Probably one of the most important macro minerals, especially around calving time. The cow will have a big demand on calcium before calving and especially during milk production. Older cows are more prone to issues with calcium around calving. This is due to their reduced ability to flush calcium from bone around calving time.
Sheep have a higher calcium requirement pre-lambing (4-0 weeks pre-lambing).
Calcium plays a key role in muscle function and immune function. Some breeds like jersey and guernseys may be more susceptible to deficiency.
Calcium also has complex interactions with other minerals pre-calving. For example we actually want low levels of calcium pre calving in the diet. High levels of magnesium and also low levels of potassium.
Also metabolic alkalosis will predispose cows to milk fever around calving time. This is where we must balance the dietary cations and anions in the pre-calving diet.