The Condition

Hypocalcaemia is an acute condition of ewes in late pregnancy and early lactation. The ewe's calcium requirements increase dramatically in late pregnancy because of the calcium demand of the rapidly growing foetal lamb and the production of colostrum, which is rich in calcium. After lambing, milk production keeps up the demand for calcium. Older ewes are more susceptible, as their ability to release calcium from the reserves in their bones declines with age.

Calcium is essential in a number of important body processes, including the proper functioning of the muscles. In cases of calcium deficiency, the ewe will become weak and collapse, and if not treated will die of heart failure (Sykes and Russell, 1991).

Although ewes fed a diet low in calcium are more at risk from hypocalcaemia, the most important predisposing factors are physical (Peet et al., 1985) or nutritional stresses (Pickard et al., 1988), to which the ewe cannot respond quickly enough. Where calcium levels provided in the diet are sufficient, ewes may still go down with hypocalcaemia because of a sudden demand for calcium through physical exertion (Peet et al., 1985). The acid-base balance of the diet (Grant et al., 1992) and magnesium status (Moate et al., 1980; Pickard et al., 1988; Tindall, 1986) may predispose the animal to hypocalcaemia. Hypocalcaemia often occurs following feed change (i.e. pasture change, change of diet, feed deprivation, etc.), which results in the sheep going off their feed for a few days.

Other dietary components may affect the absorption of calcium from the diet. Vitamin D is essential for efficient calcium absorption and its subsequent deposition in bones (Schneider et al., 1985). The vitamin is found in sun-dried fodders such as hay and manufactured in the skin by the action of sunlight. Ewes in spring lambing flocks generally have low levels of vitamin D around the time of lambing when it is most needed. Diets low in protein also adversely affect calcium absorption and deposition in bone, even if dietary calcium levels are adequate. The calcium : phosphorus ratio is also important. A ratio between 1 : 1 and 2 : 1 of calcium to phosphorus would appear to be most satisfactory, providing the overall levels of each element are within the normally accepted range. If calcium levels are corrected, phosphorus levels will generally follow suit (Sykes and Russell, 1991).