6 BULLETIN &45, U. S. DEPARTMENT OF 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- 



