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