Urinary Calcium Excretion and Blood Calcium Levels in the Bovine Near the Time of Parturition

Abstract

Parturition in the dairy cow is a time of tremendous physiological activity. During this period hormonal levels are constantly changing, for the cow not only loses the placenta, a site of hormone production, but also is preparing to secrete large quantities of milk. The latter procedure causes the cow to secrete many food nutrients at a rate much faster than she can consume them in her diet. I t is understandable that complications associated with parturition and the onset of lactation should occasionally occur. I t would be fortunate indeed if one could assume that hormonal balances and mineral metabolism at parturition and during lactation in the dairy cow were the same as that in other species. The dairy cow, however, a ruminant bred for centuries for high milk production, is different in several respects from humans and small experimental animals. Accordingly those engaged in studies of physiology of the dairy cow must, of necessity, study the dairy cow herself rather than draw inferences from studies of other species.Calcium metabolism in the bovine has always been of special interest to those engaged in research with dairy cattle because of the sizable outputs of calcium in the milk for relatively long periods of time. Added impetus was given to studies in this area when it was demonstrated about 30 years ago'2 that a marked hypocalcemia existed during parturient paresis (milk fever). In spite of this, basic information on calcium metabolism, even in the normal cow, has been slow in accumulating.Many have related it to the sudden great demand for calcium associated with the initiation of lactation. While these sizable losses of calcium are undoubtedly associated with a hypocalcemia, the onset of lactation cannot be considered the sole cause of milk fever, for many heavy producing cows do not develop this syndrome at part~riti0n.l~ Practical dairymen are vitally interested in an effective means of preventing this disorder. Recent research along these lines in Ohio20 and California9 is very encouraging. Some progress is also being made toward an understanding of calcium metabolism in the normally lactating and in the parturient bovine. This type of information is necessary before the etiology of parturient paresis can be understood.In this paper some of the more recent studies on calcium levels under varying conditions in the blood and urine of normal parturient cows and cows with parturient paresis will be discussed. In some cases we have taken the liberty of condensing data originally published in order to facilitate presentation.The specific cause of the hypocalcemia in milk fever is not known.