Although the concept of metabolic body size (kg0.75) has gained widespread use in the field of energy metabolism, its application to the growing animal has been questioned. Fasting heat production, or maintenance, rather than being a constant function of body size, has been shown to vary because of breed, sex, condition, physiological state, production level, nutrition level and environmental conditions. Data are presented to show that fasting heat production and maintenance vary with nutritional level or rate of growth in animals postweaning. Variation in these energy expenditures are related to variation in weight of metabolically active internal organs. Weights of liver and gut and fasting heat production are shown to be functions of body size and level of production. More information is needed to ascertain the primary components of energy expenditures in animals and to quantitatively relate these components to animal energy metabolism.
Twenty-seven 12-week-old barrows with average initial weight of 27 kg were randomly assigned to three treatments. The first group (HL) was fed to gain 19 kg body weight during the first 35 days (period 1) and to lose 5 kg during the second 35 days (period 2). The second group (MM) was fed to gain 7 kg during both periods 1 and 2. The third group (LH) was fed to lose 5 kg during period 1 and to gain 19 kg during period 2. At the end of the 70-day period, 7 pigs from each treatment were fasted for 30 hours and fasting heat production (FHP) was measured by indirect calorimetry. The animals were slaughtered and weights of stomach, small and large intestine, liver, pancreas, spleen, kidneys and heart were measured. Although all animals had the same final body weight, animals on the higher plane of nutrition during period 2 had significantly higher FHP and higher weights for stomach, small and large intestine, pancreas, liver and kidneys. FHP and the weights of small intestine, pancreas and liver from animals receiving the higher plane of nutrition during period 2 (LH) were 50% heavier than that from animals on low plane (HL). Positive correlations exist between FHP and weights of stomach, small intestine, large intestine, pancreas, liver and kidneys. These results indicate that prior nutritional history significantly influences FHP, which is highly correlated to weights of metabolically active organs.
1. Forty-eight intact male lambs (30 kg) were fed to gain 16 (H), 5 (M) or -6 (L) kg during a 42 d interval (period I). Lambs from each of the H and M groups were fed to gain either 16 (HH, MH), 5 (HM, MM) or -6 (HL, ML) kg and lambs from the L group were fed to gain 27 (LS), 16 (LH) or 5 (LM) kg during the ensuing 42 d (period 2).2. Fasting heat production (FHP) of four lambs from each treatment was determined at the end of period 2.3. Weights and compositions of the carcass, offal and digesta-free body as well as weights of major internal 4. Within groups of lambs of similar weight at the end of period 2, body composition was, in general, similar, 5. Within groups of lambs of similar weight, lambs that were fed at higher planes of nutrition during period 6. Neither weight of the liver, kidney, stomach, small intestine, large intestine nor daily fasting heat production 7. Regression analysis indicated that the feeding of lambs at higher planes of nutrition during period 1 resulted organs were determined for four lambs of each treatment at the end of periods 1 and 2.but FHP was greater in lambs that had been on higher planes of nutrition during period 2.2 had greater weights or proportions of liver, small intestine, large intestine and stomach.were constant functions of body-weight. Relations of these traits to body-weight changed with rate of gain.in higher maintenance requirements of those lambs during period 2.
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