Abstract
Effect of lactation on hypomagnesemia of ruminants was examined with two lactating and two non-lactating ewes. The balance between intake and discharge and the concentrations in serum of Mg, Ca and P, and the plasma level of parathyroid hormone (PTH) were compared between lactating ewes (L) and non-lactating ewes (NL) fed a control diet or a diet low in Mg and Ca (experimental diet). 1) L excreted more Mg (p<0.001) and less Ca in feces than NL when fed the control diet. They excreted more Mg (p<0.01) and more Ca (p<0.01) in feces than NL when fed the experimental diet. 2) L excreted Mg, Ca and P in milk at an almost constant level throughout the experimental period. 3) L excreted less Mg in urine than NL throughout the experimental period (p<0.01). The rate of Mg excreted in urine to Mg apparently absorbed was higher in L than in NL. 4) Body retention of Mg, Ca and P was less in L than in NL during the experimental period (p<0.05, p<0.01 and p<0.001 respectively). 5) Concentration of Mg in serum was lower in L than in NL during the experimental period (p<0.001). 6) Concentration of PTH in plasma was lower in L than in NL. It scarcely changed during the experimental period. It rose undulatingly in NL when the experimental diet was given. 7) The increase of Ca absorption accompanied with that of lactation was suggested to cause the lowering of the concentration of PTH in plasma and suppress the availability of Mg in lactating animals. Hypomagnesemia was presumed to suppress the increase of the concentration of PTH in plasma.
