620 
Joumal of Agricultural Research 
Vol. XXIX. No. 12 
Experiments reported in the present 
article and discussed to some extent in 
the preceding pages show that milk 
secretion in cows is markedly affected 
by changes in the dietary protein. 
These considerations justify the at¬ 
tempt to interpret the results which are 
to be discussed in this section on the 
hypothesis that changes in diet fre¬ 
quently affect milk yield through 
MS1Y* Ut/ME 
Fig. 6.— Experiment I. Changes in concentration of nitrogen and fat of milk. The concentration in period 
1 is taken as 100 per cent. In period 2 the protein and energy of the ration were reduced, and in period 3 
they were restored to adequacy 
In general, it may be said that milk 
secretion may be affected by changes in 
either the fat or protein of the ration, 
but much more easily by changes in 
the protein. When the milk yield is 
affected by changes in the dietary fat, 
there is generally a tendency for the 
concentration of milk nitrogen to vary 
in a direction opposite to that of the 
change made in the dietary fat, while 
this is not the case when milk yield is 
affected by changes in the dietary pro¬ 
tein. The situation is most easily 
accounted for by supposing that a 
Fig. 8.-— Experiment III. Change in the composition of milk. Concentration in period 1 is taken as 
100 per cent. In period 2, the quantity of protein in the ration was reduced without change in its quality: 
m period 3, it was restored to adequacy 
Fig. 7— Experiment II. Changes in the com¬ 
position of milk. The concentration in period 1 
is taken as 100 per cent. In period 2, the energy 
content of the ration was reduced, while the 
protein was left unchanged; in period 3, the 
energy was restored to adequacy 
change in the protein of the ration 
tends to affect milk secretion through a 
change in the precursor of milk pro¬ 
tein—namely, the free amino acids of 
the blood—while a change in the 
dietary fat tends to affect it through a 
change in the precursor of milk fat— 
namely, the phosphatide of the blood. 
changes brought about in the free 
amino acids of the blood. 
The discussion of the individual 
experiments of this series may well be 
begun with that of Experiment II, in 
which the quantity of energy (chiefly 
carbohydrate) in the ration was mark¬ 
edly reduced without making any con- 
