intercept of the regression of log HE on ME intake is used to calculate an estimate of FHP, which equates to NEm. The efficiency of utilization of ME for maintenance (km) is calculated as the ratio of NEm to MEm. These approaches have an advantage over calorimetric methods because they allow experiments to be conducted under situations more similar to those found in the beef cattle industry. They must be conducted over extended time periods, however, to allow accurate assessment of body energy changes. Accurate assessment of body composition at the beginning and end of the feeding period is required.

The NEm requirements of beef cattle have been estimated as

EBW is the average empty body weight in kilograms (Lofgreen and Garrett, 1968; Garrett, 1980). This expression was derived using data from, primarily, growing steers and heifers of British ancestry that were penned in generally nonstressful environments. Effects of activity and environment are implicitly incorporated into NEm in this system. Similarly, influences of increased feed during the feeding period, altered activity, or environmental effects differing from those at maintenance are implicitly incorporated into estimates of NEg. Application to differing situations requires appropriate adjustments.

Variation in Energy Requirements for Maintenance

Maintenance energy expenditures vary with body weight, breed or genotype, sex, age, season, temperature, physiological state, and previous nutrition. FHP or NEm is more closely related to a fractional power of EBW than to EBW1.0 (Brody, 1945; Kleiber, 1961); the most proper power has been the subject of much debate. EBW0.75, often referred to as metabolic body weight, was originally used to confer proportionality on measurements of HeE made in species differing considerably in mature weight (for example, mice to elephants). The convention generally adopted is to use EBW0.75 to scale energy requirements for body weight, even though other functions may be more appropriate for specific applications.

BREED DIFFERENCES IN MAINTENANCE

Armsby and Fries (1911) reported that “scrub” steers utilized energy less efficiently than “good” beef animals. Subsequently, numerous researchers noted differences in energy requirements or efficiencies of energy utilization among breeds of cattle. However, because of differences in procedures and approaches as well as diversity of breeds compared, direct comparison among available data is difficult. Blaxter and Wainman (1966), using calorimetry, noted that Ayrshire steers had 20 percent higher FHP (kcal/BW0.75) than black (Angus type) steers and 6 percent higher than crosses of those breeds. Results of Garrett (1971), using comparative slaughter, indicated that Holstein steers required 23 percent more feed to maintain body energy than Hereford steers. Similarly, Jenkins and Ferrell (1984b) and Ferrell and Jenkins (1985a) indicated feed required for weight or energy stasis in young bulls and heifers was greater in the Simmental breed than in those of the Hereford breed. Those data indicated MEm was, averaged across sexes, 19 percent (126 vs 106 kcal/BW0.75) greater for Simmental than Hereford cattle. Estimates reported for Simmental bulls were equal to those reported by Stetter et al. (1989). Values reported by Andersen (1980) and Byers (1982) indicated Simmental had 6 and 3 percent higher requirements than Herefords, respectively. Conversely, Old and Garrett (1987) and Andersen (1980) found maintenance requirements of Charolais and Hereford steers to be similar. Estimates for growing Friesian cattle average approximately 13 percent higher (5 to 20 percent) than for Charolais (Robelin and Geay, 1976; Vermorel et al., 1976; Geay et al., 1980; Vermorel et al., 1982). Webster et al. (1976, 1982) reported predicted basal metabolism rates of Friesian cattle to be greater than Angus (10 percent), Hereford (31 percent), or Friesian×Hereford (8 percent). Chestnutt et al. (1975) estimated maintenance requirements of Friesian to be 20 percent higher than Friesian×Hereford and 14 percent greater than Angus steers, whereas estimates of Truscott et al. (1983) were 7 percent higher for Friesian than for Hereford steers. Wurgler and Bickel (1985) found no consistent difference in estimates of maintenance requirements among Angus×Braunvieh, Braunvieh, or Friesian steers. Estimates of maintenance requirements of Limousin have been similar to those of Angus (Byers, 1982), Hereford, and Charolais (Andersen, 1980). Results of Webster et al. (1982) and Andersen (1980) indicated Chianina had about 30 percent higher energy expenditures than Angus and Hereford. Several other reports (Vercoe, 1970; Vercoe and Frisch, 1974; Patle and Mudgal, 1975; Frisch and Vercoe 1976, 1977, 1982; van der Merwe and van Rooyen, 1980; Carstens et al., 1989a) indicate that maintenance energy requirements of Bos indicus breeds of cattle, including Africander, Barzona, Brahman, and Sahiwal, are about 10 percent lower, and British crosses with those breeds about 5 percent lower than British breeds. In contrast, data of Ledger (1977) and Ledger and Sayers (1977) suggest maintenance requirements of the Boran may be about 5 percent higher than for Herefords. However, those results appear to conflict with those in the report of Rogerson et al. (1968).

Results of Jenkins and Ferrell (1983) and Ferrell and Jenkins (1984a,b,c) indicated maintenance requirements differed among genotypes of mature crossbred cows. ME required for energy stasis (kcal/BW0.75) of nonpregnant, nonlactating Jersey, Simmental, and Charolais sired cows



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