622 
Journal oj Agricultural Research 
Vol. XXIX, No. 12 
the dietary protein is increased and the 
milk yield increases without a rise in 
total plasma amino N, the concentra¬ 
tion of blood tryptophane may rise; 
and, finally, that when the milk yield 
is altered by a change in the quality of 
the dietary protein, the concentration 
of blood tryptophane may be very 
greatly altered, although the total 
plasma amino N may be unchanged. 
The correspondence between the 
changes of diet on the one hand and of 
the yield and composition of milk on 
the other, taken along with these 
results which show that the quality 
of the plasma amino acid mixture may 
vary as well as its quantity, justify 
the belief that the changes in dietary 
protein in these experiments influenced 
the yield of milk largely through changes 
in the quality of the plasma amino- 
acid mixture. 
In Experiments VII and VIII the 
quality of the dietary protein was 
changed without change in its quantity. 
protein without changing the total 
energy of the ration, there was a strong 
tendency for the concentrations of 
milk N and fat to run parallel (figs. 
8 to 12). There is no reason to think 
that changes in dietary fat had any 
influence on the concentration of milk 
fat in these experiments, for, although 
small changes were frequently made in 
the dietary fat at the same time that 
the protein was altered, there was no 
tendency toward parallelism between 
changes in dietary fat and changes in 
milk fat. In addition, the experiments 
of the Deutscher Landwirtschaftsrat 
(4) referred to above show that the 
changes which were made in the dietary 
fat in these experiments were not of 
such a nature that they could have had 
any influence either on the secretion of 
milk fat or on milk secretion in general. 
They also indicate that it was not 
through changes in the amount of fat 
formed from protein or through the 
phosphatide of the blood plasma that 
'SC 
/2a 
//o 
/oo 
90 
/=iSe /<OO 1 EEJS/OP 
2 \ 3 
1 
/V. 
'T/eOG-EM 
f?. 
— 
1 
1 
-- 
1 
20 2S 20 
JP/EC. 
4 9/4/9 24- 29 2 <S /2 /2 
i/s4H. /=20. 
Fig. 10.—Experiment VI. 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 increased and its quality improved; 
in period 3, the quantity and quality of the ration were still f urther increase^ 
The resulting changes in plasma amino 
N and in total milk yield are in fairly 
close agreement with the conclusions 
which have been drawn from the other 
experiments, and no special comment 
is required. But the concentration of 
milk N decreased in these two experi¬ 
ments when the quality of the dietary 
protein was improved, and vice versa, 
and this is not in entire accord with 
the results of the other experiments. 
The factors which control the con¬ 
centration of milk N are, however, in 
all probability numerous and com¬ 
plicated, and it is not felt that the 
lack of parallelism between quality of 
dietary protein and concentration of 
milk N in these two experiments is 
sufficient seriously to impair the force 
of the conclusions which have been 
drawn from the others. 
This part of the discussion may be 
concluded by pointing out certain other 
interesting aspects of the results. 
When changes in milk secretion were 
brought about by altering the dietary 
the changes in dietary protein affected 
the secretion of milk fat. The results, 
therefore, indicate that the secretion of 
milk fat may be influenced through the 
amino-acid mixture of the blood. It 
must be pointed out, however, that in 
Experiment I, where the protein and 
energy of the ration were changed 
together, and in Experiment II, where 
the energy was changed while the 
protein was kept constant, there was no 
marked tendency toward parallelism 
between the concentrations of milk 
protein and milk fat. Considerable 
changes in the concentration of plasma 
amino N had here no tendency to 
affect the secretion of milk fat. It 
seems not unreasonable to suppose that 
reductions in energy of a ration 
tend to cause body fat to be thrown 
out into the blood and that this in¬ 
creased supply of fat in the blood may 
overcome the tendency for the con¬ 
centration of milk fat to go down with 
that of the milk nitrogen. This is 
suggested by the work of Eckles and 
