3
Growth and Body Reserves

ENERGY AND PROTEIN REQUIREMENTS FOR GROWING CATTLE

Net energy for gain (NEg) is defined herein as the energy content of the tissue deposited, which is a function of the proportion of fat and protein in the empty body tissue gain (Garrett et al., 1959; fat contains 9.367 kcal/g and nonfat organic matter contains an average of 5.686 kcal/g). Simpfendorfer (1974) summarized data from steers of British beef breeds from birth to maturity and found that within cattle of a similar mature size, 95.6 to 98.9 percent of the variation in the chemical components and empty body energy content was associated with the variation in weight (Figure 3–1 A and B). When energy does not limit growth, the empty body contains an increasingly smaller percentage of protein and an increasingly larger percentage of fat, and reaches chemical maturity when additional weight contains little additional protein. Figure 3–1A shows that steers in this data base contained little additional protein in the gain after an SBW of 750 kg. At SBW in excess of 200 to 300 kg, there appeared to be an influence of the effect of plane of nutrition, as evidenced by the scatter of points on the plot of body fat content (Figure 3–1A).

The energy content of weight gain across a wide range of ME intakes and rates of gain was described in equation form by Garrett (1980), equations that were adapted by the Subcommittee on Beef Nutrition for use in the preceding edition of this volume (National Research Council, 1984). This data set included 72 comparative slaughter experiments conducted at the University of California between 1960 and 1980 of approximately 3,500 cattle receiving various diets. The equation developed with British-breed steers describes the relationship between retained energy (RE) and empty body weight gain (EBG) for a given empty body weight (EBW);

Eq. 3–1

Because energy is retained as either protein or fat, the composition of the gain at different weights can be estimated from RE computed in Eq. 3–1 (Garrett, 1987);

Eq. 3–2

Eq. 3–3

Using these relationships, the relationship between stage of growth (percentage of mature weight), rate of gain, and composition of gain can be computed (Table 3–1). The resulting NEg requirement in Table 3–1 for various shrunk body weights (SBW) and shrunk daily gains (SWG) are those presented in Table 1 of the 1984 edition of this volume for a medium-frame steer, except the last line shows requirements for 1.3 kg SWG rather than 1.2 kg SWG. These ranges in SWG represent those in that data base. Several relationships are shown in this table. First, energy content of the gain at a particular SWG increases with weight in a particular body size. Second, protein and fat content of the gain and expected body fat at a particular weight depend on rate of gain. Eqs. 3–1, 3–2, and 3–3 were used to compute the expected percentage of body fat at different SBW from the NE concentrations in the gain (Mcal/kg) when the 1984 National Research Council (NRC) medium-frame steer was grown from 200 kg SBW at 11.5 percent body fat at SWG of 1 kg/day (1.01 Mcal NEg/kg diet) for the first 100 kg and 1.3 kg/day (1.35 Mcal NEg/kg diet) to various SBW (Table 3–1). Eqs. 3–1 and 3–2 were used for the computations of protein and fat at various SWG, using constants of 0.891 and 0.956, respectively, for converting EBW and EBG to SBW and SWG (National Research Council, 1984). Table 3–1 shows the percentage body fat expected at various weights for the 1984 NRC medium-frame steer with typical two-phase feeding programs (grown on high-quality forage and finished on high-



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Nutrient Requirements of Beef Cattle: Seventh Revised Edition, 1996 3 Growth and Body Reserves ENERGY AND PROTEIN REQUIREMENTS FOR GROWING CATTLE Net energy for gain (NEg) is defined herein as the energy content of the tissue deposited, which is a function of the proportion of fat and protein in the empty body tissue gain (Garrett et al., 1959; fat contains 9.367 kcal/g and nonfat organic matter contains an average of 5.686 kcal/g). Simpfendorfer (1974) summarized data from steers of British beef breeds from birth to maturity and found that within cattle of a similar mature size, 95.6 to 98.9 percent of the variation in the chemical components and empty body energy content was associated with the variation in weight (Figure 3–1 A and B). When energy does not limit growth, the empty body contains an increasingly smaller percentage of protein and an increasingly larger percentage of fat, and reaches chemical maturity when additional weight contains little additional protein. Figure 3–1A shows that steers in this data base contained little additional protein in the gain after an SBW of 750 kg. At SBW in excess of 200 to 300 kg, there appeared to be an influence of the effect of plane of nutrition, as evidenced by the scatter of points on the plot of body fat content (Figure 3–1A). The energy content of weight gain across a wide range of ME intakes and rates of gain was described in equation form by Garrett (1980), equations that were adapted by the Subcommittee on Beef Nutrition for use in the preceding edition of this volume (National Research Council, 1984). This data set included 72 comparative slaughter experiments conducted at the University of California between 1960 and 1980 of approximately 3,500 cattle receiving various diets. The equation developed with British-breed steers describes the relationship between retained energy (RE) and empty body weight gain (EBG) for a given empty body weight (EBW); Eq. 3–1 Because energy is retained as either protein or fat, the composition of the gain at different weights can be estimated from RE computed in Eq. 3–1 (Garrett, 1987); Eq. 3–2 Eq. 3–3 Using these relationships, the relationship between stage of growth (percentage of mature weight), rate of gain, and composition of gain can be computed (Table 3–1). The resulting NEg requirement in Table 3–1 for various shrunk body weights (SBW) and shrunk daily gains (SWG) are those presented in Table 1 of the 1984 edition of this volume for a medium-frame steer, except the last line shows requirements for 1.3 kg SWG rather than 1.2 kg SWG. These ranges in SWG represent those in that data base. Several relationships are shown in this table. First, energy content of the gain at a particular SWG increases with weight in a particular body size. Second, protein and fat content of the gain and expected body fat at a particular weight depend on rate of gain. Eqs. 3–1, 3–2, and 3–3 were used to compute the expected percentage of body fat at different SBW from the NE concentrations in the gain (Mcal/kg) when the 1984 National Research Council (NRC) medium-frame steer was grown from 200 kg SBW at 11.5 percent body fat at SWG of 1 kg/day (1.01 Mcal NEg/kg diet) for the first 100 kg and 1.3 kg/day (1.35 Mcal NEg/kg diet) to various SBW (Table 3–1). Eqs. 3–1 and 3–2 were used for the computations of protein and fat at various SWG, using constants of 0.891 and 0.956, respectively, for converting EBW and EBG to SBW and SWG (National Research Council, 1984). Table 3–1 shows the percentage body fat expected at various weights for the 1984 NRC medium-frame steer with typical two-phase feeding programs (grown on high-quality forage and finished on high-

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Nutrient Requirements of Beef Cattle: Seventh Revised Edition, 1996 FIGURE 3–1 Relationship between empty body weight (kg) and body fat (kg) in male castrates of British beef breeds. A: From Simpfendorfer (1974). B: From Simpfendorfer (1974); superimposed points are from Lofgreen and Garrett (1968); Fox et al. (1972); Jesse et al. (1976); Crickenberger et al. (1978); Harpster (1978); Lomas et al. (1982); and Woody et al. (1983). energy grain diets). Table 3–1 shows that even at low rates of gain and early stages of growth, some fat is deposited and both protein and fat are synthesized as rate of gain increases. Lightweight (90 kg) Holstein calves restricted to 0.23 to 0.53 kg ADG/day had 14.2 to 16.5 percent fat in the gain, respectively (Abdalla et al., 1988), which agrees with the values in Table 3–1. Phospholipids are required for cellular membrane growth (Murray et al., 1988). As energy intake above maintenance increases, protein synthesis rate becomes first limiting, and excess energy is deposited as fat; this dilutes body content of protein, ash, and water, which are deposited in nearly constant ratios to each other at a particular age (Garrett, 1987). To predict NEg required for SBW and SWG, EBW and EBG were converted to 4 percent shrunk liveweight gain with the following equations developed for use in the 1984 edition of this volume from the Garrett (1980) body composition data base: Eq. 3–4 Eq. 3–5 or with constants of 0.891 * SBW and 0.956 * SWG. These equations were rearranged to predict EBG and SWG; Eq. 3–6 Eq. 3–7 In the rearranged equations, RE is equivalent to NE available for gain. Thus, if intake is known, the net energy required for gain (NEFG) may be calculated as (DMI—feed required for maintenance) * diet NEg. NEFG can then be substituted into Eqs. 3–6 and 3–7 for RE to predict ADG. Given the relationship between energy retained and protein content of gain, protein content of SWG is given as (National Research Council, 1984): Eq. 3–8 The weight at which cattle reach the same chemical composition differs depending on mature size and sex; hence, composition is different even when the weight is the same (Fortin et al., 1980; Figure 3–2 A and B). Each type reached 28 percent body fat (equivalent body composition) at different weights (Figure 3–2A). Figure 3–2B shows a similar plot for empty body protein, with the end

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Nutrient Requirements of Beef Cattle: Seventh Revised Edition, 1996 TABLE 3–1 Relationship of Stage of Growth and Rate of Gain to Body Composition, Based on NRC 1984 Medium-Frame Steer Shrunk ADG, kg Shrunk body weight, kg 200 250 300 350 400 450 500   NEg required, Mcal/da 0.6 1.68 1.99 2.28 2.56 2.83 3.09 3.34 0.8 2.31 2.73 3.13 3.51 3.88 4.24 4.59 1.0 2.95 3.48 4.00 4.49 4.96 5.42 5.86 1.3 3.93 4.65 5.33 5.98 6.61 7.22 7.81   Protein in gain, percentb 0.6 20.4 19.5 18.8 18.0 17.3 16.6 16.0 0.8 18.7 17.6 16.5 15.5 14.6 13.6 12.7 1.0 17.0 15.6 14.2 13.0 11.7 10.5 9.3 1.3 14.4 12.5 10.7 9.0 7.3 5.7 4.2   Fat in gain, percentc 0.6 5.9 9.7 13.2 16.6 19.9 23.1 26.2 0.8 13.6 18.7 23.6 28.2 32.8 37.1 41.4 1.0 21.4 27.9 34.1 40.1 45.6 51.5 56.9 1.3 22.3 29.0 35.4 41.5 47.4 53.2 58.7   Body fat, percent 0.6 11.6 10.8 10.9 11.5 12.3 13.4 14.5 0.8 11.6 12.5 13.9 15.6 17.5 19.4 21.4 1.0 11.6 14.2 17.0 19.9 22.8 25.6 28.5 1.3 11.6 14.4 17.4 20.4 23.4 26.4 29.3 1 then 1.3 11.6 14.2 17.0 20.1 23.1 26.1 29.1 aComputed from the 1984 NRC equation which was determined from 72 comparative slaughter experiments (Garrett, 1980); retained energy (RE)=0.0635 EBW0.75 EBG1.097, where EBW is 0.891 SBW and EBG is .956 SBG. bComputed from the equations of Garrett (1987), which were determined from the 1984 NRC data base; proportion of fat in the shrunk body weight gain=0.122 RE-0.146, and proportion of protein=0.248-0.0264 RE. The proportion of fat and protein in the gain is for the body weight and ADG the RE is computed for. cPercent body fat was determined when grown at 1 kg ADG to 300 kg and 1.3 kg ADG to each subsequent weight as described above. of the line corresponding to the weight at 28 percent body fat. Weight at the same 12th rib lipid content varied 170 kg among steers of different biological types (Cundiff et al., 1981). The first NRC net energy system (National Research Council, 1976) used the Lofgreen and Garrett (1968) system to predict energy requirements, which was based on British breed steers given an estrogenic implant. From 1970 to 1990, larger mature-size European breed sires were increasingly used with the U.S. base British breed cow herd, resulting in the development of more diverse types of cows in the United States. This change, along with the use of sire evaluation programs that led to selection for larger body size to achieve greater absolute daily gain, resulted in an increase in average steer slaughter weights. The preceding edition of this volume (National Research Council, 1984) provided equations for medium- and large-frame cattle to adjust requirements for these changes. The current population of beef cattle in the United States varies widely in biological type and slaughter weight. By 1991, steers slaughtered averaged 542 kg, 48 percent choice with a weight range of 399 to 644 kg (M.Berwin, U.S. Department of Agriculture Market News data, Des Moines, IA, personal communication, 1992). All systems developed since the NRC 1984 system use some type of size-scaling approach to adjust for differences in weight at a given composition. The Commonwealth Scientific and Industrial Research Organization (CSIRO) system (Commonwealth Scientific and Industrial Research Organization, 1990) uses one table of energy requirements for proportion of a standard reference weight, then gives a table of “standard reference weights” for different breed types. This standard reference weight is defined as the weight at which skeletal development is complete and the empty body contains 25 percent fat, which corresponds to a condition score 3 on a 0 to 5 scale. Oltjen et al. (1986) developed a mechanistic model to predict protein accretion from initial and mature DNA content, with the residual between net energy available for gain and that required for protein synthesis assumed to be deposited as fat. The animal’s current weight as a proportion of mature weight is used to adjust for differences in mature size and use of implants. The Institut National de la Recherche Agronomique (INRA) system (Institut National de la Recherche Agronomique, 1989) uses allometric relationships between the EBW and SBW, the weight of the chemical components, and the weight of the fat-free body mass to predict energy and protein requirements. Coefficients in the equations are

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Nutrient Requirements of Beef Cattle: Seventh Revised Edition, 1996 FIGURE 3–2 Relationship between empty body weight (kg) and body fat (%) in Angus and Holstein heifers, steers, and bulls; composition differs even when weight is the same. A: Each type reached 28 percent body fat (equivalent body composition) at different weights. B: A similar plot for empty body protein; the end of the line corresponds to the weight at 28 percent body fat. parameters from the Gompertz equation (Taylor, 1968), which represents changes in liveweight with time. Initial and final weights with growth curve coefficients are given for six classes of bulls, two classes of steers, and two classes of heifers for finishing cattle, and two classes each for male and female growing cattle. The amount of lipids deposited daily is proportional to the daily liveweight gain raised to the power 1.8. Daily gain of protein is calculated from the gain in the fat-free body mass because protein content of fat free gain varies little with type of animal, growth rate, or feeding level (Garrett, 1987). Byers et al. (1989) developed an equation for steers similar to that of NRC (1984), except weight is replaced by proportional weight (current weight/dam mature weight). A different exponent is used for “no growth regulator” (nonGR); the growth regulator (GR) equation assumes use of an estrogenic implant. Fox et al. (1992) developed a system to interrelate the Beef Improvement Federation (BIF) frame-size system for describing breeding females and the USDA system for describing feeder cattle with energy and protein requirements. Dam mature weight is predicted from the BIF (1986) frame sizes of 1 to 9, which is assumed to be the same as the weight at which a similar frame size steer is 28 percent body fat (USDA low-choice grade). That weight is subsequently divided into the frame size of steer assumed to represent NRC 1984, the medium-framed steer equa-

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Nutrient Requirements of Beef Cattle: Seventh Revised Edition, 1996 tion, to obtain an adjustment factor that is used to compute the weight at which other frame sizes and sexes are equivalent in body composition. This approach is similar to the CSIRO (1990) standard reference weight system. Based on input from industry specialists and land-grant university research and extension animal scientists, this subcommittee decided to use the NRC 1984 net energy system and the body weights and energy content of gain represented by the medium-frame steer equation as a standard reference base because of its widespread acceptance, success with its use, and the large body-composition data base underlying that system. The focus of this revision was on refining that system so that energy and protein requirements can be predicted for the wide ranges in body sizes of breeding and feeder cattle in North America, including both Bos taurus and Bos indicus types. Because neither their actual composition nor mature weight is known, body composition and subsequent NE requirements must be predicted from estimated mature cow weight for breeding cattle or final weight and grade of feeder cattle. Because of the large number of breed types used, the widespread use of crossbreeding, anabolic implants, steers rather than bulls, feeding systems, and carcass grading systems used in North America, the European and CSIRO systems used to predict energy and protein requirements are not readily adaptable to North American conditions. Other proposed systems (Oltjen et al., 1986; Byers et al., 1989; Institut National de la Recherche Agronomique, 1989; Commonwealth Scientific and Industrial Research Organization, 1990; Agricultural and Food Research Council, 1993) either did not account for as much of the variation with the validation data set described later or are not sufficiently complete to allow prediction of requirements from common descriptions of cattle and all conditions that must be taken into account in North America (bulls, steers, and heifers; various implant combinations; wide variations in body size, feeding systems, and final weights and grades). The system developed for predicting energy and protein requirements for growing cattle assumes cattle have a similar body composition at the same degree of maturity, based on the evaluations presented previously. The NRC 1984 medium-frame steer equation (Eq. 3–1) is used as the standard reference base to compute the energy content of gain at various stages of growth and rates of gain for all cattle types. This is accomplished by adjusting the body weights of cattle of various body sizes and sexes to a weight at which they are equivalent in body composition to the steers in the Garrett (1980) data base, as described by Tylutki et al. (1994): Eq. 3–9 EQSBW is weight equivalent to the 1984 NRC medium-frame size steer, SBW is shrunk body weight being evaluated, SRW is standard reference weight for the expected final body fat (Table 3–2), and FSBW is final shrunk body weight at the expected final body fat (Table 3–2). These values were determined by averaging the percent body fat within all cattle in each of three marbling categories in the energy and protein retained validation data (Harpster, 1978; Danner et al., 1980; Lomas et al., 1982; Woody et al., 1983). Body fat percent averaged 27.8 (±3.4), 26.8 (±3), and 25.2 (±2.91) for those pens in the small, slight, or trace marbling categories, respectively. In comparison, the body fat data of Perry et al. (1991a,b) and Ainslie et al. (1992) averaged 28.4 percent (±4.1) for those in the small-marbling category. These steers had been selected to be a cross section of the current breed types and body sizes used in the United States. This variable SRW allows adapting the system to both U.S. and Canadian grading systems and determining SRW for marketing cattle at different end points. For breeding herd replacement heifers, FSBW is expected mature weight (MW). When computed as shown in Table 3–1 for heifers grown at 0.6 to 0.8 kg/day, accumulated fat content was 18 to 22 percent at the 28 percent fat steer SRW. Therefore, the SRW for breeding herd replacement heifers was assumed to be the same as the 1984 medium-frame steer fed to 28 percent fat. This approach is supported by a summary of the U.S. Meat Animal Research Center (MARC) data (Smith et al., 1976; Cundiff et al., 1981; Jenkins and Ferrell, 1984) in which mature weights of heifer mates averaged 10 percent more than implanted steer mates finished on high-energy diets TABLE 3–2 Standard Reference Weights for Different Final Body Compositions   Average Marbling Score Traces Slight Small Body fat, percent SEa 25.2±2.9 26.8±3.0 27.8±3.4 Standard reference weight, kgb 435 462 478 aThe means and standard errors (SE) shown for body fat in each marbling score category were determined by averaging the percentage body fat across all cattle in each of three marbling categories in the energy and protein retained validation data (Harpster, 1978; Danner et al., 1980; Lomas et al., 1982; and Woody et al., 1983). In a second comparison evaluation, the body fat data of Perry et al. (1991a, 1991b) and Ainslie et al. (1992) averaged 28.4 percent (±4.1) for those in the small marbling category. These relate to the current USDA and Canadian grading standards, respectively, as follows: traces, standard or A; slight, select or AA; and small, choice or AAA. bThe standard reference body weights (SBW basis) were determined from the NEg concentrations in the gain (Mcal/kg) when the reference animal (1984 NRC medium frame steer) was grown from 200 kg SBW at 11.5 percent body fat at SWG of 1 kg/day (1.01 Mcal NEg/kg diet) for the first 100 kg and 1.3 kg/day (1.35 Mcal NEg/kg diet) until the percentage body fat in table 2 was reached. Eq. 1 and 2 were used for the computations, using constants of .891 and .956, respectively for converting EBW and EWG to SBW and SWG. The SRW and FSBW (mature weight) of replacement heifers (18 to 22 percent fat) is assumed to be the same as the 28 percent fat weight as implanted steer mates, based on the data of Smith et al. (1976), Cundiff et al. (1981), Jenkins and Ferrell (1984), and Harpster (1978) and accumulated fat content when heifers are grown at replacement heifer rates (Table 3–1). Breeding bulls are assumed to be 67 percent greater than cows, giving an SRW of 800 kg.

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Nutrient Requirements of Beef Cattle: Seventh Revised Edition, 1996 after weaning. Based on MARC data, breeding bulls are assumed to be 67 percent heavier at maturity than cows, giving an SRW of 800 kg, which is the mature weight of a bull with the same genotype as the 1984 NRC medium-frame steer. The EQSBW computed from the SRW/FSBW multiplier is then used in Eq. 3–7 to compute the NEg requirement. If Eq. 3–1 or 3–6 is used, SBW is adjusted to EBW with Eq. 3–4. Alternatively, the equation of Williams et al. (1992; EBW=full BW * [1-gut fill], where gut fill is 0.0534+0.329 * fractional forage NDF) can be used to predict EBW from unshrunk liveweight. Predicted gut fill is then corrected with multipliers for full BW, physical form of forage, and fraction of concentrates. Because a table of requirements can be generated for any body size using the computer disk provided, only one example is shown (533 kg FSBW to represent the average steer in the United States). A similar table can be computed and printed for any body size with the computer disk containing the model. In this representative example, an FSBW change of 35 kg alters the NEg requirement by approximately 5 percent. Heifers and bulls with similar parents as the steers represented in this table have 18 percent greater and lesser, respectively, NEg requirements at the same weight as these steers. This system requires accurate estimation of FSBW. Most cattle feeders are experienced with results expected with feedlot finishing on a high-energy diet of backgrounded calves or yearlings that Representative Example of Requirements This example, a 320-kg steer with an FSBW of 600 kg (or herd replacement heifer with an MW of 600 kg) has an EQSBW of (478/600) * 320=255 kg. A 320-kg heifer with an FSBW of 480 has an EQSBW of (478/480) * 320=319 kg. The predicted SWG for the 320-kg steer consuming 5 Mcal NEg is (Eq. 3–7); 13.91 * 50.9116 * 255-0.6837=13.91 * 4.337 * 0.02263=1.365 kg/day. The SWG of the heifer consuming the same amount of energy will be 13.91 * 50.9116 * 319-0.6837=1.17 kg/day. To compute NEg requirement in this example 320-kg steer using Eq. 3–1 (0.891 * SBW to compute EBW and 0.956 * SWG to compute EBG): 255 * 0.891=227 kg EBW; 1.365 * 0.956=1.305 EBG; RE=0.0635 * 2270.75 * 1.3051.097=0.0635 * 58.5 * 1.339=4.97 Mcal. Assuming NEm requirement is 0.077 SBW0.75, the NEm requirement is (0.077 * 3200.75)=5.83 Mcal/day. Net protein requirement for gain is then (Eq. 3–8); 268-(29.4 * (5/1.365)) * 1.365=147 g/day. This value is then divided by the efficiency of use of absorbed protein to obtain the metabolizable protein required for gain (0.83-(0.00114 * EQSBW)), which is added to the metabolizable protein required for maintenance (3.8 * SBW0.75) to obtain the total metabolizable protein required. For the 320-kg steer, MP=147/(0.83-0.00114 ((478/600) * 320))+(3.8 * 3200.75)=560 g. have received an estrogenic implant. Guidelines for other conditions are reduce FSBW 25 to 45 kg for nonuse of an estrogenic implant, increase FSBW 25 to 45 kg for use of an implant containing trenbolone acetate (TBA) plus estrogen, increase FSBW 25 to 45 kg for extended periods at slow rates of gain, and decrease FSBW 25 to 45 kg for continuous use of a high-energy diet from weaning. Anabolic Agents A variety of anabolic agents are available for use in steers and heifers destined for slaughter to enhance growth rate, feed efficiency, and lean tissue accretion. Trade names, active ingredients, and restrictions on animal use for products currently available in North America are given in Table 3–3. With the exception of melengestrerol acetate (MGA), which is added to the feed, these products are implanted into the ear. They have been approved for use by the Food and Drug Administration in the United States and the Bureau of Veterinary Drugs in Canada, although not all of the products listed in Table 3–3 are approved in both countries. The mode of action of anabolic agents is not completely understood but, in the final analysis, they enhance the rate of protein accretion in the body (National Research Council, 1994). Effects of these agents on growth, body, and carcass composition have also been reviewed (Galbraith and Topps, 1981; Unruh, 1986). These products enhance rate of gain and feed intake. Rate of gain is usually enhanced more than intake, and feed efficiency is also improved. Their effect on nutrient utilization is minimal, so their impact on requirements can TABLE 3–3 Anabolic Agents Used for Growing and Finishing Cattle in North America Trade Name Active Ingredients Animal Use Compudose Estradiol Steers over 270 kg Finaplix Trenbolone acetate Steers or heifers Forplix Zeranol Trenbolone acetate Steers or heifers Implus-H Estradiol benzoate Testosterone Heifers Implus-S Estradiol benzoate Progesterone Steers MGA Melengesterol acetate Heifers Ralgro, Magnum Zeranol Steers or heifers Revalor Estradiol Trenbolone acetate Steers or heifers Synovex—C Estradiol benzoate Progesterone Suckling calves Synovex—H Estradiol benzoate Testosterone Heifers over 180 kg Synovex—S Estradiol benzoate Progesterone Steers over 180 kg

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Nutrient Requirements of Beef Cattle: Seventh Revised Edition, 1996 be accounted for by their effect on protein, fat, and energy accretion, which is taken into account by adjusting slaughter weight at constant finish. Effects on dry matter intake have also been quantified and are discussed in Chapter 7. All anabolic implants that contain an estrogenic substance yield similar increases in performance when evaluated under similar conditions (Byers et al., 1989). Nearly all the increase in weight gain can be accounted for by an increased growth of lean tissue and skeleton (Trenkle, 1990). Recent studies (Trenkle, 1990; Perry et al., 1991b; Bartle et al., 1992) indicate that compared to not using an implant, estrogenic implants increase protein content of gain equivalent to a 35-kg change in FSBW, whereas estradiol and trenbolone acetate (TBA) combination implants alter the protein content of gain equivalent to a change of approximately 70 kg in FSBW. If the FSBW is reduced by 25 to 45 kg when no implant is given or is increased approximately 25 to 45 kg if TBA+estrogen is given, the NEg requirement is changed by approximately 5 percent. This change is consistent with the 5 percent increase in net energy requirement when estrogenic implants were not in use (National Research Council, 1984) and the 4 percent adjustment in net energy requirement for nonuse of an anabolic implant in the model of Oltjen et al. (1986). Use of the two EBG exponents (GR and nonGR) in the equation of Byers et al. (1989) results in an 18 percent greater NEg requirement at 750 g ADG and a 20 percent greater NEg requirement at 1,500 g daily gain without an estrogenic implant compared with continuous use of an estrogenic implant. Solis et al. (1988) found, however, that the continuous use of an estrogenic implant in steers increased final weight at a similar composition by 25 kg as a result of 4.4 percentage units less fat in the gain over the growth period. These results are consistent with the recommendations given here for adjusting FSBW for the use of anabolic implants. Ionophore Effects Ionophores are polyether compounds included in diets of growing and finishing cattle to improve feed efficiency and animal health. Four products are currently licensed in North America, referred to by chemical name as lasalocid, laidlomycin propionate, monensin, and salinomycin. Lasalocid and monensin are licensed in both the United States and Canada, laidlomycin propionate is licensed in the United States, and salinomycin is licensed in Canada. The ionophore’s mechanisms of action are initiated by channeling ions through cell membranes (Bergen and Bates, 1984), and they have a marked effect on microbial cells in particular. There is a shift in volatile fatty acids produced in the rumen toward more propionate with corresponding reductions in acetate and butyrate. Measurements with rumensin in vivo have shown that it increases propionate production by 49 and 76 percent for high-roughage and high-concentrate diets, respectively (Van Maanen et al., 1978). This magnitude of response implies a significant improvement in the capture of feed energy during ruminal fermentation with less methane produced. Thus, metabolizable and net energy values of feeds should increase when ionophores are consumed. In a comparative slaughter trial, Byers (1980) found that the efficiency of energy use for maintenance was increased 5.7 percent by monensin with no effect on efficiency for gain. Delfino et al. (1988) made a similar observation with respect to lasalocid; they observed a 10 percent improvement in NEm of the feed with no effect on NEg. In a review of feedlot data, Raun (1990) reported that for cattle fed high-concentrate diets (average 15.7 percent forage), rumensin increased feed efficiency by 5.6 percent and gain by 1.8 percent but decreased dry matter intake by 4 percent. Simulations using the model in this publication (Chapter 10), with a 90 percent concentrate diet, showed that a 12 percent increase in NEm concentration of the diet with a 4 percent reduction in intake gave a 5.3 and 1.5 percent improvement in feed efficiency and gain, respectively. With lower energy rations (40 percent concentrate only), Goodrich et al. (1984) concluded that monensin increased feed efficiency and gain by 7.5 and 1.6 percent, respectively, with 6.4 percent lower intake. Simulation of these results using a 12 percent enhancement of ration NEm with monensin gave 7.9 and 4.5 percent improvements in feed efficiency and gain, respectively. These simulations confirm observed results that the proportional response in feed efficiency and gain to including monensin decreases as ration energy level increases. There are insufficient data available to develop individual recommendations for each ionophore and its effect on NEm. Thus, for all ionophores it is recommended that the NEm concentration of the diet be increased by 12 percent. Ionophores have characteristic effects on intake; and this is discussed in Chapter 7. Several reports have suggested that ionophores can improve energetic efficiency in cows and breeding animals. However, data are inconsistent (for review, see Sprott et al., 1988). Ionophores can have significant effects on nutrients other than energy. In general, they enhance absorption of nitrogen, magnesium, phosphorus, zinc, and selenium with inconsistent effects on calcium, potassium, and sodium. For further information see Chapter 5 and the review by Spears (1990). From experimental data on the simultaneous use of anabolic agents and ionophores, the subcommittee has concluded that interaction is minimal. Thus, it is recommended that adjustments made to slaughter weight based

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Nutrient Requirements of Beef Cattle: Seventh Revised Edition, 1996 on use of anabolic agents are independent of ionophore use and adjustments made to ration NEm based on use of ionophores are independent of anabolic agents. Their effects on feed intake have been considered to be additive. Previous Plane of Nutrition Effects Energy intake above maintenance can vary considerably, depending on diet fed during early growth in stocker and backgrounding programs. Table 3–1 indicates that a reduced intake above maintenance results in a greater proportion of protein in the gain at a particular weight, which is supported by several studies (Fox and Black, 1984; Abdalla et al., 1988; Byers et al., 1989) and the model by Keele et al. (1992). When thin cattle are placed on a high-energy diet, however, compensatory fat deposition occurs. Most of the improved efficiency of gain results from a decreased maintenance requirement and increased feed intake (Fox and Black, 1984; Ferrell et al., 1986; Carstens et al., 1987; Abdalla et al., 1988). As discussed in the maintenance requirement section, it is assumed NEm requirement is 20 percent lower in a very thin animal (CS 1), is increased 20 percent in a very fleshy animal (CS 9), and changes 5 percent per condition score. The NEm adjustment for previous nutrition (COMP) is thus computed as Eq. 3–10 where CS is body condition score. The effect of plane of nutrition is taken into account by the rate of gain function (increased fat deposition with increased rate of gain) and EQSBW in the primary equations. Thus, the user determines the expected final weight and body fat, and the model computes EQSBW to use in computing NEg required as shown in Eq. 3–9. The change in efficiency of energy utilization is accounted for by a reduced NEm requirement and increased DMI above maintenance. Effects of Special Dietary Factors Diet composition and level of intake differences will cause the composition of the ME (ruminal volatile fatty acids, intestinally digested carbohydrate, and fat) to vary (Ferrell, 1988), which can affect the composition of gain (Fox and Black, 1984). Most of these effects will alter rate of gain, which is taken into account by the primary equations; however, fat distribution may be altered, which could affect carcass grade (Fox and Black, 1984). Unique Breed Effects Most of the unique breed effects on NEg requirements are accounted for by differences in the weight at which different breeds reach a given chemical composition (Harpster, 1978; Cundiff et al., 1986; Institut National de la Recherche Agronomique, 1989). Nonetheless, breeds can differ in fat distribution, which can influence carcass grade (Cundiff et al., 1986; Perry et al., 1991a). Validation of Energy and Protein Requirement System The standard reference weight (SRW) approach was validated and compared to the 1984 NRC system with three distinctly different data sets that were completely independent of those used to develop the NRC systems—the one presented in this publication and the one developed for the preceding edition of this volume (National Research Council, 1984). The Oltjen et al. (1986) model was also compared to the other two with the first two data sets. For the 1984 NRC system, cattle with frame sizes larger than 6 were considered large-framed. For this publication, the standard reference weight (478 kg) was divided by the pen mean weight at 28 percent body fat to obtain the body size adjustment factor, which was then applied to the actual weight for use in the standard reference equations to predict energy and protein retained. Data set 1 (Harpster, 1978; Danner et al., 1980; Lomas et al., 1982; Woody et al., 1983) included 82 pen observations (65 pens of steers and 17 pens of heifers) with body composition determined by the same procedures used by Garrett (1980) in developing the NRC 1984 system. Included were FSBW representative of the range in cattle fed in North America; all silage to all corn-based diets; no anabolic implant, estrogen only or estrogen+TBA; and Bos taurus breed types representative of those fed in North America (British, European, Holstein, and their crosses). Data set 2 included 142 serially slaughtered (whole body chemical analysis by component; Fortin et al., 1980; Anrique et al., 1990) nonimplanted steers, heifers, and bulls ranging widely in body size. A detailed description of these data sets, validation procedures, and results were published by Tylutki et al. (1994), except the SRW has been increased from 467 to 478 kg. In nearly every subclass, the system developed for this publication accounted for more of the variation and had less bias than did the other two systems. Nearly identical results were obtained between the 1984 NRC and present systems when energy retained was used to predict SWG in Eq. 3–7; this equation is the one most commonly used to predict ADG. Figure 3–3 shows the results when all subclasses were combined. The present model accounted for 94 percent of the variation with a 2 percent overprediction bias for retained energy and 91 percent of the variation in retained protein with a 2 percent underprediction bias. Figure 3–3 shows that use of the NRC 1984 medium-frame steer as a standard reference base results in accurate prediction of net energy requirements for growth across wide variations in cattle breed, body size, implant, and nutritional management systems.

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Nutrient Requirements of Beef Cattle: Seventh Revised Edition, 1996 FIGURE 3–3 Use of the NRC 1984 medium-frame steer as a standard reference base results in accurate prediction of net energy requirements for growth across wide variations in cattle breed, body size, implant, and nutritional management systems.

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Nutrient Requirements of Beef Cattle: Seventh Revised Edition, 1996 Data set 3 included ADG predicted, using model level 2, from independent trials with 96 different diets fed to a total of 943 Bos indicus (Nellore breed) steers and bulls in which ME intake and body composition were determined (Lanna et al., 1996). FSBW was determined from final EBW fat content. For the NRC 1984 and the present systems, the r2 was 0.58 and 0.72, and the bias was -20 percent and -2 percent respectively. These validations indicate that given the accuracies obtained, problems with predicting net energy and protein requirements and SWG are likely to include one of the following: choosing the wrong FSBW short-term, transitory effects of previous nutrition, gut fill, or anabolic implants, variation in NEm requirement, variation in ME value assigned to the feed because of variations in feed composition and extent of ruminal or intestinal digestion, variation in NE and NEg derived from the ME because of variation in end products of digestion and their metabolizability, and variations in gut fill. AMINO ACID REQUIREMENTS In recent studies, abomasal infusion of high-quality sources of amino acids significantly increased nitrogen balance in steers, despite the fact that they were fed diets balanced to optimize ruminal fermentation and to provide protein in excess of NRC requirements (Houseknecht et al., 1992; Robinson et al., 1994). These studies indicate that protein accretion was constrained by quantity and/or proportionality of amino acids absorbed. Amino acid requirements for tissue growth are a function of the percentage of each amino acid in the net protein accretion and thus depend on the accuracy of prediction of protein retained. Ainslie et al. (1993) summarized various studies that have determined essential amino acid content of tissue protein in selected muscles (Hogan, 1974; Evans and Patterson, 1985), in daily accretion (Early et al., 1990), or in the whole empty body (Williams, 1978; Rohr and Lebzien, 1991; Ainslie et al., 1993). In a sensitivity analysis with model predicted vs first-limiting amino acid allowable gain, the average of the three whole empty body studies gave the least bias (Fox et al., 1995). These average values (average of Williams, 1978; Rohr and Lebzien, 1991; Ainslie et al., 1993) are as follows (g/100 g empty body protein); arginine, 3.3; histidine, 2.5; isoleucine, 2.8; leucine, 6.7; lysine, 6.4; methionine, 2.0; phenylalanine, 3.5; threonine, 3.9; and valine, 4.0. Tryptophan values were not given because of limitations in assay procedures. A number of recent studies have evaluated tissue amino acid requirements by measuring net flux of essential amino acids across the hind limb of growing steers (Merchen and Titgemeyer, 1992; Byrem et al., 1993; Boisclair et al., 1994; Robinson et al., 1995). The proportionality of individual amino acid uptake did not markedly change when protein accretion was increased by infusing various compounds (bovine somatotropin, cimaterol, or casein). The proportions of the essential amino acids in the net flux in these studies followed the same trends as suggested by the tissue composition values listed above. The above studies and the data previously cited in this section suggest that both quantity and proportionality of amino acid availability are important to achieve maximum energy allowable ADG. In a first NRC attempt to accomplish this for cattle, the model level 2, as described in Chapter 10, has been provided to allow the user to estimate both quantity and proportion of essential amino acids required by the animal and supplied by the diet. The critical steps involved are the prediction of microbial growth and composition; amount and composition of diet protein escaping ruminal degradation; intestinal digestion and absorption; and net flux of absorbed amino acids into tissue. Because of limitations in the ability to predict each of these components, the estimates of amino acid balances provided should be used only as a guide. The subcommittee has taken this step to provide a structure that is intended to stimulate research that will improve the ability to predict amino acid balances, which should lead to increased efficiency of energy and protein utilization in cattle. Net daily tissue synthesis of protein represents a balance between synthesis and degradation (Oltjen et al., 1986; Early et al., 1990; Lobley, 1992). Lobley (1992) indicated that a 500-kg steer with a net daily protein accretion of 150 g actually degrades and resynthesizes at least another 2,550 g. Thus, balancing for daily net accretion accounts for only about 5.5 percent of the total daily protein synthesis. Protein metabolism is very dynamic, and a kinetic approach is needed to accurately predict amino acid requirements. Small changes in either the rate of synthesis or degradation can cause great alterations in the rate of gain, and the relative maintenance requirement changes with level of production. Lobley (1992), however, concluded that the precision of kinetic methods is critical; a 2 percent change in synthesis rate would alter net protein accretion 20 to 40 percent, and many of the procedures are not accurate within 4 to 5 percent. When combined with a system that has limitations in predicting absorbed amino acids from microbial and feed sources, errors could be greatly magnified with an inadequate mechanistic metabolism model. Given present knowledge, the subcommittee decided that protein and amino acids required for growth should be based on net daily accretion values that have been actually measured. Maintenance requirements for protein have been measured with metabolism trials (Institut National

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Nutrient Requirements of Beef Cattle: Seventh Revised Edition, 1996 de la Recherche Agronomique, 1989) or in growth trials beginning at or slightly above maintenance (Wilkerson et al., 1993). Net daily protein and amino acid accretion have been measured and validated in the comparative slaughter studies reported here. However, this subcommittee recommends that models such as that of Oltjen et al. (1986) be developed, refined, and validated so that in the future this approach can be used to allow more accurate prediction of daily amino acid requirements. ENERGY AND PROTEIN REQUIREMENTS FOR BREEDING HERD REPLACEMENTS No rate of gain requirement has been given in previous NRC publications for growing cattle because they are used to market available forage at early stages of growth, which results in wide variations in rate of gain before feedlot finishing. However, replacement heifer growth rate that results in first parturition at 2 years of age is most economical (Gill and Allaire, 1976). In addition, inadequate size at first parturition may limit milk production and conception during first lactation. Excess energy intake, however, can have negative effects on mammary development. For example, excessive energy intake had a negative effect on mammary parenchyma (ductular epithelial tissue; Harrison et al., 1983; Foldager and Serjsen, 1987). Because puberty is associated with weight, parenchyma tissue growth, which is not linearly related to body growth, may be truncated before full ductal development as a result of excess energy intake before puberty (Van Amburgh et al., 1991). Excess energy intake, as evidenced by overconditioning from 2 to 3 months of age until after conception, should be avoided. Numerous data are available to support the concept of a genetically determined threshold age and weight at which bulls or heifers attain puberty (for reviews, see Robinson, 1990; Ferrell, 1991; Dunn and Moss, 1992; Patterson et al., 1992; Schillo et al., 1992). Joubert (1963) proposed that heifers would not attain puberty until they reached a given degree of physiological maturity, which is similar to the “target weight” concept proposed by Lamond (1970). Simply stated, the concept is to feed replacement heifers to attain a preselected or target weight at a given age (Spitzer et al., 1975; Dziuk and Bellows, 1983; Wiltbank et al., 1985). In general, heifers of typical beef breeds (e.g., Angus, Charolais, Hereford, Limousin) are expected to attain puberty at about 60 percent of mature weight (Laster et al., 1972, 1976, 1979; Stewart et al., 1980; Ferrell, 1982; Sacco et al., 1987; Martin et al., 1992; Gregory et al., 1992). Heifers of dual purpose or dairy breeds (e.g., Braunvieh, Brown Swiss, Friesian, Gelbvieh, Red Poll) tend to attain puberty at a younger age and lower weight relative to mature weight (about 55 percent of mature weight) than those of beef breeds. Conversely, heifers of Bos indicus breeds (e.g., Brahman, Nellore, Sahiwal) generally attain puberty at older ages and heavier weight, and at a slightly higher percentage of mature weight (65 percent) as compared to European beef breeds. The following model was developed to compute target weights and growth rates for breeding herd replacement heifers, using the data summarized in Chapter 4 (target breeding weights are 60 and 65 percent of mature weight for Bos taurus and Bos indicus, respectively). Then the equations described previously are used to predict net energy and protein requirements for growth. Based on the data summarized by Gregory et al. (1992) it is assumed that target first calving weights are 80 percent of mature weight, which is the 6 breed average for 2-year-old as a percentage of 6-year-old weight in this MARC data base. Target calving weight factors for 3 and 4 year olds (0.92 and 0.96, respectively) are from the model described by Fox et al. (1992). Predicting target weights and rates of gain: where: MW is mature weight, kg; LW is liveweight, kg; TPW is target puberty weight, kg; TCW1 is target calving weight, kg at 24 months; TCW2 is target calving weight, kg at 36 months; TCW3 is target calving weight, kg at 48 months; TCW4 is target calving weight, kg at >48 months; TCWx is current target calving weight, kg; TCWxx is next target calving weight, kg; TCA is target calving age in days; TPA is target puberty age in days; BPADG is prepubertal target ADG, kg/day; APADG is postpubertal target ADG, kg/day; ACADG is after calving target ADG, kg/day Tage is heifer age, days; CI is calving interval, days. The equations in the previous section are used to compute requirements for the target ADG, and adjustments to reach these targets because of previous nutrition are made by determining ADG and NEg requirements needed to

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Nutrient Requirements of Beef Cattle: Seventh Revised Edition, 1996 achieve the targets. For pregnant animals, gain due to gravid uterus growth should be added to predicted daily gain (SWG), as follows: where CBW is calf birth weight, kg. For pregnant heifers, weight of fetal and associated uterine tissue should be deducted from EQEBW to compute growth requirements. The conceptus weight (CW) can be calculated as follows: where, CW is conceptus weight, kg; and t is days pregnant. Net energy requirement for optimal growth of breeding heifer replacements can be determined for these rates of growth with the primary net energy requirement equations, using expected mature weight as FSBW. ENERGY AND PROTEIN RESERVES OF BEEF COWS In utilizing available forage, beef cows usually do not consume the amount of energy that matches their requirements for maintenance, gestation, or milk production. Reserves are depleted when forage quality and (or) quantity declines because of weather, overstocking, or inadequate forage management, but are replenished when these conditions improve. In addition, most beef cows are not housed and must continually adjust energy balance for changes in environmental conditions. Optimum management of energy reserves is critical to economic success with cows. Whether too fat or thin, cows at either extreme are at risk from metabolic problems and diseases, decreased milk yield, low conception rates, and difficult calving (Ferguson and Otto, 1989). Overconditoning is expensive and can lead to calving problems and lower dry matter intake during early lactation. Conversely, thin cows may not have sufficient reserves for maximum milk production and will not likely rebreed on schedule. To maintain a 12-month calving interval, cows must be bred by 83 days after calving (365 minus an average gestation length of 282 days). Dairy cows usually ovulate the first dominant follicle, but beef cows average three dominant follicles being produced before ovulation, depending on the suppressive effects of suckling, body condition, or energy intake (Roche et al., 1992). Both postcalving cow condition score and energy balance control ovulation (Wright et al., 1992). Conception rates reach near maximum at body condition score 5 (Wright et al., 1992). Ovulation occurs in dairy cattle 7 to 14 days after the energy balance nadir is reached during early lactation (Butler and Canfield, 1989). Beef cows in adequate body condition with adequate energy intake may have a similar response because the negative effects of suckling may be offset by the lower energy demands of beef cows (W.R.Butler, Cornell University, personal communication, 1992). Allowing for three ovulations (assuming the first ovulation goes undetected), and allowing for two observed ovulations and inseminations for conception, the first ovulation must occur 41 days after calving. To allow this, the feeding program must be managed so that maximum negative energy balance during early lactation is reached by about 31 days after calving (41 days to first ovulation minus 10 days for ovulation after maximum negative energy balance). If the cow is too fat, intake will be lower and reserves will be used longer during early lactation, resulting in an extended time to maximum negative energy balance. Even if thin cows consume enough to meet requirements by 31 days, a feedback mechanism mediated through hormonal changes seems to inhibit ovulation if body condition is inadequate (Roche et al., 1992). Additional signals relative to the need for a given body condition before ovulation appear to occur in cows nursing calves. In previous NRC publications, changes in energy reserves were accounted for by allowing for weight gain or loss. However, in practice, few producers weigh beef cows to determine if their feeding program is allowing for the appropriate energy balance. Energy reserves are more often managed by observing body condition changes, and all systems developed since the last NRC publication use condition scores (CS) to describe energy reserves. Body condition score is closely related to body fat and energy content (Wagner, 1984; Houghton et al., 1990; Fox et al., 1992; Buskirk et al., 1992). The CSIRO nutrient requirement recommendations (Commonwealth Scientific Industrial Research Organization, 1990) adapted the 0 to 5 body condition scoring system of Wright and Russel (1984a,b). In their system, a CS change of 1 contains 83 kg body weight change, which contains 6.4 Mcal/kg for British breeds and 5.5 Mcal/kg for large European breeds; this is equivalent to 55 kg and 330 Mcal/CS on a 1 to 9 scale. The INRA (1989) nutrient requirement recommendations use a 0 to 5 system also and assume 6 Mcal lost/kg weight loss, which is equivalent to 332 Mcal/CS on a 9-point scale. The Oklahoma (Cantrell et al., 1982; Wagner, 1984; Selk et al., 1988) and Colorado groups (Whitman, 1975) developed a 9-point system for condition scoring. The Purdue group (Houghton et al., 1990) used a 5-point scale with minus, average, and plus within each point, which in effect approximates the dairy 1 to 5 system; both are similar to a continuous 9-point scale. Empty body lipid was 3.1, 8.7, 14.9, 21.5 and 27.2, respectively, for CS 1 to 5, which they proposed correspond to CS 2, 5, and 8 on the 1 to 9 scale. Empty body weights averaged 75 kg per increase in condition score, which is equivalent to 50 kg/CS on a 9-point system. The Texas group (Herd and Sprott, 1986) used a 9-point scale and reported 0, 4, 8, 12, 16, 24, 28,

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Nutrient Requirements of Beef Cattle: Seventh Revised Edition, 1996 and 32 percent body fat, respectively, for CS 1–9. The Cornell group (Fox et al., 1992) used the Oklahoma 9-point scoring system and 14 studies of body composition in cows to develop a model to predict weight and energy lost or gained with changes in age, mature size, and condition score. In a 455-kg vs a 682-kg mature cow with a CS of 5, a loss of 1 CS from 5 to 4 is associated with 30 kg and 167 Mcal vs 45 kg and 257 Mcal, respectively, which is 5.6 Mcal/kg. From CS 2 to CS 1, the weight lost contains 4.4 Mcal/kg. The Purdue group (Buskirk et al., 1992) predicted from body weight and CS changes energy content of tissue gain (or loss) at each CS to be 2.16, 2.89, 3.62, 4.34, 5.07, 5.8, 6.53, 7.26, and 7.98 Mcal/kg for CS 1 to 9, respectively. Their CS 5 value of 5.07 compares to the CSIRO (1990) value of 6.4 for British breeds and 5.5 for European breeds; the INRA (1989) value of 6; and the Fox et al. (1992) value of 5.6 Mcal/kg weight change at a CS of 5, which reaches a maximum of 5.7 at CS 9 and declines to 4.4 by CS 2, on a 1 to 9 scale. The Buskirk et al. (1992) system assumes a linear decline in energy content of gain as weight is lost, which implies proportional protein and fat in the gain or loss with changes in weight as occurs during growth. The other systems (Institut National de la Recherche Agronomique, 1989; Commonwealth Scientific Industrial Research Organization, 1990; National Research Council, 1989; Fox et al., 1992) assume a hierarchical loss of fat energy first in mature animals using and replenishing reserves. Another difference is that the Buskirk et al. (1992) system uses NEg values of feeds to meet NE reserves requirements, whereas the CSIRO, INRA, and NRC systems as well as others (Moe, 1981; Fox et al., 1992) assume higher efficiencies of use of ME for energy reserves than for growth. The model below was developed from a body composition data set provided by MARC (C.L.Ferrell, personal communication, 1995). Body condition score, body weight, and body composition are used to calculate energy reserves. The equations were developed from data on chemical body composition and visual appraisal of condition scores (1 to 9 scoring system) from 105 mature cows of diverse breed types and body sizes. Characteristics of the data set were EBW=0.851 * SBW; mean EBW, 546 (range 302 to 757) kg; percentage empty body fat, 19.3 (range 4.03 to 31.2); percentage empty body protein, 15.3 (range 13.2 to 18.0); and body condition score, 5.56 (range 2.25 to 8.0). The developed equations were validated on an independent data set of 65 mature cows (data from C.L.Ferrell, MARC, personal communication, 1995). The validation data set consisted of 9 year old cows of diverse sire breeds and Angus or Hereford dams with mean EBW, 471 (range 338 to 619) kg; mean percentage empty body fat, 20.3 (range 8.5 to 31.3); mean percentage empty body protein, 18.2 (range 13.9 to 21.3); and mean condition score, 4.9 (range 3.0 to 7.5). The resulting best-fit equations to describe relationships between CS and empty body percentage fat, protein water, and ash were linear (Figure 3–4). A zero intercept model was used to describe the relationship between percent empty body fat and CS. The mean SBW change associated with a CS change was computed as 44 kg. It is assumed that for a particular cow the ash mass does not change when condition score changes. In the validation of this model, CS accounted for 67, 52, and 66 percent of the variation in body fat, body protein, and body energy, respectively. 1. Body composition is computed for the current CS: AF=0.037683 * CS; r2=0.67. AP=0.200886-0.0066762 * CS; r2=0.52. AW=0.766637-0.034506 * CS; r2=0.67. AA=0.078982-0.00438 * CS; r2=0.66. EBW=0.851 * SBW TA=AA * EBW where: AF=proportion of empty body fat AP=proportion of empty body protein FIGURE 3–4 Relationship of empty body weight, protein, ash, and fat (as percentage) of body condition score in mature cows.

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Nutrient Requirements of Beef Cattle: Seventh Revised Edition, 1996 AW=proportion of empty body water AA=proportion of empty body ash SBW=shrunk body weight, kg EBW=empty body weight, kg TA=total ash, kg 2. For CS=1 ash, fat, and protein composition are as follows: AA1=0.074602 AF1=0.037683 AP1=0.194208 where: AA1 is proportion of empty body ash @ CS=1 AF1 is proportion of empty body fat @ CS=1 AP1 is proportion of empty body protein @ CS=1 3. Assuming that ash mass does not vary with condition score, EBW and component body mass at condition score 1 is calculated: EBW1=TA/AA1 TF=AF * EBW TP=AP * EBW TF1=EBW1 * AF1 TP1=EBW1 * AP1 where: EBW1 is Calculated empty body weight at CS=1, kg TF is total fat, kg TP is total protein, kg TF1 is total body fat @ CS=1, kg TP1 is total body protein @ CS=1, kg 4. Mobilizable energy and protein are computed: FM=(TF-TF1) PM=(TP-TP1) ER=9.4FM+5.7PM where: FM is mobilizable fat, kg PM is mobilizable protein, kg ER is energy reserves, Mcal 5. EBW, AF and AP are computed for the next CS to compute energy and protein gain or loss to reach the next CS: EBW=TA/AA where: EBW is EBW at the next score TA is total kg ash at the current score AA is proportion of ash at the next score AF, AP, TF and TP are computed as in steps 1 and 3 for the next CS and FM, PM, and ER are computed as the difference between the next and current scores. Table 3–4 gives CS descriptions and Table 3–5 shows the percentage composition and SBW change associated with each CS computed with this model. This model predicts energy reserves to be a constant 5.82 Mcal/kg liveweight loss, which compares to the 1989 NRC dairy value of 6 Mcal/kg, the CSIRO values of 6.4 for British breeds and 5.5 for European breeds, the INRA value of 6 and the AFRC value of 4.54. Protein loss is predicted to be 81 g/kg, compared to 117, 135, 138, and 160 g/kg weight loss for the Buskirk et al. (1992), CSIRO (1990), AFRC (1993), and NRC (1985) systems. SBW is predicted to be 76.5, TABLE 3–4 Cow Condition Score Condition Score Body Fat, percenta Appearance of Cowb 1 3.77 Emaciated—Bone structure of shoulder, ribs, back, hooks and pins sharp to touch and easily visible. Little evidence of fat deposits or muscling. 2 7.54 Very thin—Little evidence of fat deposits but some muscling in hindquarters. The spinous processes feel sharp to the touch and are easily seen, with space between them. 3 11.30 Thin—Beginning of fat cover over the loin, back, and foreribs. Backbone still highly visible. Processes of the spine can be identified individually by touch and may still be visible. Spaces between the processes are less pronounced. 4 15.07 Borderline—Foreribs not noticeable; 12th and 13th ribs still noticeable to the eye, particularly in cattle with a big spring of rib and ribs wide apart. The transverse spinous processes can be identified only by palpation (with slight pressure) to feel rounded rather than sharp. Full but straightness of muscling in the hindquarters. 5 18.89 Moderate—12th and 13th ribs not visible to the eye unless animal has been shrunk. The transverse spinous processes can only be felt with firm pressure to feel rounded—not noticeable to the eye. Spaces between processes not visible and only distinguishable with firm pressure. Areas on each side of the tail head are fairly well filled but not mounded. 6 22.61 Good—Ribs fully covered, not noticeable to the eye. Hindquarters plump and full. Noticeable sponginess to covering of foreribs and on each side of the tail head. Firm pressure now required to feel transverse process. 7 26.38 Very good—Ends of the spinous processes can only be felt with very firm pressure. Spaces between processes can barely be distinguished at all. Abundant fat cover on either side of tail head with some patchiness evident. 8 30.15 Fat—Animal taking on a smooth, blocky appearance; bone structure disappearing from sight. Fat cover thick and spongy with patchiness likely. 9 33.91 Very fat—Bone structure not seen or easily felt. Tail head buried in fat. Animal’s mobility may actually be impaired by excess amount of fat. aBased on the model presented in this chapter. bAdapted from Herd and Sprott, 1986.

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Nutrient Requirements of Beef Cattle: Seventh Revised Edition, 1996 TABLE 3–5 Empty Body (EB) Chemical Composition at Different Condition Scores (CS) CS Percent in EB SBW, percent of CS 5a Fat Protein Ash Water 1 3.77 19.42 7.46 69.35 77 2 7.54 18.75 7.02 66.69 81 3 11.30 18.09 6.58 64.03 87 4 15.07 17.04 6.15 61.36 93 5 18.84 16.75 5.71 58.70 100 6 22.61 16.08 5.27 56.04 108 7 26.38 15.42 4.83 53.37 118 8 30.15 14.75 4.39 50.71 130 9 33.91 14.08 3.96 48.05 144 aWeight change from CS5 weight can be estimated from the difference between CS5 weight and CS5 weight * percent of CS5 weight for the CS in question. Net energy reserves provided, or required to change CS, is kg weight change * 5.82. 81.3, 86.7, 92.9, 108.3, 118.1, 129.9, and 144.3 percent of a CS 5 cow for CS 1, 2, 3, 4, 6, 7, 8, and 9, respectively. A 500 kg cow is predicted to weigh 465, 434, 407, and 383 kg at CS 4, 3, 2, and 1 with weight losses of 35, 31, 27, and 24 kg for CS 5, 4, 3 and 2, respectively. Corresponding values for a 650 kg cow are 604, 564, 528, and 497 kg SBW at CS 4, 3, 2 and 1 with weight losses per CS of 46, 40, 35 and 31 kg for CS 5, 4, 3 and 2, respectively. Table 3–6 gives Mcal mobilized in moving to the next lower score, or required to move from the next lower score, to the one being considered for cows with different mature sizes. These cows are within the range included in the data base used to develop the regression equations (433 to 887 kg SBW). Diet NEm replaced by mobilized reserves, or required to replenish reserves, are computed by assuming 1 Mcal of mobilized tissue will replace 0.8 Mcal of diet NEm, and 1 Mcal of diet NEm will provide 1 Mcal of tissue NE, based on Moe (1981) and NRC (1989). For example, a 500 kg cow at CS 5 will mobilize 207 Mcal in declining to a CS 4. If NEm intake is deficient 3 Mcal/day, this cow will lose 1 CS in (207 * 0.8)/3=55 days. If consuming 3 Mcal NEm above daily requirements, this cow will move back to a CS 5 in 207/3=69 days. The weakest link in this model is the prediction of body TABLE 3–6 Energy Reserves for Cows with Different Body Sizes and Condition Scores CS Mcal NE Required or Provided for Each CSa at CS 5 Mature Weight 400 450 500 550 600 650 700 750 800 2 112 126 140 154 168 182 196 210 223 3 126 141 157 173 189 204 220 236 251 4 144 162 180 198 217 235 253 271 289 5 165 186 207 227 248 269 289 310 331 6 193 217 242 266 290 314 338 362 386 7 228 267 285 314 342 371 399 428 456 8 275 309 343 378 412 446 481 515 549 9 335 377 419 461 503 545 587 629 670 aRepresents the energy mobilized in moving to the next lower score, or required to move from the next lower score to this one. Each kg of SBW change contains 5.82 Mcal, and SBW at CS 1, 2, 3, 6, 1, 8, and 9 are 76.5, 81.3, 86.7, 92.9, 108.3, 118.1, 129.9, and 144.3 percent of CS 5 weight, respectively. weight change associated with each CS change. 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