52 REPORT OF DEPARTMENT OF ANIMAL HUSBANDRY OF THE 
The foregoing figures make it plain that the excess of diges- 
tible protein in the ration over and above that needed for the 
milk, rather than any demand for milk secretion, largely deter- 
mines the extent to which protein suffers destructive changes. 
In the case of the grade Jersey the daily protein supply was 
gradually diminished from 1.8 lbs. to .9 lb. and with Cow 12 
from 2.6 lbs. to 1.6 Ibs. The most marked result was to depress 
the daily protein decomposition from .86 Ib. to .46 lb. in one case 
and from 1.2 lbs. to .60 lb. in the other case. When the food 
protein was increased to the original quantity the reverse process 
took place. While the protein utilized for milk secretion was 
somewhat lessened when the food protein was decreased, the 
depression of milk yield was not at all commensurate with the 
drop in the digested protein. This accommodation of the cow’s 
metabolic process to a diminished protein supply mostly by a 
decrease in protein decomposition rather than wholly by depress- 
ing the milk production, indicates what the writer has previously 
suggested, that protein exchange outside the formation of the 
nitrogen compounds of the milk has no constructive function and 
that so large a quantity of protein as 2.5 lbs. daily for the aver- 
age cow is justified only on the theory that the balance above 
what is really utilized in the miik and what would be required 
for maintenance in a non-producing animal acts to stimulate 
milk secretion rather than to support it. These comments are 
not to be taken as implying that a liberal supply of protein is not 
promotive of generous milk secretion. 
A view unsupported by valid evidence, which has been more 
or less current and has been to some extent accepted in a pop- 
ular way, is that the fat supply in the food modifies butter fat 
formation to an important extent. It is already shown in the 
context that with Cow 10 a large increase in the food fat did not 
affect the proportion of fat in the milk or the amount secreted. 
No inquiry was made as to whether the constitution of the but- 
ter fat mixture was in any way affected. 
