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Nutrient Requirements of Beef Cattle: Seventh Revised Edition, 1996
and 32 percent body fat, respectively, for CS 1–9. The Cornell group (Fox et al., 1992) used the Oklahoma 9-point scoring system and 14 studies of body composition in cows to develop a model to predict weight and energy lost or gained with changes in age, mature size, and condition score.
In a 455-kg vs a 682-kg mature cow with a CS of 5, a loss of 1 CS from 5 to 4 is associated with 30 kg and 167 Mcal vs 45 kg and 257 Mcal, respectively, which is 5.6 Mcal/kg. From CS 2 to CS 1, the weight lost contains 4.4 Mcal/kg. The Purdue group (Buskirk et al., 1992) predicted from body weight and CS changes energy content of tissue gain (or loss) at each CS to be 2.16, 2.89, 3.62, 4.34, 5.07, 5.8, 6.53, 7.26, and 7.98 Mcal/kg for CS 1 to 9, respectively. Their CS 5 value of 5.07 compares to the CSIRO (1990) value of 6.4 for British breeds and 5.5 for European breeds; the INRA (1989) value of 6; and the Fox et al. (1992) value of 5.6 Mcal/kg weight change at a CS of 5, which reaches a maximum of 5.7 at CS 9 and declines to 4.4 by CS 2, on a 1 to 9 scale. The Buskirk et al. (1992) system assumes a linear decline in energy content of gain as weight is lost, which implies proportional protein and fat in the gain or loss with changes in weight as occurs during growth. The other systems (Institut National de la Recherche Agronomique, 1989; Commonwealth Scientific Industrial Research Organization, 1990; National Research Council, 1989; Fox et al., 1992) assume a hierarchical loss of fat energy first in mature animals using and replenishing reserves. Another difference is that the Buskirk et al. (1992) system uses NEg values of feeds to meet NE reserves requirements, whereas the CSIRO, INRA, and NRC systems as well as others (Moe, 1981; Fox et al., 1992) assume higher efficiencies of use of ME for energy reserves than for growth.
The model below was developed from a body composition data set provided by MARC (C.L.Ferrell, personal communication, 1995). Body condition score, body weight, and body composition are used to calculate energy reserves. The equations were developed from data on chemical body composition and visual appraisal of condition scores (1 to 9 scoring system) from 105 mature cows of diverse breed types and body sizes. Characteristics of the data set were EBW=0.851 * SBW; mean EBW, 546 (range 302 to 757) kg; percentage empty body fat, 19.3 (range 4.03 to 31.2); percentage empty body protein, 15.3 (range 13.2 to 18.0); and body condition score, 5.56 (range 2.25 to 8.0). The developed equations were validated on an independent data set of 65 mature cows (data from C.L.Ferrell, MARC, personal communication, 1995). The validation data set consisted of 9 year old cows of diverse sire breeds and Angus or Hereford dams with mean EBW, 471 (range 338 to 619) kg; mean percentage empty body fat, 20.3 (range 8.5 to 31.3); mean percentage empty body protein, 18.2 (range 13.9 to 21.3); and mean condition score, 4.9 (range 3.0 to 7.5). The resulting best-fit equations to describe relationships between CS and empty body percentage fat, protein water, and ash were linear (Figure 3–4). A zero intercept model was used to describe the relationship between percent empty body fat and CS. The mean SBW change associated with a CS change was computed as 44 kg. It is assumed that for a particular cow the ash mass does not change when condition score changes. In the validation of this model, CS accounted for 67, 52, and 66 percent of the variation in body fat, body protein, and body energy, respectively.
1. Body composition is computed for the current CS:
AF=0.037683 * CS; r2=0.67.
AP=0.200886-0.0066762 * CS; r2=0.52.
AW=0.766637-0.034506 * CS; r2=0.67.
AA=0.078982-0.00438 * CS; r2=0.66.
EBW=0.851 * SBW
TA=AA * EBW
AF=proportion of empty body fat
AP=proportion of empty body protein
FIGURE 3–4 Relationship of empty body weight, protein, ash, and fat (as percentage) of body condition score in mature cows.