[Magnesium in the blood and urine of cows during various stages of the reproductive interval being fed summer and winter feed rations].

Magnesium concentration was investigated in the blood plasma and urine of 3460 cows in the course of four green-feeding seasons and four winter feeding seasons in 1980 to 1984. A decrease in the magnesium content in blood plasma was found to intensify in the last six weeks of gravidity. In the first stage of lactation the magnesium concentrations of plasma begin to grow to reach a relative maximum in the cows in the second stage of lactation. These regularities apply to both the summer and winter feeding seasons. The magnesium content in urine has a similar pattern. The summer values of the magnesiemia of whole cow populations are significantly higher than the winter values. Tendencies to lower magnesium concentrations generally prevail in both blood plasma and urine; these tendencies are most conspicuous in the cows in the last six weeks to parturition. As a rule, the concentrations of magnesium in urine were lower in summer than in the season of winter feed rations. As for the contents of the blood plasma magnesium of the cow populations as well as individual cows, there are considerable differences between various stages but the trends are the same in all groups. In the green feeding season the range of fluctuation of the magnesiemia values is larger than in the winter feeding season. The same tendency of changes in individual groups was also recorded in the magnesium concentrations in the urine of cows in different stages. However, the direction of the changes did not always correspond to the direction of the changes in the blood plasma. A practical requirement has been derived from the results of the study that magnesium contents should be increased in the feed rations given to high-pregnant cows and cows in the first lactation.