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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