6 BULLETIN 945, U. S. DEPARTMENT OE AGRICULTURE. 
to more than very small variations or that the concentration of either 
of these elements contained in any of the soft tissues undergoes 
more than insignificant changes. Evidence has also been adduced to 
indicate that calcium assimilation in cows is likely to be seriously 
interfered with for a period of at least eight days by the mere col- 
lection of their urine and feces by attendants as practiced in the 
experiments of Forbes, of Hart, and of ourselves (13), 
It is likely, therefore, that any considerable deficiency of either 
calcium or phosphorus in the rations of a milking cow will bring 
about the loss of both elements from the animal's bones if continued 
for more than two or three weeks, and that a cow which has suf- 
fered from the lack of either during any considerable part of her 
lactation period will find herself depleted in both when she reaches 
the end of that period. 
In recently published articles from this laboratory (12) (13) it 
has been shown that the phosphorus content of the blood plasma of 
cows is highly variable, and that it is likely to be low in the plasma 
of the Beltsville herd toward the end of their periods of pregnancy. 
This suggests that the cows of the Beltsville herd usually reach the 
end of their lactation period with their phosphorus stores depleted, 
and that the rations fed during the dry period are not sufficient to 
restore them. For the reasons which have been given it is likely 
that the calcium stores of the Beltsville cows are also depleted during 
their lactation periods, and that neither the calcium nor the phos- 
phorus stores can be restored to their proper level during the dry 
period unless the cows are fed rations which make it possible for 
them to assimilate liberal quantities of both elements. 
In the articles just mentioned, certain other facts regarding cal- 
cium and phosphorus metabolism were brought to light. It was 
shown that the concentration of calcium in cows' blood plasma is 
much more constant than that of phosphorus. It is usually easy to 
raise the concentration of plasma phosphorus by increasing the 
amount of phosphorus in the rations — either by feeding more grain 
or by adding sodium phosphate to the ration. But the changes, 
brought about in the concentration of plasma calcium by analogous 
procedures or by any other influences that we have encountered so 
far, are comparatively insignificant and usually fall within the limits 
of error of our determinations. 
It has seemed likely, therefore, that changing the amount of phos- 
phorus in the ration would have more immediate and easily deter- 
minable effect* on the changes which go on in a cow's udder shortly 
before her calf is born than changing the amount of calcium. The 
experiments herein reported were planned with this idea in mind. 
But it was essential that both the control and the experimental ani- 
