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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