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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