MILK PRODUCTION AND BODY INCREASE OF DAIRY COWS 
17 
of the corresponding excretions which alone can be of use to the 
animal. 
In other words this difference is the portion of the feed energy 
which can become useful for maintenance (in the more inclusive 
sense) work, growth, or other constructive metabolism. It is to this 
difference that the term "metabolizable energy" has been given. 
In the literature this portion of the feed has been variously desig- 
nated, as available energy (as distinguishable from net available 
energy), fuel value, physiological heat value, etc. From what has 
been said it must not be inferred that all this difference, or metaboliza- 
ble energy of the feed, can be utilized for maintenance (in the nar- 
rower sense) or production; rather, it is understood that some of 
this energy must be utilized in the work of digesting the feed, and 
forming and eliminating the excreta. 
Table II shows a somewhat unusual case of a loss of nitrogen ac- 
companied by a gain of carbon (cow 631, periods I and II). In all 
the other experimental periods nitrogen and carbon were either 
gained or lost together. This is discussed more fully under gain or 
loss of body protein and fat. 
The end products of protein destruction in the body contain 
chemical energy, and in order to eliminate the nitrogen factor in the 
computation of metabolizable energy the animal body is computed 
to nitrogen equilibrium by the application of a correction for the 
gain or loss of body nitrogen. The factor used for this correction is 
that proposed by Rubner and referred to by Armsby (5) ; namely 
7.45 Calories per gram of nitrogen. This energy is added in the case 
of gain, and subtracted in the case of nitrogen loss. Thus, a con- 
siderable part of the gross energy of the feed protein can not be 
utilized by the organism. This correction of the energy for the 
dailv gain or loss of bodv nitrogen is found in Table 12: 
Table 12. — Correctio 
Yljoi 
* gain or loss of body nit 
•ogen 
Cow 
Xo. 
Pe- 
riod 
Nitrogen 
gain 
Correc- 
tion 
Cow 
Xo. 
Pe- 
riod 
Nitrogen 
gain 
Correc- 
tion 
Cow 
No. 
Pe- 
riod 
Nitrogen 
gain 
Correc- 
tion 
631 
lui 
Grams 
-4.0 
-6.0 
+19.3 
Calories 
-29.8 
-44.7 
+143. 8 
615 
(ill 
Grams \ Calories 
+3.4 i +25.3 
-1.2 | -8.9 
-7.1 [ -52.9 
579 
U 
Grams 
+9.5 
+1.8 
Calories 
+70.8 
+ 13.4 
GAIN OR LOSS OF BODY PROTEIN AND FAT 
Since the glycogen supply of the body does not fluctuate materially 
under normal conditions of feeding, changes in the nitrogen and 
carbon of the body may be ascribed to changes in the protein and 
fat content without appreciable error. From the nitrogen and car- 
bon balances it is therefore possible to compute the gain or loss of 
body protein and fat. Whether nitrogen is gained or lost it is con- 
sidered to be accompanied by such an amount of carbon as is con- 
tained in an equivalent amount of protein. Thus tjie gain or loss 
in fat can be computed from the carbon balance only after the car- 
bon content of the gain or loss of protein has been set aside. From 
this it follows that in case there is a utilization of a part of the non- 
nitrogenous fraction of katabolizing tissue protein the amount so 
utilized is not accounted for and constitutes an error in the compu- 
100177— 24f 3 
