Calcium Metabolism in Dairy Cows as Studied with Ca45

Abstract

Summary Radiocalcium was used to study Ca metabolism in dairy cows at parturition and at various stages of lactation. Data are presented on the appearance of Ca 45 in the blood, milk, feces, and urine at various times following single oral and intravenous doses. The blood-milk pattern also is given for a cow fed Ca 45 daily over several months of a lactation period. When Ca 45 was injected into two dairy cows in single doses at about 20 days prepartum, there was an indication that much of it was deposited in the mammary glands, where it remained until the animals were milked. Radiocalcium intravenously administered during lactation was deposited in the milk in amounts directly correlated with the total amount of milk calcium secreted. During 4 days after injection, a cow producing about 4 kg. milk per day secreted 19 per cent of the Ca 45 dose, whereas the same cow and another one producing about 7 kg. per day secreted 34 per cent. Following oral administration these cows secreted from 4 to 6 per cent of the Ca 45 in their milk in 4 days at daily levels of milk production which were about the same as those following intravenous injection. Evidence is presented that Ca ions are readily interchangeable between the various Ca fractions of blood, and on this basis a simple method for quantitative estimation of these fractions is suggested. These data on Ca fractions, as well as the blood and milk specific activity relationships following oral and intravenous administration, are presented and discussed in terms of precursor-product relationships.