Slow release bolus for high pregnant cows, stays active for a long time in the rumen. The active substances help with the absorption and regulation of calcium and magnesium. This bolus helps to prevent milk fever. Because of its mass the bolus stays in the rumen where it slowly releases 1,25-dihydroxyvitamine D3 glycoside. This active form of vitamin D3 is easily absorbed by the gut. This form of vitamin D3 is already active and doesn't have to be activated by the liver or kidney. Its helps with the absorption and regulation of calcium and magnesium. This bolus is administered 2 days before the expected calf moment and delivers Calcitrol to the cow in a period of 10 days. This allows the cow to increase the level of calcium in the blood from its own metabolism. The efficacy of Calcitrol for use in dairy cows against dairy disease is endorsed, among other things, in an EFSA report (European food safety authority).