for maintenance) of steers and heifers is similar. ARC (1980) and CSIRO (1990) similarly concluded fasting metabolism of castrate males and heifers was similar.

Ferrell and Jenkins (1985a) estimated similar FHP (kcal/BW0.75/day) for Hereford bulls (70.4) and heifers (69.3), but estimates for Simmental bulls (80.8) were 9 percent higher than for Simmental heifers (74.1). When expressed as ME required for maintenance, Hereford bulls and heifers differed by only 2 percent, but estimates for Simmental bulls were 16.5 percent higher than for Simmental heifers. Pooled across breeds, estimated ME required for energy stasis was 12 percent higher for intact males than for females (123 vs 110 kcal ME/BW0.75/day). Webster et al. (1977) reported that Hereford×Friesian bulls had predicted basal metabolism values about 20 percent higher than steers of the same breed cross. In a subsequent report (Webster et al., 1982), values presented indicated bulls had 13 to 15 percent higher predicted basal metabolism than steers. Geay et al. (1980) also suggested higher maintenance requirements of bulls than heifers. ARC (1980) and CSIRO (1990), cited the report of Graham (1968) as indicating rams had 18 percent higher fasting metabolism than wethers and ewes. However, Bull et al. (1976) and Ferrell et al. (1979) estimated the ME required for maintenance of rams to be only 2 to 3 percent higher than for ewe lambs. The average of available data, if the sheep data of Bull et al. (1976) and Ferrell et al. (1979) are excluded, support the conclusion of ARC (1980) and CSIRO (1990) that maintenance requirements of bulls are 15 percent higher than that of steers or heifers of the same genotype.

AGE EFFECTS ON MAINTENANCE

The concept that maintenance per unit of size declines with age in cattle and sheep (Blaxter, 1962; Graham et al., 1974) has been generally accepted. Data from sheep, predominately castrate males, generally support this view (Graham and Searle, 1972a,b; Graham, 1980). The equation of Graham et al. (1974) indicated maintenance decreased exponentially and was related to age by the relationship e-0.08age, which indicates the decrease was 8 percent per year. The generalized equation reported by Corbett et al. (1985) for sheep and cattle, which was later adopted by CSIRO (1990), indicates maintenance decreases 3 percent per year. CSIRO (1990) indicated a minimum of 84 percent of initial values to be attained at about 6 years. Young et al. (1989) noted metabolic rate deviated substantially from allometric relationships; deviations were greatest during times of highest relative growth rate. They further suggested that significant deviations may also occur in association with other productive functions. Data reported from cattle are less consistent. Blaxter et al. (1966) found little influence of age (15 to 81 weeks), other than that associated with weight, on maintenance of steers. Results of Blaxter and Wainman (1966), Taylor et al. (1981) and Birkelo et al. (1989) were consistent with those findings. Vermorel et al. (1980) indicated maintenance requirements of cattle changed little between 5 and 34 weeks of age, but data of Carstens et al. (1989a) indicate a 6 percent decrease in FHP and an 8 percent decrease in ME required for maintenance between 9 and 20 months. Conversely, data reported by Tyrrell and Reynolds (1988) indicated ME required for maintenance (kcal/SBW0.75) increased 14 percent in beef heifers as weight increased from 275 to 475 kg. To our knowledge, direct comparisons of mature, productive females to younger or nonreproducing animals are not available. Indirect evidence (see above) suggests that maintenance of mature, productive cows is not less than that of younger, growing animals postweaning.

SEASONAL EFFECTS ON MAINTENANCE

Although, typically, effects of season have been associated with effects of temperature, it has become increasingly evident that season per se may have significant effects on maintenance requirements of cattle and sheep. Christopherson et al. (1979), Blaxter and Boyne (1982), and Webster et al. (1982) noted lower maintenance requirements of sheep, cattle, and bison during the fall of the year. Predicted basal metabolism of cattle was 90.3, 92.0, 78.9, and 86.3 kcal/BW0.75 during weeks 0 to 16, 17 to 32, 33 to 48 and 49 to 52, respectively, in Scotland (Webster et al., 1982). Data reported from Colorado by Birkelo et al. (1989) indicate FHP during fall, winter, and spring measurements were 90.7, 95.6, and 96.2 percent of FHP measured during the summer, but MEm did not consistently follow this pattern. Estimates of energy required for weight stasis of mature cows by Byers et al. (1985) for fall, winter, and spring were 86, 86, and 92 percent and those for energy stasis were 94, 102, and 100 percent of estimates made during the summer. Laurenz et al. (1991) reported similar effects of season on energy required for weight stasis of Angus and Simmental cows and for energy stasis of Angus cows but a dissimilar pattern for energy stasis of Simmental cows. Byers and Carstens (1991) reported further observations and indicated that as cow fatness increased, maintenance requirements increased during the spring and summer but decreased during the fall and winter. Walker et al. (1991) clearly demonstrated that seasonal effects in ewes are related to photoperiod. Possible season/genotype or latitude effects have not been quantified.

TEMPERATURE EFFECTS ON MAINTENANCE

For a detailed review, the reader is referred to the report, Effect of Environment on Nutrient Requirements of Domestic Animals (National Research Council, 1981b). Heat production in cattle arises from tissue metabolism and from



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