CALCIUM, PHOSPHORUS, AND MAGNESIUM ABSORPTION AND SECRETION IN THE BOVINE DIGESTIVE-TRACT AS INFLUENCED BY DIETARY CONCENTRATION OF THESE ELEMENTS

Effect of dietary concentration of Ca, P, and Mg on the net movement of these elements in the gastro-intestinal tract of cattle was investigated in a metabolic study where eight different diets were fed to two Jersey cows and two steers cannulated in the duodenum and ileum. The major site of Ca absorption was prior to the small intestine (SI) with little net secretion or absorption of Ca in the SI and large intestine (LI). Absorption or secretion of Ca was dependent on Ca intake. Mg absorption prior to the proximal duodenum occurred under each dietary regimen and a positive relationship was found between net absorption of Mg before the SI and Mg intake. Net absorption of Mg also occurred in the SI while both net absorption and secretion of Mg were found in the LI. Secretion of P before the SI was high and the major site of net absorption was the SI. Both net absorption and secretion of P occurred in the LI. Ca, P, and Mg requirements of Jersey cow weighing 300 +/- 20 kg, were calculated using regression equations relating intake to retention and found to be 23, 10.7, and 5.9 g d-1 respectively.