Loose mineral or blocks. Blocks are not all the same. The percentage of magnesium needed in the supplement to obtain this level of consumption varies with intake of the supplement. Magnesium oxide and magnesium sulfate contain more readily available magnesium than magnesite or dolomite limestone. Look at the bottom of the mineral tag under the ingredients heading to determine the source of magnesium in a particular mineral.
Low consumption means low protection.
Many producers report poor consumption of high magnesium minerals. This is why it is advisable to start cows on high magnesium minerals six to eight weeks prior to periods of critical need. Some producers elect to leave high magnesium minerals with cows all year. Never give cows a choice between high magnesium mineral and other minerals. Monitor consumption of the mineral.