RELATION BETWEEN THE DIET. THE COMPOSITION OF 
THE BLOOD, AND THE SECRETION OF MILK OF DAIRY 
COWS 1 
By C. A. Cary t , Organic Chemist, and Edward B. Meigs, Physiologist, Research 
Laboratories, Bureau of Dairying, United States Department of Agriculture 
INTRODUCTION 
It has been shown that with milking 
cows the lactose, fat, and proteins of 
milk are made in the mammary gland 
from glucose (10), 2 phosphatide ( 11), 
and amino acids (3 ), respectively, 
which are taken directly from the 
plasma of the blood. Meigs (12) has 
pointed out that within the limits of 
the experimental errors involved the 
mammary gland takes these pre¬ 
cursors out of the plasma in approxi¬ 
mately the proportion in which the 
corresponding constituents occur in 
the milk. This, as he points out, 
furnishes strong evidence for the view 
that the lactose, fat, and proteins of 
milk are, respectively, derived en¬ 
tirely from the dextrose, phosphatide, 
and free amino acids of the blood. 
Abrupt changes in the rations of 
milking cows frequently produce de¬ 
cided changes in the yield and com¬ 
position of milk. The concentration 
of some of the milk precursors in the 
blood plasma may be determined with 
a fair degree of accuracy. A study of 
the composition of the rations, blood, 
and milk simultaneously might, there¬ 
fore, be expected to throw much light 
on important problems in the physi¬ 
ology of milk secretion. It would also 
throw light upon the relation between 
the diet and the composition of the 
blood with animals where the process 
of absorption on a given ration is 
practically constant. The cow fur¬ 
nishes an excellent subject in many 
respects for such a study. 
The plasma amino N in milking cows 
is low (2, 3). This is especially true 
when they are fed according to the 
ordinary standards of feeding. It is 
seldom so high as 3 mg. per 100 c. c. 
On an average, 25 per cent of this is 
taken out in a single passage of the 
blood through the gland. This means 
a very great depletion of some of the 
individual amino acids. It would 
seem likely, a priori, that the yield 
and composition of milk might, there¬ 
fore, frequently be affected and limited 
by changes in either the concentration 
or composition of this amino-acid mix¬ 
ture. Hence, especial attention in this 
study has been given to the relation of 
the amino acids of the blood to the diet 
and the secretion of milk. 
In the experiments to be reported 
milking cows were used. In their 
rations the protein was varied in 
amount or quality or both, and the 
energy content was varied either alone 
or together with the amount of protein. 
The amino N of the blood and plasma 
was determined at intervals. In some 
experiments it was found possible to 
follow simultaneously the amino N 
and the concentration of a particular 
amino acid (tryptophane) in the blood 
and thus to get some idea of the quali¬ 
tative changes occurring in the blood 
mixture of amino acids. The glucose 
and lipoid P of the blood and plasma 
were determined in some experiments. 
The yield of milk and its content of N 
and fat were followed throughout. 
The milk lactose was determined in 
some experiments. 
EXPERIMENTAL ROUTINE 
The experiments, with the exception 
of No. VI, were begun with cows on 
rations which were adequate, or nearly 
adequate, in both protein and energy, 
according to the commonly accepted 
feeding standards for maintenance and 
milk yield (7, p. 133). The initial 
periods on approximately adequate 
rations were followed by periods in 
which the rations were made markedly 
inadequate in either energy or protein 
content, or both, the protein mixture 
in the ration being changed in either 
quantity or composition, or both. 
These periods of inadequate feeding 
were generally followed by periods in 
which the adequacy of the rations was 
again restored. The changes in ra¬ 
tions were made abruptly and without 
transition periods. 
Except for the hay, the feed was 
given in two equal portions, one be¬ 
tween 4 and 5 a. m. and the other 
between 1 and 2 p. m. The daily 
allowance of hay was fed at the after¬ 
noon feeding. Each cow was milked 
at the same time daily (4.30 to 4.45 
1 Received for publication May 12,1924—issued March, 1925. 
2 Reference is made by number (italic) to “Literature cited,” p. 624. 
Journal of Agricultural Research, 
Washington, D. C. 
( 603 ) 
Vol. XXi'X, No. 12 
Dec. 15,1924 
Key No. A-88 
