Differences in maternal body weight (12–15 kg) were induced in two groups (n = 43–50) of ewes during pregnancy by controlled allowance of pasture. Following parturition equal numbers (9–12) were allocated to one of three herbage allowances in a 2 x 3 factorial design during 42 days of lactation. This design was employed on two occasions; in one (Expt 1) the sheep were machine-milked twice daily, in the second (Expt 2) they each suckled two lambs. Balances of Ca and P in the body were determined by comparative slaughter during early pregnancy, at parturition and after 42 days of lactation. Herbage Ca intake and milk Ca output were determined on a weekly basis.

Mean daily milk production during the 42 days of lactation ranged from 1·48 to 2·68 kg/day amongst treatment groups. The general trend, irrespective of level of milk production, was for negative Ca balances, ranging from 0·71 to 1·42 g Ca/day. It is argued that variation in apparent absorbability (0·17–0·46) and in rate of absorption (30–113 mg/kg W per day) of Ca among groups suggest that these factors are not in themselves responsible for the poor Ca balances observed.

It is suggested that demand for Ca from the diet during early lactation is reduced as a consequence of bone Ca resorption due to matrix osteoporosis resulting from a protein deficiency state. Negative Ca balances in sheep undernourished during pregnancy were similarly attributed to poor protein balances.

Although negative P balances were observed these were not attributed to low P intake.