Units of metabolic body size for comparisons amongst adult sheep and cattle.

Abstract

Proc. Aust. Soc. Anim. Prod. (1972) 9: 352 UNITS OF METABOLIC BODY SIZE FOR COMPARISONS AMONGST ADULT SHEEP AND CATTLE N. McC. GRAHAM* Summary It is shown that liveweight (W) in kgO*' is not a valid unit of metabolic body size in comparisons between sheep and cattle, W'.D is recommended for this purpose alone. For comparisons within these species, neither W nor W'*' should be used but W's13 is acceptable. Fat-free weight in kg'aD may serve both purposes. 1. INTRODUCTION When animals of dissimilar weights are compared, prdwtive functions are commonly expressed per kgop73; this is based largely on the premise that fasting metabolism (i.e. net energy requirement for maintenance) per kg'.T3 is constant (Brody 1945). It has been suggested that kg3i4 is more appropriate than kg's7' (Kleiber 1965), but the two units are indistinguishable in practice. Brody collated information from many sources, and concluded 'that the maintenance needs for energy, protein and dietary catalysts . . . vary in the same manner as basal energy and endogenous nitrogen metabolism, namely with approximately W'Y He also stated 'that milk production and egg production [vary] with W'sY rather than with WI-O, and that the ratio of maximal food-energy intake to basal metabolism is independent of the size of the animal. There are ,exceptions in detail, but this appears to be the general rule'. It is unfortunate for those interested in farm animals that sheep and cattle are examples of such 'exceptions in detail'. Thus in the data on fasting metabolism which Brody lists, cattle were above the' interspecific mean and sheep below. Methodology has been improved, but recent observations confirm that metabolic rate is much higher in cattle than in sheep; 351 versus 243 kJ/24h/kg0e7' (84 and 58 kcal) (Blaxter and Wainman 1966), to be compared with the mammalian average of 293 kJ (70 kcal) /24h/kg0.73 (Brody 1945). Furthermore, it has been found that fasting heat production per kg'm7' varies inversely with live weight in adult sheep (Graham 1967) ; the existence of such systematic, as opposed to random, variation in metabolic rate is an obstacle to the intraspecific use of Brody's unit of metabolic body size. Blaxter and Wainman ( 1966) mentioned that metabolism per kg'yg was more nearly constant as between sheep and cattle than metabolism per kg0*73, but they did not elaborate on this nor recommend the use of W'yg as a scaler. * Division of Animal Physiology, CSIRO, Ian Clunies Ross Animal Research Laboratory, P.O. Box 239, Blacktown, N.S.W., 2148. Studies of adult sheep have shown metabolism per kg of fat-free weight to be constant (Graham 1967), but there has been no assessment of this relationship in cattle. In the present paper, the available information on fasting metabolism is used to derive units of metabolic body size for purely practical application in comparisons amongst adult sheep and cattle. II. SOURCE OF DATA Published information for adult sheep and cattle was used, subject to measurements of fasting metabolism meeting the modem methodological criteria discussed by Marston ( 1948) and by Blaxter ( 1962)) viz. the animals were trained for calorimetry and had been fasted for 2-5 days, the environment was thermoneutral and the period of measurement was at least 24 h. The only acceptabie relevant data for adult cattle comprised 30 sets of observations (liveweight and fasting metabolism) on Aberdeen-Angus, Shorthorn and Shorthorn x Ayrshne crossbreds of weights 3 15-585 kg (Forbes et crZ. 1928, 193 1; Blaxter and Wainman 1964, 196.6). Estimates of body fat content were made on the basis of the following equation derived from the work of Haecker ( 1920) and Moulton, Trowbridge and Haig ( 1922) on similar types of animal:F = 95 W1y9 x lo-' (residual standard deviation 23 per cent) where F is total body fat (ether extract) and W is liveweight, both in kg. There is more extensive information for sheep, and 30 sets (to match the cattle data) were chosen in which fat-free weight had actually been measured in vivo (Graham 1967). The values used represented the whole weight range (26-64 kg), and metabolic rates were close to the average, weight for weight, in the published material (Marston 1948; Blaxter 1962; Graham 1967). III. ANALYSIS AND DISCUSSION In his analysis of data for diverse species, Brody ( 1945) demonstrated that the relationship between metabolism and weight could properly be derived from the linear equation relating the logarithms of the variables. This procedure was followed, although its validity could not be proven with the limited amount of data for only two species. Analysis of the pooled data for sheep and cattle showed that metabolic rates averaged 121 -F- 13 kJ (29 2 3 kcal) /24h/kg0eg2 of total weight (W) or 15 1 f- 13 kJ (36 t- 3 kcal) /24h/kg0eg1 of fat-free weight (L) . The extreme polarity of the data precludes a useful statement about the errors of the exponents. The derivation of weight exponents for each species separately is unsatisfactory because of the inevitably narrow range of liveweight; therefore the applicability of the above averages, and of some alternative expressions, to each species was examined. Since metabolism per unit of metabolic body size should not vary systematically with liveweight, this examination focussed upon the correlation between Iiveweight and each expression of metabolic rate. It is apparent from Table 1 that simple liveweight is a poor unit of metabolic body size because correlation between it and metabolic rate was quite high (Y, 353 0.6 to - 0.8). Fat-free weight is a good unit for sheep, but it seems from Table 1 to be less satisfactory for cattle and inappropriate for comparisons between TABLE 1 Fasting metabolic rate per weight unit and its correlation with liveweight in adult sheep and cattle sheep and cattle. However, since the estimation of fat-free weight in the cattle cannot be regarded as very reliable, it can only be suggested that access to actual data would probably simplify the situation. Brody's unit evidently has some justification in comparisons amongst sheep or amongst cattle but not, as already stated, in comparisons between the twoI species. Although W'sg is an ideal unit of metabolic body size for comparing sheep with cattle, the correlations in Table 1 show that it is no better than simple weight otherwise. Subject to the reservations stated above, the expression most suited to both purposes is Lo? Two of these alternative units were tested in application to energy balance data for 3 sheep and 3 steers, all of which were given identical diets at several levels of feeding (Blaxter and Wainman 1961) . Regression of energy gain on metabolizable energy for either the sheep or the cattle accounted for 84 per cent of the variance in energy gain when the scaler was Wo*73 and 89 per cent when it was W'sg However when the data for the two species were pooled, only 72 per cent `of the variance was taken up by regression with W'sT' as the unit whereas 90 per cent was removed when it was W'sg, a substantial improvement. In another direct comparison between sheep and cattle, Blaxter, Wainman and Davidson ( 1966) fed adult wethers and steers identical diets ad lib. With 3 forages, maximum intake of digestible energy per kJ of measured maintenance requirement was 11 per cent less to 2 per cent more, depending on digestibility, in the steers. However, intake per kg'aT3 was 14 per cent to 38 per cent more in the steers than in the sheep. Liveweights were not given, but assuming they were 50 and 5,OO kg for the particular breed types used (Blaxter and Wainman 1966), it may be shown that intake per kg'eg was 10 per cent to 18 per cent less 354 in the steers. On this basis, Woe9 could be judged a more suitable unit than W'-53 when appetite is in question. However, in the absence of prior knowledge of the control of appetite, the indiscriminate or exclusive use of any unit of liveweight is liable to be misleading. IV. CONCLUSION It is concluded that the available data do not support the use of Wo*73 as a unit of metabolic body size in comparisons between sheep and cattle; there is some justification for W Oeg . It is difficult to demonstrate the correct usage for within-species comparisons; simple Iiveweight and W'ag are definitely unsuitable, but `wO.73 is probably satisfactory in practice. There is some evidence to indicate that fat-free weight in kg'eg may be an adequate unit for both situations. It is suggested that, pending the acquisition of much more information, particularly for cattle, the present analysis offers a more rational basis for choosing a unit of metabolic body size when making comparisons amongst sheep and cattle than does Brody's ( 1945) 'mouse to elephant' curve for basal metabolism. V. REFERENCES Blaxter, K. L. (1962). British Journal of Nutrition, 16: 615. Blaxter, K. L., and Wainman, F. W. (1961). Journal of Agricultural Science, Cambridge. 57: 419. Blaxter, K. L., and Wainman, F. W. (1964). Journal of Agricultural Science, Cambridge, 63: 113. Blaxter, K. L., and Wainman, F. W. (1966). British Journal of Nutrition, 20: 103. Blaxter, K. L., Wainman, F. W., and Davidson, J. L. (1966). AnimaZ Production, 8: 75. Brody, S. ( 1945). 'Bioenergetics and Growth' [Reinhold: New York]. Forbes, E. B., Braman, W. W., and Kriss, M. (1928). Journal of Agricultural Research, 37: 253. Forbes, E. B., Braman, W. W., and Kriss, M. (1931). 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