referred to as metabolic body weight, was originally used to confer proportionality on measurements of HeE made in species differing considerably in mature weight (for example, mice to elephants). The convention generally adopted is to use EBW0.75 to scale energy requirements for body weight, even though other functions may be more appropriate for specific applications. BREED DIFFERENCES IN MAINTENANCE Armsby and Fries (1911) reported that “scrub” steers utilized energy less efficiently than “good” beef animals. Subsequently, numerous researchers noted differences in energy requirements or efficiencies of energy utilization among breeds of cattle. However, because of differences in procedures and approaches as well as diversity of breeds compared, direct comparison among available data is difficult. Blaxter and Wainman (1966), using calorimetry, noted that Ayrshire steers had 20 percent higher FHP (kcal/BW0.75) than black (Angus type) steers and 6 percent higher than crosses of those breeds. Results of Garrett (1971), using comparative slaughter, indicated that Holstein steers required 23 percent more feed to maintain body energy than Hereford steers. Similarly, Jenkins and Ferrell (1984b) and Ferrell and Jenkins (1985a) indicated feed required for weight or energy stasis in young bulls and heifers was greater in the Simmental breed than in those of the Hereford breed. Those data indicated MEm was, averaged across sexes, 19 percent (126 vs 106 kcal/BW0.75) greater for Simmental than Hereford cattle. Estimates reported for Simmental bulls were equal to those reported by Stetter et al. (1989). Values reported by Andersen (1980) and Byers (1982) indicated Simmental had 6 and 3 percent higher requirements than Herefords, respectively. Conversely, Old and Garrett (1987) and Andersen (1980) found maintenance requirements of Charolais and Hereford steers to be similar. Estimates for growing Friesian cattle average approximately 13 percent higher (5 to 20 percent) than for Charolais (Robelin and Geay, 1976; Vermorel et al., 1976; Geay et al., 1980; Vermorel et al., 1982). Webster et al. (1976, 1982) reported predicted basal metabolism rates of Friesian cattle to be greater than Angus (10 percent), Hereford (31 percent), or Friesian×Hereford (8 percent). Chestnutt et al. (1975) estimated maintenance requirements of Friesian to be 20 percent higher than Friesian×Hereford and 14 percent greater than Angus steers, whereas estimates of Truscott et al. (1983) were 7 percent higher for Friesian than for Hereford steers. Wurgler and Bickel (1985) found no consistent difference in estimates of maintenance requirements among Angus×Braunvieh, Braunvieh, or Friesian steers. Estimates of maintenance requirements of Limousin have been similar to those of Angus (Byers, 1982), Hereford, and Charolais (Andersen, 1980). Results of Webster et al. (1982) and Andersen (1980) indicated Chianina had about 30 percent higher energy expenditures than Angus and Hereford. Several other reports (Vercoe, 1970; Vercoe and Frisch, 1974; Patle and Mudgal, 1975; Frisch and Vercoe 1976, 1977, 1982; van der Merwe and van Rooyen, 1980; Carstens et al., 1989a) indicate that maintenance energy requirements of Bos indicus breeds of cattle, including Africander, Barzona, Brahman, and Sahiwal, are about 10 percent lower, and British crosses with those breeds about 5 percent lower than British breeds. In contrast, data of Ledger (1977) and Ledger and Sayers (1977) suggest maintenance requirements of the Boran may be about 5 percent higher than for Herefords. However, those results appear to conflict with those in the report of Rogerson et al. (1968). Results of Jenkins and Ferrell (1983) and Ferrell and Jenkins (1984a,b,c) indicated maintenance requirements differed among genotypes of mature crossbred cows. ME required for energy stasis (kcal/BW0.75) of nonpregnant, nonlactating Jersey, Simmental, and Charolais sired cows

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Nutrient Requirements of Beef Cattle: Seventh Revised Edition, 1996 (from Angus or Hereford dams) was 112, 123, and 99 percent that of Angus-Hereford (130 kcal/BW0.75) cross cows. Similarly, the results of Lemenager et al. (1980) suggested that energy needs of Simmental×Hereford cows was about 25 percent higher than Hereford cows during gestation, whereas Angus×Hereford and Charolais×Hereford required about 5 and 7 percent more than Herefords. Laurenz et al. (1991) reported that Simmental cows required 21 percent more ME (kcal/BW0.75) than Angus cows. Klosterman et al. (1968) observed no difference in estimated energy requirements to maintain weight of mature nonpregnant nonlactating Hereford and Charolais cows when adjusted for body condition. Similarly, when adjusted for body condition, Hereford×Friesian and White Shorthorn×Galloway cows required similar amounts of energy to maintain liveweight (Russel and Wright, 1983). Estimates of ME (kcal/BW0.75) for energy stasis of nonpregnant, nonlactating Red Poll, Brown Swiss, Gelbvieh, Maine Anjou, and Chianina sired cows (C.L. Ferrell and T.G.Jenkins, unpublished data) were 112, 122, 117, 113, and 108 percent of values for Angus-Hereford (126 kcal/BW0.75) cross cows. Similar values were reported for weight stasis of those cows, with the exception of Gelbvieh and Chianina, which were higher (Ferrell and Jenkins, 1987). In that study, ME (kcal/BW0.75) required for weight stasis of purebred Angus, Hereford, and Brown Swiss were 116, 115 and 155 percent of that estimated for Angus-Hereford crossbreds (119 kcal/BW0.75). Results of Taylor and Young (1968) and Taylor et al. (1986) indicated energy required (recalculated as kcal/BW0.75) for long-term weight equilibrium of British Friesian, Jersey, and Ayrshire cows to be 20 percent higher than that of Angus and Hereford cows. Energy required by Dexter cows was 9 percent higher than the average of Angus and Hereford cows. Thompson et al. (1983) reported estimates indicating ME required for energy stasis was 9 percent higher in Angus×Holstein than in Angus×Hereford cows. Ritzman and Benedict (1938) observed no difference between energy required by Jersey and Holstein cows, whereas Brody (1945) observed slightly higher requirements by Holstein cows than Jersey cows. Solis et al. (1988) reported estimates of ME required for weight and energy stasis for 15 breed or breed crosses from a 5-breed diallel. Simple correlation between the two estimates was 0.84 and the slope of the linear regression was 0.99, indicating good agreement between the two estimates. When pooled, estimates of ME required for energy stasis were 104, 96, 96, 112, and 106 kcal/BW0.75/day for 1/2 Angus, 1/2 Brahman, 1/2 Hereford, 1/2 Holstein, and 1/2 Jersey cows, respectively. Most of these reports observed differences between or among breeds compared and serve to document that considerable variation exists in maintenance requirements among cattle germ plasm resources. However, because of the diversity of breeds, methodologies, conditions, etc., direct comparisons between studies are often tenuous. As a result, the subcommittee selected studies in which British breeds or British breed crosses were compared with other breeds or breed crosses and expressed the results as relative values. It is believed the following generalizations can be made with some confidence, based on the data reviewed in the preceding paragraphs. In growing cattle, Bos indicus breeds of cattle (for example, Africander, Barzona, Brahman, Sahiwal) require about 10 percent less energy than beef breeds of Bos taurus cattle (for example, Angus, Hereford, Shorthorn, Charolais, Limousin) for maintenance, with crossbreds being intermediate. Conversely, dairy or dual-purpose breeds of Bos taurus cattle (for example, Ayrshire, Brown Swiss, Braunvieh, Friesian, Holstein, Simmental) apparently require about 20 percent more energy than beef breeds, with crosses being intermediate. Data involving straightbred, mature cows are more limited. However, available data with straightbreds combined with those of crossbreds, indicate that relative differences between breeds in mature cows is similar to that observed in growing animals. This may be generalized further to indicate, in both adult and growing cattle, that a positive relationship exists between maintenance requirement and genetic potential for measures of productivity (for example, rate of growth or milk production; Webster et al., 1977; Taylor et al., 1986; Ferrell and Jenkins, 1987; Montano-Bermudez et al., 1990). Consistent with this concept, available data also suggest that animals having genetic potential for high-productivity may have less advantage or be at a disadvantage in nutritionally or environmentally restrictive environments (Kennedy and Chirchir, 1971; Baker et al., 1973; Frisch, 1973; Moran, 1976; O’Donovan et al., 1978; Jenkins and Ferrell, 1984b; Ferrell and Jenkins, 1985a,b; Jenkins et al., 1986). This concept is further supported by the reports of Peacock et al. (1976), Ledger and Sayers (1977), and Frisch and Vercoe (1977). Frisch and Vercoe (1980, 1982) have subsequently shown that selection for increased growth in a high-stress environment results in decreased FHP. Results from these and other studies show that correlated responses to selection may result in a genotype/environment interaction. Selection may result in a population of animals highly adapted to a specific environment but less adapted to different environments and with decreased adaptability to environmental changes (Frisch and Vercoe, 1977; Taylor et al., 1986; Jenkins et al., 1991). SEX DIFFERENCES IN MAINTENANCE Garrett (1970) found little difference in estimated fasting HE or ME required for maintenance between steers and heifers. Subsequently, Garrett (1980), in a study based on comparative slaughter experiments involving 341 heifers and 708 steers, concluded that FHP (net energy required

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Nutrient Requirements of Beef Cattle: Seventh Revised Edition, 1996 for maintenance) of steers and heifers is similar. ARC (1980) and CSIRO (1990) similarly concluded fasting metabolism of castrate males and heifers was similar. Ferrell and Jenkins (1985a) estimated similar FHP (kcal/BW0.75/day) for Hereford bulls (70.4) and heifers (69.3), but estimates for Simmental bulls (80.8) were 9 percent higher than for Simmental heifers (74.1). When expressed as ME required for maintenance, Hereford bulls and heifers differed by only 2 percent, but estimates for Simmental bulls were 16.5 percent higher than for Simmental heifers. Pooled across breeds, estimated ME required for energy stasis was 12 percent higher for intact males than for females (123 vs 110 kcal ME/BW0.75/day). Webster et al. (1977) reported that Hereford×Friesian bulls had predicted basal metabolism values about 20 percent higher than steers of the same breed cross. In a subsequent report (Webster et al., 1982), values presented indicated bulls had 13 to 15 percent higher predicted basal metabolism than steers. Geay et al. (1980) also suggested higher maintenance requirements of bulls than heifers. ARC (1980) and CSIRO (1990), cited the report of Graham (1968) as indicating rams had 18 percent higher fasting metabolism than wethers and ewes. However, Bull et al. (1976) and Ferrell et al. (1979) estimated the ME required for maintenance of rams to be only 2 to 3 percent higher than for ewe lambs. The average of available data, if the sheep data of Bull et al. (1976) and Ferrell et al. (1979) are excluded, support the conclusion of ARC (1980) and CSIRO (1990) that maintenance requirements of bulls are 15 percent higher than that of steers or heifers of the same genotype. AGE EFFECTS ON MAINTENANCE The concept that maintenance per unit of size declines with age in cattle and sheep (Blaxter, 1962; Graham et al., 1974) has been generally accepted. Data from sheep, predominately castrate males, generally support this view (Graham and Searle, 1972a,b; Graham, 1980). The equation of Graham et al. (1974) indicated maintenance decreased exponentially and was related to age by the relationship e-0.08age, which indicates the decrease was 8 percent per year. The generalized equation reported by Corbett et al. (1985) for sheep and cattle, which was later adopted by CSIRO (1990), indicates maintenance decreases 3 percent per year. CSIRO (1990) indicated a minimum of 84 percent of initial values to be attained at about 6 years. Young et al. (1989) noted metabolic rate deviated substantially from allometric relationships; deviations were greatest during times of highest relative growth rate. They further suggested that significant deviations may also occur in association with other productive functions. Data reported from cattle are less consistent. Blaxter et al. (1966) found little influence of age (15 to 81 weeks), other than that associated with weight, on maintenance of steers. Results of Blaxter and Wainman (1966), Taylor et al. (1981) and Birkelo et al. (1989) were consistent with those findings. Vermorel et al. (1980) indicated maintenance requirements of cattle changed little between 5 and 34 weeks of age, but data of Carstens et al. (1989a) indicate a 6 percent decrease in FHP and an 8 percent decrease in ME required for maintenance between 9 and 20 months. Conversely, data reported by Tyrrell and Reynolds (1988) indicated ME required for maintenance (kcal/SBW0.75) increased 14 percent in beef heifers as weight increased from 275 to 475 kg. To our knowledge, direct comparisons of mature, productive females to younger or nonreproducing animals are not available. Indirect evidence (see above) suggests that maintenance of mature, productive cows is not less than that of younger, growing animals postweaning. SEASONAL EFFECTS ON MAINTENANCE Although, typically, effects of season have been associated with effects of temperature, it has become increasingly evident that season per se may have significant effects on maintenance requirements of cattle and sheep. Christopherson et al. (1979), Blaxter and Boyne (1982), and Webster et al. (1982) noted lower maintenance requirements of sheep, cattle, and bison during the fall of the year. Predicted basal metabolism of cattle was 90.3, 92.0, 78.9, and 86.3 kcal/BW0.75 during weeks 0 to 16, 17 to 32, 33 to 48 and 49 to 52, respectively, in Scotland (Webster et al., 1982). Data reported from Colorado by Birkelo et al. (1989) indicate FHP during fall, winter, and spring measurements were 90.7, 95.6, and 96.2 percent of FHP measured during the summer, but MEm did not consistently follow this pattern. Estimates of energy required for weight stasis of mature cows by Byers et al. (1985) for fall, winter, and spring were 86, 86, and 92 percent and those for energy stasis were 94, 102, and 100 percent of estimates made during the summer. Laurenz et al. (1991) reported similar effects of season on energy required for weight stasis of Angus and Simmental cows and for energy stasis of Angus cows but a dissimilar pattern for energy stasis of Simmental cows. Byers and Carstens (1991) reported further observations and indicated that as cow fatness increased, maintenance requirements increased during the spring and summer but decreased during the fall and winter. Walker et al. (1991) clearly demonstrated that seasonal effects in ewes are related to photoperiod. Possible season/genotype or latitude effects have not been quantified. TEMPERATURE EFFECTS ON MAINTENANCE For a detailed review, the reader is referred to the report, Effect of Environment on Nutrient Requirements of Domestic Animals (National Research Council, 1981b). Heat production in cattle arises from tissue metabolism and from

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Nutrient Requirements of Beef Cattle: Seventh Revised Edition, 1996 fermentation in the digestive tract. Animals dissipate heat by evaporation, radiation, convection, and conduction. Both heat production and dissipation are regulated to maintain a nearly constant body temperature. Within the zone of thermoneutrality, HE is essentially independent of temperature and is determined by feed intake and the efficiency of use; body temperature control is primarily via regulation of heat dissipation. When effective ambient temperature increases above the zone of thermoneutrality—that is, higher than the upper critical temperature (UCT)—productivity decreases, primarily as a result of reduced feed intake. In addition, elevated body temperature results in increased tissue metabolic rate and increased “work” of dissipating heat (for example, increased respiration and heart rates); consequently, energy requirements for maintenance increase. Conversely, when effective ambient temperature decreases below the zone of thermoneutrality—that is, below the lower critical temperature (LCT)—HE produced from “normal” tissue metabolism and fermentation is inadequate to maintain body temperature. As a result, animal metabolism must increase to provide adequate heat to maintain body temperature. Consequently energy requirements for maintenance increase. Both UCT and LCT vary with the rate of heat production in thermoneutral conditions and the animals ability to dissipate or conserve heat. As noted in other sections of this report, heat production of animals in thermoneutral conditions may differ substantially as functions of feed intake, physiological state, genotype, sex, and activity. The word acclimatization is used to describe adaptive changes in response to changes in the climatic conditions and include behavioral as well as physiological changes. Behavioral modification includes using variation in terrain or other topographical features such as windbreaks, huddling in groups, or changing posture to minimize heat loss in cold and during decreased activity, seeking shade to decrease exposure to radiant heat, seeking a hill to increase exposure to wind, or wading in water to increase heat dissipation in high temperatures. Physiological adaptations include changes in basal metabolism, respiration rate, distribution of blood flow to skin and lungs, feed and water consumption, rate of passage of feed through the digestive tract, hair coat, and body composition. Physiological changes usually associated with acute temperature changes include shivering and sweating as well as acute changes in feed and water consumption, respiration rate, heart rate, and activity. It should also be noted that animals differ greatly in their behavioral responses and in their ability to physiologically adapt to the thermal environment. Genotype differences are particularly evident in this regard. Recognizing the importance of adaptation, the National Research Council committee (1981b), relying primarily on the results of Young (1975a,b), concluded that required NEm of cattle adapted to the thermal environment is related to the previous ambient (air) temperature (Tp, °C) in the following manner: This equation indicates that the NEm requirement of cattle changes by 0.0007 Mcal/BW0.75 for each degree that previous ambient temperature differed from 20° C. It should be noted that these corrections for previous temperature are largely opposite the photoperiod effect discussed previously. Heat or cold stress occur when effective ambient temperature is higher than UCT or less than LCT. UCT and LCT are functions of how much heat the animal produces and how much heat is lost to the environment. HE of the animal may be calculated as shown previously: where ME is ME intake and RE is retained energy, which may include NEg, NEl, NEy, etc. (all expressed relative to BW0.75). Cold Stress Both environmental and animal factors contribute to differences in heat loss from the animal. Environmental factors include air movement, precipitation, humidity, contact surfaces, and thermal radiation. Although results are not totally satisfactory, numerous efforts have been made to integrate these effects with animal responses. Factors contributing to differences in animal heat loss from conduction, convection, and radiation are surface area (SA), which includes surface or external insulation (EI), and internal or tissue insulation (TI). Evaporative losses are affected by respiration volume as well as SA, EI, and TI. Respiratory losses, although not quantified by National Research Council (1981b), represent 5 to 25 percent and total evaporative heat losses represent 20 to 80 percent of total heat losses (Ehrlemark, 1991). Surface area is related to body weight by the equation thus, TI (°C/Mcal/m2/day) is primarily a function of subcutaneous fat and skin thicknesses. Typical values are 2.5 for a newborn calf, 6.5 for a 1-month old calf, 5.5 to 8.0 for yearling cattle and 6.0 to 12 for adult cattle. EI is provided by hair coat plus the layer of air surrounding the body. Thus, external insulation is related to hair depth. However, the effectiveness of hair as external insulation is influenced by wind, precipitation, mud, and hide thickness. These effects have been described as follows:

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Nutrient Requirements of Beef Cattle: Seventh Revised Edition, 1996 where EI is expressed as °C/Mcal/m2, WIND is wind speed (kph), and HAIR is effective hair depth (cm). MUD and HIDE are adjustments for mud and hide thickness. Total insulation (IN) is and LCT may be calculated as (National Research Council, 1981b): where LCT, IN, and HE/SA are as described previously. The term He represents the minimal total evaporative heat loss and is estimated (Ehrlemark, 1991) as: The animal can receive or lose heat by solar or long-wave radiation. The net impact of thermal radiation on the animal depends on the difference between the combined solar and long-wave radiation received by the animal and the long-wave radiation emitted by the animal. For animals in bright sunlight, a net gain of heat by thermal radiation usually exists, resulting in an increased effective ambient temperature (EAT) of 3° to 5° C (National Research Council, 1981b). In bright sunlight, this effect lowers LCT by 3° to 5° C. Conversely, CSIRO (1990) have indicated that the rate of heat loss by long-wave radiation increases on cold clear nights resulting in an increase in the LCT. Within the temperature range of -10° to 10° C this effect is about 5°C. The increase in energy required to maintain productivity in an environment colder than the animal’s LCT may be estimated as where MEc is the increase in maintenance energy requirement (Mcal/day), SA is surface area (m2), LCT is lower critical temperature (°C), EAT is effective ambient temperature (°C) adjusted for thermal radiation, and IN is total insulation (°C/Mcal/m2/day). Total net energy for maintenance under conditions of cold stress (NEmc) becomes Heat Stress If ambient temperature and thermal radiation exceed the temperature of the skin surface, the animal cannot lose heat by sensible means (conduction, convection, and radiation) and will gain heat by these routes. Evaporative heat loss occurs from the skin (cutaneous) or through respiration. The effectiveness of both cutaneous and respiratory evaporative heat loss diminishes as relative humidity (RH) of the air increases and is totally ineffective when RH=100. Animals can store some heat in their bodies during the day and dissipate the stored heat during cooler daytime periods or at night, if the animal’s heat production exceeds its ability to dissipate heat; but if hyperthermia persists, animals cannot survive. There has been much study of the various aspects of heat stress on animal performance, but there are no established bases for quantitative description of effects. Ehrlemark (1991), for example, developed a regression of respiratory heat loss on the ratio of ambient temperature minus LCT to body temperature minus LCT but did not include cutaneous evaporative heat loss or the influence of RH. It is generally agreed that adjustments to maintenance energy requirement for heat stress should be based on the severity of heat stress; however, severity can vary considerably among animals, depending on animal behavior, acclimatization, diet, level of productivity, radiant heat load, or genotype. The type and intensity of panting by an animal can provide an index for appropriate adjustment in maintenance requirement—an increase of 7 percent when there is rapid shallow breathing and 11 to 25 percent when there is deep, open-mouth panting (National Research Council, 1981b). With severe heat, feed consumption is reduced and consequently metabolic heat production and productivity are reduced. EFFECTS OF PHYSIOLOGICAL STATE ON MAINTENANCE Total heat production increases during gestation (Brody, 1945). Although indirect evidence is available to suggest maintenance requirements of cows increase during gestation (Brody, 1945; Kleiber, 1961; Ferrell and Reynolds, 1985), an increase has not been directly measurable by comparative slaughter evaluations (Ferrell et al., 1976). Increased heat production associated with pregnancy, for the purpose of estimating energy requirements, may be assumed to be attributable to the productive process of pregnancy. In contrast, Moe et al. (1970) estimated ME requirements for maintenance to be 22 percent higher in lactating than in nonlactating cows (primarily Holstein). A similar difference (23 percent) was reported by Flatt et al. (1969), whereas Ritzman and Benedict (1938) reported a larger (49 percent) difference. Neville and McCullough (1969) and Neville (1974) using Hereford cows and different approaches, estimated the maintenance requirement of lactating cows to be more than 30 percent higher than nonlactating cows. The reports of Patle and Mudgal (1975, 1977) agree with those observations, whereas data of Ferrell and Jenkins (1985b, 1987; and unpublished data) suggest a difference of 10 to 20 percent. Taken in total, available data indicate maintenance requirements of lactating cows to be about 20 percent higher than those of nonlactating cows.

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Nutrient Requirements of Beef Cattle: Seventh Revised Edition, 1996 EFFECTS OF ACTIVITY ON MAINTENANCE Few data are available regarding efficiency of ME use for muscular work. In addition, it may be debated whether activity is a maintenance or productive function. It is highly probable that grazing cattle walk considerably further than penned animals and, therefore, expend more energy for work; however, the extent to which grazing animals expend more energy standing, changing positions, eating, or ruminating than penned cattle is not well documented. It is recognized that energy expenditure for work by grazing cattle is influenced by numerous factors including herbage quality and availability, topography, weather, distribution of water, genotype, or interactions among these factors. Variation among individuals may be substantial. In a review of available literature, CSIRO (1990) estimated the increase in maintenance energy requirements of grazing as compared to penned cattle to be 10 to 20 percent in best grazing conditions and about 50 percent for cattle on extensive, hilly pastures where animals walk considerable distances to preferred grazing areas and water. An alternative approach to estimate the NE required for activity (NEma; CSIRO, 1990) was devised as follows: where DMI is dry matter intake from pasture (kg/day); D is digestibility of dry matter (as a decimal); T is terrain (level, 1.01; undulating, 1.5; or hilly, 2.0), and GF is green forage availability (ton/ha). If no green forage is available, replacement of GF with total forage available (TF) was suggested on the premise that selectivity, hence distance walked, decreases when no green forage is available. Effects of Previous Nutrition/Compensatory Gain The phenomenon of compensatory gain is described as a period of faster or more efficient rate of growth following a period of nutritional or environmental stress. Numerous reports are available to document this phenomena in cattle and other species (Wilson and Osbourn, 1960; Carrol et al., 1963; Lawrence and Pierce, 1964; Hironaka and Kozub, 1973; Lopez-Sanbidet and Verde, 1976; O’Donovan, 1984; Hovell et al., 1987; Abdalla et al., 1988; Drouillard et al., 1991). The response to previous nutritional deprivation is highly variable, however. Data are available, for example, that show that at similar body weights, body fat is decreased (Smith et al., 1977; Mader et al., 1989; Carstens et al., 1991), not changed (Fox et al., 1972; Burton et al., 1974; Rompala et al., 1985) or increased (Searle and Graham, 1975; Tudor et al., 1980; Abdalla et al., 1988) after a period of realimentation. Differences among animal genotypes; severity, nature, and duration of restriction; and nutritional regime and interval of measurement of the response during realimentation are among the many variables contributing to differences. A major component of compensatory growth by animals given abundant feed after a period of restriction is increased feed intake. This component is discussed in more detail in a later section. This response will cause increased gut fill and liveweight, but there is also evidence for higher efficiency of energy use. Several reports (Graham and Searle, 1979; Thompsen et al., 1980; Carstens et al., 1991) have provided evidence to suggest higher net efficiency of ME use for body energy gain. The duration of these effects is subject to debate, however (Butler-Hogg, 1984; Ryan et al., 1993a,b). Results of studies reported by Marston (1948) have contributed to an understanding of the other possible mechanisms involved in compensatory growth. Those results showed that level of feed intake may affect the metabolic rate of sheep and cattle. These and other reports (Graham and Searle, 1972a,b; Graham et al., 1974; Graham and Searle, 1975; Thomson et al., 1980; Ferrell and Koong, 1987; Ferrell et al., 1986) have shown that fasting heat production decreases in response to decreased feed intake. Similarly, several reports (Wilson and Osbourn, 1960; Walker and Garrett, 1970; Foot and Tulloh, 1977; Ledger, 1977; Ledger and Sayers, 1977; Gray and McCracken, 1980; Andersen, 1980; Corbett et al., 1982) have shown that maintenance in rats, swine, cattle, and sheep is decreased after periods of decreased nutritional intake. Some of the possible explanations for altered metabolism associated with different planes of nutrition have been discussed by Milligan and Summers (1986), Ferrell (1988), and Johnson et al. (1990). Briefly, metabolic bases for changes include altered rates of ion pumping and metabolite cycling (Milligan and Summers, 1986; Harris et al., 1989; Summers et al., 1988; McBride and Kelly, 1990; Lobley et al., 1992) and altered size and metabolic rate of visceral organs (Canas et al., 1982; Koong et al., 1982, 1985; Burrin et al., 1989). There is much, although not total, support for the general conclusion that maintenance is reduced during and for some time after a period of feed restriction (Graham and Searle, 1972a; Thorbek and Henckel, 1976; Andersen, 1980; Ledger and Sayers, 1977; Schnyder et al., 1982; Stetter et al., 1989); however, reports on the extent of reduction have been variable, and range from about 10 percent to more than 50 percent. Little definitive information is available regarding the duration of the reduced maintenance or, stated another way, the length of time that an animal exhibits compensatory gain after it has access to abundant feed is not well defined. Further, critical description of animals such that expected degree of compensation can be predicted with confidence, without knowing their genotype and history (the nature and severity of restriction, etc.) is lacking. Because of these types of

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Nutrient Requirements of Beef Cattle: Seventh Revised Edition, 1996 problems, generalizations are difficult, although several mathematical descriptions have been proposed (Baldwin et al., 1980; Corbett et al., 1985; Koong et al., 1985; Frisch and Vercoe, 1977). A reduction in maintenance of 20 percent for a compensating animal seems a reasonable generalization (Thorbek and Henckel, 1976; Crabtree et al., 1976; Frisch and Vercoe, 1977; Andersen, 1980; Baldwin et al., 1980; Thomson et al., 1980; Vermorel et al., 1982; Schnyder et al., 1982; Koong et al., 1982, 1985; Koong and Nienaber, 1987; Webster et al., 1982; Wurgler and Bickel, 1985; Ferrell et al., 1986; Birkelo et al., 1989; Burrin et al., 1989; Carstens et al., 1989b). The duration of reduced maintenance is subject to the extent and duration of restricted growth and to nutritional regimen during the recovery periods; typically, 60 to 90 days of compensation is expected. Use of Energy from Weight Loss Animals, particularly in a pasture or range situation, intermittently lose body weight when feed quantity or quality is inadequate to meet the animal’s nutrient requirements. Available data indicate composition of liveweight loss is approximately equal to the composition of liveweight gain in animals (Agricultural Research Council, 1980; Commonwealth Scientific and Industrial Research Organization, 1990). Thus, the energy content of liveweight loss and gain are similar. Energy content and composition of weight gain are discussed in subsequent sections. Buskirk et al. (1992) argued that the energy content of empty body weight gain in mature cows varies, depending on cow body condition. They estimated energy content of empty body weight change in cows with body condition scores (1 to 5 scale) of 1, 2, 3, 4, and 5 to be 2.57, 3.82, 5.06, 6.32, and 7.57 Mcal/kg, respectively. Similarly, CSIRO (1990) adopted relationships established by Hulme et al. (1986) that indicate energy content of liveweight change in dairy cattle increases linearly from 3.0 to 7.1 Mcal/kg as condition score increases from 1 to 8 (on a scale of 1 to 8). Composition of weight change in mature cows is discussed in greater detail in Chapter 3. Although limited data are available, data from sheep (Marston, 1948), dairy cows (Flatt et al., 1965; Moe et al., 1970) and beef cows (Russel and Wright, 1983) indicate the efficiency of use of energy from body tissue loss for maintenance or milk production to be 77 to 84 percent with the mean being approximately 80 percent. REFERENCES Abdalla, H.O., D.G.Fox, and M.L.Thonney. 1988. Compensatory gain by Holstein calves after underfeeding protein. J. Anim. Sci. 66:2687–2695. Agricultural and Food Research Council. 1993. Energy and Protein Requirements of Ruminants. Wallingford, U.K.: CAB International. Agricultural Research Council. 1965. The Nutrient Requirements of Farm Livestock. No. 2. Ruminants. London, U.K.: Agricultural Research Council. Agricultural Research Council. 1980. The Nutrient Requirements of Ruminant Livestock: Technical Review. Farnham Royal, U.K.: Commonwealth Agricultural Bureaux. Andersen, B.B. 1980. Feeding trials describing net requirements for maintenance as dependent on weight, feeding level, sex and genotype. Ann. Zootech. 29:85–92. Armsby, H.P., and J.A.Fries. 1911. The Influence of Type and of Age upon the Utilization of Feed by Cattle. Bull. No. 128. Washington, D.C.: U.S. Department of Agriculture, Bureau of Animal Industry. Baker, F.S., Jr., A.Z.Palmer, and J.W.Carpenter. 1973. Brahman×European crosses vs British breeds. Pp. 227–284 in Crossbreeding Beef Cattle, Series 2, M.Koger, T.J.Cunha, and P.C.Wainick, eds. Gainesville, Fla.: University of Florida Press. Baldwin, R.L., N.E.Smith, J.Taylor, and M.Sharp. 1980. Manipulating metabolic parameters to improve growth rate and milk secretion. J. Anim. Sci. 51:1416–1428. Birkelo, C.P., D.E.Johnson, and H.W.Phetteplace. 1989. Plane of nutrition and season effects on energy maintenance requirements of beef cattle. Energy Metab. Proc. Symp. 43:263–266. Blaxter, K.L. 1962. The Energy Metabolism of Ruminants. Springfield, Ill.: Charles C Thomas. Blaxter, K.L., and A.W.Boyne. 1982. Fasting and maintenance metabolism of sheep. J. Agric. Sci. Camb. 99:611–620. Blaxter, K.L., and F.W.Wainman. 1966. The fasting metabolism of cattle. Br. J. Nutr. 20:103–111. Blaxter, K.L., J.L.Clapperton, and F.W.Wainman. 1966. Utilization of the energy and protein of the same diet by cattle of different ages. J. Agric. Sci. Camb. 67:67–75. Brody, S. 1945. Bioenergetics and Growth. New York: Hafner. Bull, L.S., H.F.Tyrrell, and J.T.Reid. 1976. Energy utilization by growing male and female sheep and rats, by comparative slaughter and respiration techniques. Energy Metab. Proc. Symp. 19:137–140. Burrin, D.G., C.L.Ferrell, and R.A.Britton. 1989. 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Nutrient Requirements of Beef Cattle: Seventh Revised Edition, 1996 Carstens, G.E., D.E.Johnson, M.A.Ellenberger, and J.D.Tatam. 1991. Physical and chemical components of the empty body during compensatory growth in beef steers. J. Anim. Sci. 69:3251–3264. Chestnutt, D.M.B., R.Marsh, J.G.Wilson, T.A.Steqart, T.A.McCillough, and T.McCallion. 1975. Effects of breed of cattle on energy requirements for growth. Anim. Prod. 21:109–119. Christopherson, R.J., R.J.Hudson, and M.K.Christopherson. 1979. Seasonal energy expenditures and thermoregulatory response of bison and cattle. Can J. Anim. Sci. 59:611–617. Commonwealth Scientific and Industrial Research Organization. 1990. Feeding Standards for Australian Livestock: Ruminants. East Melbourne, Victoria, Australia: CSIRO Publications. Corbett, J.L., E.P.Furnival, and R.S.Pickering. 1982. Energy expenditure at pasture of shorn and unshorn border Leichester ewes during late pregnancy and lactation. Energy Metab. Proc. Symp. 29:34–37. Corbett, J.L., M.Freer, and N.M.Graham. 1985. A generalized equation to predict the varying maintenance metabolism of sheep and cattle. Energy Metab. Proc. Symp. 32:62–65. Crabtree, R.M., M.Kay, and A.J.F.Webster. 1976. The net availabilities of ME for body gain of two pelleted diets offered to Hereford×Friesian castrate males over different live-weight ranges. Anim. Prod. 22:156–157. Drouillard, J.S., C.L.Ferrell, T.J.Klopfenstein, and R.A.Britton. 1991. Compensatory growth following metabolizable protein or energy restrictions in beef steers. J. Anim. Sci. 69:811–818. Ehrlemark, A. 1991. Heat and Moisture Dissipation from Cattle: Measurements and Simulation Model. Ph.D. dissertation. Swedish University of Agricultural Sciences, Uppsala, Sweden. Ferrell, C.L. 1988. Contribution of visceral organs to animal energy expenditures. J. Anim. Sci. 66(Suppl. 3):23–34. Ferrell, C.L., and T.G.Jenkins. 1984a. A note on energy requirements for maintenance of lean and fat Angus, Hereford and Simmental cows. Anim. Prod. 35:305–309. Ferrell, C.L., and T.G.Jenkins. 1984b. Relationships among various body components of mature cows. J. Anim. Sci. 58:222–233. Ferrell, C.L., and T.G.Jenkins. 1984c. Energy utilization by mature, nonpregnant, nonlactating cows of different breeds. J. Anim. Sci. 58:234–243. Ferrell, C.L., and T.G.Jenkins. 1985a. Energy utilization by Hereford and Simmental males and females. Anim. Prod. 41:53–61. Ferrell, C.L., and T.G.Jenkins. 1985b. Cow type and the nutritional environment: Nutritional aspects. J. Anim. Sci. 61:725–741. Ferrell, C.L., and L.P.Reynolds. 1985. Oxidative metabolism of gravid uterine tissues of the cow . Energy Metab. Proc. Symp. 32:298–301. Ferrell, C.L., and T.G.Jenkins. 1987. Influence of biological type on energy requirements. Pp. 1–7 in Proceedings of the Grazing Livestock Nutrition Conference. Misc. Publ. Stillwater, Okla.: Agricultural Experiment. Station, Oklahoma State University. Ferrell, C.L., and L.J.Koong. 1987. Response of body organs of lambs to differing nutritional treatments. Energy Metab. Proc. Symp. 32:26–29. Ferrell, C.L., W.N.Garrett, N.Hinman, and G.Gritchting. 1976. Energy utilization by pregnant and nonpregnant heifers. J. Anim. Sci. 42:937–950. Ferrell, C.L., J.D.Crouse, R.A.Field, and J.L.Chant. 1979. Effects of sex, diet and stage of growth upon energy utilization by lambs. J. Anim. Sci. 49:790–801. Ferrell, C.L., L.J.Koong, and J.A.Nienaber. 1986. Effect of previous nutrition on body composition and maintenance energy costs of growing lambs. Br. J. Nutr. 56:595–605. Flatt, W.P., L.A.Moore, N.W.Hooven, and R.D.Plowman. 1965. Energy metabolism studies with a high-producing lactating dairy cow. J. Dairy Sci. 48:797–798. Flatt, W.P., P.W.Moe, A.W.Munson, and T.Cooper. 1969. Energy utilization by high-producing dairy cows. II. Summary of energy balance experiments with lactating Holstein cows. Energy Metab. Proc. Symp. 12:235–239. Foot, J.Z., and N.M.Tulloh. 1977. Effects of two paths of liveweight change on efficiency of feed use and on body composition of Angus steers. J. Agric. Sci. Camb. 88:135–142. Fox, D.G., R.R.Johnson, R.L.Preston, T.R.Dockerty, and E.W. Klosterman. 1972. Protein and energy utilization during compensatory growth in beef cattle. J. Anim. Sci. 34:310–318. Frisch, J.E. 1973. Comparative drought resistance of Bos indicus and Bos taurus crossbred herds in Central Queensland. 2. Relative mortality rates, calf birth weights and weights and weight changes of breeding cows. Aust. J. Exp. Agric. Anim. Husb. 13:117–126. Frisch, J.E., and J.E.Vercoe. 1976. Maintenance requirements, fasting metabolism and body composition in different cattle breeds . Energy Metab. Proc. Symp. 19:209–212. Frisch, J.E., and J.E.Vercoe. 1977. 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Sci. 57(Suppl. 2):478–497. Geay, Y., J.Robelin, and M.Vermorel. 1980. Influence of the ME content of the diet on energy utilization for growth in bulls and heifers. Energy Metab. Proc. Symp. 26:9–12. Graham, N.M. 1968. Effects of undernutrition in late pregnancy on the nitrogen and energy metabolism of ewes. Aust. J. Agric. Res. 19:555–565. Graham, N.M. 1980. Variation in energy and nitrogen utilization by sheep between weaning and maturity. Aust. J. Agric. Res. 31:335–345. Graham, N.M., and T.W.Searle. 1972a. Balance of energy and matter in growing sheep at several ages, body weights and planes of nutrition. Aust. J. Agric. Res. 23:97–108. Graham, N.M., and T.W.Searle. 1972b. Growth in sheep. II. Efficiency of energy and nitrogen utilization from birth to 2 years. J. Agric. Sci. Camb. 79:383–389. Graham, N.M., and T.W.Searle. 1975. Studies on weaner sheep during and after a period of weight stasis. I. Energy and nitrogen utilization. Aust. J. Agric. Res. 26:343–353. 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