(from Angus or Hereford dams) was 112, 123, and 99 percent that of Angus-Hereford (130 kcal/BW0.75) cross cows. Similarly, the results of Lemenager et al. (1980) suggested that energy needs of Simmental×Hereford cows was about 25 percent higher than Hereford cows during gestation, whereas Angus×Hereford and Charolais×Hereford required about 5 and 7 percent more than Herefords. Laurenz et al. (1991) reported that Simmental cows required 21 percent more ME (kcal/BW0.75) than Angus cows. Klosterman et al. (1968) observed no difference in estimated energy requirements to maintain weight of mature nonpregnant nonlactating Hereford and Charolais cows when adjusted for body condition. Similarly, when adjusted for body condition, Hereford×Friesian and White Shorthorn×Galloway cows required similar amounts of energy to maintain liveweight (Russel and Wright, 1983). Estimates of ME (kcal/BW0.75) for energy stasis of nonpregnant, nonlactating Red Poll, Brown Swiss, Gelbvieh, Maine Anjou, and Chianina sired cows (C.L. Ferrell and T.G.Jenkins, unpublished data) were 112, 122, 117, 113, and 108 percent of values for Angus-Hereford (126 kcal/BW0.75) cross cows. Similar values were reported for weight stasis of those cows, with the exception of Gelbvieh and Chianina, which were higher (Ferrell and Jenkins, 1987). In that study, ME (kcal/BW0.75) required for weight stasis of purebred Angus, Hereford, and Brown Swiss were 116, 115 and 155 percent of that estimated for Angus-Hereford crossbreds (119 kcal/BW0.75). Results of Taylor and Young (1968) and Taylor et al. (1986) indicated energy required (recalculated as kcal/BW0.75) for long-term weight equilibrium of British Friesian, Jersey, and Ayrshire cows to be 20 percent higher than that of Angus and Hereford cows. Energy required by Dexter cows was 9 percent higher than the average of Angus and Hereford cows. Thompson et al. (1983) reported estimates indicating ME required for energy stasis was 9 percent higher in Angus×Holstein than in Angus×Hereford cows. Ritzman and Benedict (1938) observed no difference between energy required by Jersey and Holstein cows, whereas Brody (1945) observed slightly higher requirements by Holstein cows than Jersey cows. Solis et al. (1988) reported estimates of ME required for weight and energy stasis for 15 breed or breed crosses from a 5-breed diallel. Simple correlation between the two estimates was 0.84 and the slope of the linear regression was 0.99, indicating good agreement between the two estimates. When pooled, estimates of ME required for energy stasis were 104, 96, 96, 112, and 106 kcal/BW0.75/day for 1/2 Angus, 1/2 Brahman, 1/2 Hereford, 1/2 Holstein, and 1/2 Jersey cows, respectively.

Most of these reports observed differences between or among breeds compared and serve to document that considerable variation exists in maintenance requirements among cattle germ plasm resources. However, because of the diversity of breeds, methodologies, conditions, etc., direct comparisons between studies are often tenuous. As a result, the subcommittee selected studies in which British breeds or British breed crosses were compared with other breeds or breed crosses and expressed the results as relative values. It is believed the following generalizations can be made with some confidence, based on the data reviewed in the preceding paragraphs. In growing cattle, Bos indicus breeds of cattle (for example, Africander, Barzona, Brahman, Sahiwal) require about 10 percent less energy than beef breeds of Bos taurus cattle (for example, Angus, Hereford, Shorthorn, Charolais, Limousin) for maintenance, with crossbreds being intermediate. Conversely, dairy or dual-purpose breeds of Bos taurus cattle (for example, Ayrshire, Brown Swiss, Braunvieh, Friesian, Holstein, Simmental) apparently require about 20 percent more energy than beef breeds, with crosses being intermediate. Data involving straightbred, mature cows are more limited. However, available data with straightbreds combined with those of crossbreds, indicate that relative differences between breeds in mature cows is similar to that observed in growing animals. This may be generalized further to indicate, in both adult and growing cattle, that a positive relationship exists between maintenance requirement and genetic potential for measures of productivity (for example, rate of growth or milk production; Webster et al., 1977; Taylor et al., 1986; Ferrell and Jenkins, 1987; Montano-Bermudez et al., 1990).

Consistent with this concept, available data also suggest that animals having genetic potential for high-productivity may have less advantage or be at a disadvantage in nutritionally or environmentally restrictive environments (Kennedy and Chirchir, 1971; Baker et al., 1973; Frisch, 1973; Moran, 1976; O’Donovan et al., 1978; Jenkins and Ferrell, 1984b; Ferrell and Jenkins, 1985a,b; Jenkins et al., 1986). This concept is further supported by the reports of Peacock et al. (1976), Ledger and Sayers (1977), and Frisch and Vercoe (1977). Frisch and Vercoe (1980, 1982) have subsequently shown that selection for increased growth in a high-stress environment results in decreased FHP. Results from these and other studies show that correlated responses to selection may result in a genotype/environment interaction. Selection may result in a population of animals highly adapted to a specific environment but less adapted to different environments and with decreased adaptability to environmental changes (Frisch and Vercoe, 1977; Taylor et al., 1986; Jenkins et al., 1991).

SEX DIFFERENCES IN MAINTENANCE

Garrett (1970) found little difference in estimated fasting HE or ME required for maintenance between steers and heifers. Subsequently, Garrett (1980), in a study based on comparative slaughter experiments involving 341 heifers and 708 steers, concluded that FHP (net energy required



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