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.
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
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.
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).
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).
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.