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The flow of energy through the body is illustrated in Fig. 4.6. Energy starts as con-

sumption energy or intake energy. Part of this is digested in the gut and passes through

the gut wall as digestible energy, the rest being passed out in the feces as fecal energy.

Part of the digestible energy is lost in the urine, and the remainder, called metabolic

or assimilated energy, can then be used for work. The work energy can be divided

into two: respiration energy which is used for the basic maintenance of the body (rest-

ing energy) and for activity, and production energy for growth and reproduction.

The flow chart for protein is similar except that protein is normally used only for

production. Protein is not used in respiration except under special conditions of food

shortage when protein is broken down (catabolized) to provide energy.

Metabolic energy (M) can be measured in two ways:

1 in the laboratory by measuring resting energy and activity to obtain the respira-

tion component (R), and from growth and population studies to obtain production

(P), so that:

M =R +P

2 in the field by measuring consumption (C), fecal (F), and urinary (U) outputs, so

that:

M =C −F −U

Basal metabolism is the energy needed for basic body functions. The energy comes

from oxidation of fats, proteins, and carbohydrates to produce water and carbon

dioxide. Thus maintenance energy can be measured from expired air volume and

composition because intake air has a stable composition of 20.94% oxygen, 0.03%

carbon dioxide, and 79.03% nitrogen. Since 6 moles of carbon dioxide and water are

46 Chapter 4

1.5

1.0

0.5

0

• 0.5

25

20

15

10

5

0

40

20

5 10152025SOND J FMAMJ J AOS^0

1977 1978

Fecal crude protein (%)

Rate of gain (kg / day) Snow depth (cm)

Fecal crude protein (%)

(b)

(a)

Fig. 4.5(a) Body weight gain of male elk () and calves () in Alberta can be predicted from the percentage of fecal crude
protein. (b) Seasonal changes in the percentage of fecal crude protein are related to snow depth. (After Gates and Hudson
1981.)

4.5 Basal metabolic rate and food requirement

4.5.1Energy flow

4.5.2Basal
metabolic rate