Calcium and phosphorus requirements of the ewe during pregnancy and lactation. 2. Phosphorus.

Mineral balance and radioisotope studies have been carried out to test the adequacy of the recent Agricultural Research Council (1980) recommendations for calcium and phosphorus for pregnant and lactating ewes. At the same time, P metabolism was compared throughout pregnancy and lactation in ewes fed according to these recommendations and in ewes fed a plentiful supply of dietary Ca and P. Bone mineral stores were mobilized in late pregnancy and early lactation, irrespective of the rate of P absorption. These stores were then replaced in mid- to late lactation in ewes given the plentiful Ca and P intake but not in the ewes given the restricted intake. Results suggest that these changes in bone stores occurred as a result of changes in Ca requirements rather than in P requirements, and that accretion of P into bone or resorption of P from bone occurred merely as a consequence of this change in Ca requirements. Immediate demands for P for maintenance and fetal or milk production do not reflect net P demands, which also take into account changes in bone P metabolism. The rate of P absorption was directly related to net demands and the rate of endogenous excretion was inversely related to net demands. Absorption and endogenous faecal loss of P were also both directly related to P intake. A lack of dietary Ca, particularly in mid- to late lactation, makes it impossible to draw conclusions on the adequacy of the Agricultural Research Council (1980) recommendations for P. Results do suggest, however, that P requirements ought to be calculated according to net demands for P rather than immediate demands.