Partition of Calcium Metabolism in Dairy Cows

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Abstract
A series of digestion trials was conducted with Jersey cows following a single intravenous injection of radioactive calcium. The relative importance of absorption and metabolic fecal secretion of calcium were studied. These trials were made during the dry period, during lactation and pregnancy, with changes in the calcium and phosphorus content of the ration, and with a calcium chelating agent added to the diet. The cows were unable to maintain calcium balance when the dietary Ca/P ratio was less than unity. Cows responded to changes in calcium or phosphorus in the diet by adjusting gain (absorption) and loss (metabolic fecal secretion). The changes in absorption were much greater than in secretion. Cows on low-calcium (high-phosphorus) diets utilized dietary calcium less efficiently than cows on higher calcium diets. This effect may be due to the high-phosphorus content of these rations. In other respects, our results are in keeping with the concept of calcium homeostasis. The lactating cow utilized dietary calcium more efficiently than the non-lactating cow; the pregnant cow more so than the non-pregnant cow.