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Basal Metabolism Basal metabolism (HeE) is the result of chemical change that occurs in the cells of an animal in the fasting and resting state using just enough-~nergy to maintain vital cellular activity, respiration, and circulation. Thor tbe.measure- ment of basal metabolism,gbasal metaboIic~rate3, the animal must be in a thermoneutral environment`, ia ;postabsorptive .statc, resting, but conscious, quiescent,-and without stress. (There is difficulty determining when ruminant animals reach the postabsorptive state.) The:HeE established under minimal heat output may have the connotation that values have plateaued, when actually, as fasting continues, HeE is in a slow decline. Thus, length of time for the fast is an important criterion to specify. There is much confusion and disagreement as to when resting me- tabolism ends and basal metabolism begins for each species. Passage rate of food through the tract could be a factor on which to base this decision. In all cases, the length of the fasting period should be specified. A common bench- mark of fasting metabolism is when the respiratory quotient (RQ) reflects the catabolism of fat. Experimentally this requires from 48 to 144 h fasting to achieve. Generally HeE is determined when the animal is not thermally stressed (no panting, no gular response, no sweating, no shivering) and is technically where heat production has plateaued at a minimal level. Thus a range of tem- peratures may exist over which HeE can be determined. The formula for calculating rate of metabolic heat production by ruminants via indirect calorimetry (Brouwer, 1965) is as follows: 22
Basal Metabolism 23 HeE (kcal) = 3.866 x O2 (liters) + 1.200 x CO2 (liters)-0.518 x CH4 (liters)- 1.431 N (grams in urine) Although O2 measurements alone are not as accurate as adjusting for CO2 production and the other factors, calculations using 4.7 kcal/liter O2 are ac- ceptable because of a minimal technical error in the calculated value (Whit- tow, 19761. This assumes an RQ for a fasting animal of 0.71, and ignores the losses of methane (CH4) and nitrogen (N) in nonruminants. Recent studies, based on oxygen consumption, have failed to establish a TNZ in poultry. Heat production (HE) was found to decline continuously as the ambient temperature increased (O'Neill et al., 1970; Romijn and Vreugdenhil, 1969; van Kampen, 1974~. Using 20°C as a baseline, the HE of chickens was 13.4 percent higher at 15°C and 7.5 percent lower at 25°C (Figure 4), temperatures that are, respectively, presumably outside and within the TNZ. In all cases chickens were exposed to constant temperature environments. Similar data for beef cows are shown (Figure 4), although resting metabolic rate here is based on the previous month's average ambient temperature. The use of this temperature is necessary because cattle, unlike chickens, are usually exposed to widely fluctuating natural environments. There is supportive data for growing calves (Christopherson et al., 1979; Webster et al., 1969) of a shift in resting thermoneutral heat production as a consequence of prior cold exposure. Data from sheep (Slee, 1968; Webster et al., 1969) also suggest that resting metabolic rate is influenced by temper- ature to which the animals were exposed prior to the metabolic measurement. Data from two cows (Young, 1975a) in controlled temperature chambers with 8 weeks exposure to a near-constant temperature are shown in Figure 4 with each point representing results from a single animal as an average of measurements made over several weeks. Other data to support the overall ef- fects were from groups of cows, one group housed (4 cows) and one group outside (8 cows) during the Alberta winter. Measurements are group aver- ages, and prior exposure temperature is average outside temperature during the month prior to metabolic rate measurements. On the basis of metabolic heat production, there appears to be no obvious plateau that can be easily identified as thermoneutral. This creates doubts about using minimum metabolic heat production to define thermoneutrality. The equation for estimating HeE appears to be readily solvable, but its der- ivation and meaning are complex. Surface area of an animal is an important component because of its role in heat loss, and surface area is positively cor- related to heat production. In turn, the HeE is empirically highly correlated to metabolic body weight, i.e., WE, where x is the exponential power to which the value W is raised. For a detailed discussion on the derivation of HeE - aW`, the reader is referred to Kleiber (1961) and Mount (1968~.
24 FARM ANIMALS AND THE ENVIRONMENT 160 140 - cut - ~ ]20 - Y 110 UJ 100 J 0 90 At: UJ 80 by en .^ u.l /u fir 60 · ~ Beef Cow · I ndividuals, Young, 1 975a · Groups, Young, 1975b \Laying Hen `~.~ -30 -20 -10 0 10 20 EFFECTIVE AMBIENT TEMPERATURE OF PRIOR EXPOSURE ( C) 30 FIGURE 4. Basal metabolic heat production for laying fowl (van Kam- pen and Romijn, 1970) and resting metabolic heat production at 22 h fast- ing for the beef cow (Young, 1975a). Values for HeE have considerable variability even among similar breeds within species. Table 3 lists HeE values from numerous sources determined by several experimental approaches, including gaseous exchange, heat out- put measurements, and comparative slaughter. Kleiber (1961) determined that M (as a measure of HeE) = 70 kcal/W0 75/day as the best approximation of HeE for homeotherms, where M = kcal and W = body weight in kilo- grams. The European Association for Animal Production adopted in 1964 the three-quarters power of body weight as the interspecies reference base (Mount, 1968~. Recognizing the many routes by which energy transfer can occur in ani- mals, one can foresee that many factors influence HeE, such as prior plane of nutrition, rate of feed intake, environment, age, activity, disease and infec
Basal Metabolism 25 TABLE 3 Basal Metabolic Rates of Various Farm Species from Various Sources to Indicate Older and Contemporary Values in the Literature Species HeE - aWX/day Source Rabbit 64.7 W075 Kleiber, 1961 Goat 54.4 W075 Kleiber, 1961 Sheep 72.4 W°7s Kleiber, 1961 Cow 69.8 W075 Kleiber, 1961 Cattle, beef heifer 75.4 W075 Kleiber, 1961 Pig 68.1 W075 Mount, 1968 Sheep, old wether 58.5 W073 Blaxter, 1962 Cattle, beef 77.0 W075 Lofgreen and Garrett, 1968 Cattle, dairy 117.0 W075 Cited by Mount, 1968 Cattle, several breeds 93.0 W075 Blaxter and Wainman, 1961 Poultry 78.0 W075 Whittow, 1976 lion, sex, breed, species, type and extent of pelage, and others. Even in ho- meotherms body temperature tends to rise as the animal has greater difficulty removing heat, and with the rise in core temperature comes a progressively increasing O2 utilization, which increases on the basis of Qua = 2.0 (Ames et al., 1971~. Thus, a rise of 10°C in body temperature theoretically results in a twofold increase in oxygen utilization, as revealed by a linear plot on a semi- log scale of O2 utilization (log) versus core temperature (arithmetic). Obvi- ously, animals succumb should the core temperature rise several degrees above normal, but the expression of Qua reveals how increasingly more diffi- cult it is for the animal to rid itself of excess heat when relatively small rises in core temperature increase heat production. However, certain breeds of an- imals are more efficient in coping with heat, and their resistance depends to some extent on how much of a burden is HeE. For example, light breed chickens (White Leghorns) have a higher UCT and greater heat loss at ambi- ent temperatures of -5 to 32.5°C than heavy breeds such as Barred Rocks (Ota and McNally, 1961) and appear to be more tolerant of heat. The greater resistance of light breeds to hot weather is partially explained by a reduced HeE (Burman and Snaper, 1965~. Minimizing the proportion of metabolizable energy expended for HeE would be advantageous in maximizing energy available for product syn- thesis. As an example, a diurnal fluctuation of HeE in poultry is well docu- mented, since heat production at night may be as much as 18-30 percent less for the chicken (King and Farner, 19614. By offering laying chickens their diets as single meals in the evening rather than in the morning, efficiency of production is higher (Simon, 1973), attributed to the lower HeE at that time (Balnave, 19741. Based on O2 consumption, cold-acclimated animals gener
26 FARM ANIMALS AND THE ENVIRONMENT ally have higher heat production at temperatures above those at which accli- mation occurred (Young, 1975a,b). This is a response to acclimation in ani- mals with a lower UCT. Such information has practical application in a consideration of alternatives for maximizing efficiency of energy expenditure for maintenance and weight gain in colder climates where shelter versus feed or fuel weigh heavily in farming budgets. Verstegen and Van der Hel (1974) showed that heat loss is less and LCT lower for pigs raised on asphalt or straw, than on concrete, when cold environments are encountered. Thus, the animal tends to adjust its metabolism to tolerate colder environments. Basal metabolism is as much as 45 percent higher in molting hens (Perek and Sulman, 1945), and this expenditure of energy accounts for up to 82 per- cent of the ME intake. Unfeathered male chickens have HeE threefold greater than males at 22°C, while at 29 and 34°C, the unfeathered birds had HeE values 1.5 and 1.0 times, respectively, those of feathered males (O'Neill et al., 1971). Apparently, prior plane of nutrition influences HeE. Male chickens previ- ously fed a high-energy diet have a higher HeE than those given low-energy diets (Mellen et al., 1954), and the effect persists for some time after dietary energy is lowered (Freeman, 1963~. High-energy diets generally produce greater weight gain with greater efficiency, so their use for broiler produc- tion, despite a higher HeE, has received greater acceptability. Dukes (1947) reported that chickens with transmissible lymphomatosis had a slight increase in HeE but that a marked increase was detected in two cases of lymphocytoma. A febrile condition in animals that raises the core temperature would be reflected in the HeE. The behavior component of animals is closely related to the level of basal functions, and the interplay of behavior with the stage of the reproductive cy- cle and managerial systems also modifies HeE. Such husbandry practices as shearing sheep and the molting of hens are similar in influencing heat pro- duction at basal conditions. Thus, there are many parallels of responses among the wide diversity of animals that serve mankind.