624 
Journal of Agricultural Research 
Vol. XXIX, No. 12 
tinuous process and where there is a 
constant demand tor energy for milk 
secretion, the level of plasma amino N 
is frequently more influenced by (a) the 
energy content of the ration, (b) the 
quality of the dietary protein, and (c) 
the nutritional condition of the animal 
than by the actual quantity of protein 
in the ration. 
7. Under appropriate conditions very 
great changes may occur in the con¬ 
centration of the free tryptophane of 
the blood, while at the same time little 
or no change occurs in the level of total 
amino N. These alterations in the com¬ 
position of blood mixture of amino acids 
may continue for weeks. They occur 
when either the quantity or quality of 
the dietary protein is changed. 
8. Sharp reductions in the quantity 
of energy in the ration or in either 
the quantity or quality of the protein 
are immediately followed by reductions 
in milk yield. The composition of the 
milk also undergoes changes which may 
reasonably be attributed to the changes 
in diet. When the original ration is 
again substituted for the reduced 
ration, there is a general tendency for 
the yield and composition of the milk 
to return to their original status. A 
study of these changes in the rations 
and milk, along with those occurring 
simultaneously in the amino N, trypto¬ 
phane, and other blood constituents 
which have been determined in these 
experiments, justifies the working hy¬ 
pothesis that changes in dietary pro¬ 
tein and energy affect milk secretion 
largely by inducing changes in the 
quantity and quality of the amino- 
acid mixture circulating in the blood 
plasma. 
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