1. Two experiments are described in which kinetic aspects of calcium metabolism were studied in housed lactating sheep consuming different fresh herbage species. The importance of protein supply was also investigated.
2. In Expt. 1, two groups (n 4) were offered, ad lib., a freshly cut ryegrass (Lolium perenne L.)-white clover (Trifolium repens L.) pasture containing 5·48 g Ca/kg dry matter (DM). One group was supplemented daily with 100 g protected casein. A third group (n 4) was offered, ad lib., freshly cut oats-Tama ryegass (Lolium multiflorum L.) herbage which had a lower Ca content of 3·07 g Ca/kg DM. Stable Ca and nitrogen balances were carried out during the first 7 weeks of lactation. At this stage 180 μCi45Ca were administered for Ca kinetic studies.
3. In Expt 2, eight sheep were offered, ad lib., a fresh ryegrass–white clover pasture, and paired on the basis of their udder size. One member of each pair was supplemented daily with 100 g casein via the abomasum and the amount of milk removed was equalized between pairs. Ca and N balances (12 d) and Ca kinetic studies (280 μCi 45Ca) were carried out during weeks 2 and 5 of lactation.
4. Rate of absorption of Ca increased, while rate of Ca secretion in milk and resorption from bone decreased as lactation progressed. Ca balances changed from negative to positive as lactation progressed in sheep offered ryegrass–white clover, but, while improving, were always negative in sheep offered oats–Tama ryegrass. Protein supplementation increased (18%) milk production of the ewes in Expt 1 and their retention of N in Expt 2.
5. The proportion of utilized Ca derived from the diet, as opposed to the skeleton, tended to increase as a result of protein supplementation.
6. Availability of Ca from ryegrass–white clover ranged from 0.19 to 0.32, even though only 50% of the net Ca requirement was derived from the diet. Availability of Ca from the oats–Tama ryegrass diet was similar, though in this case less than 20% of the net Ca requirement was derived from the diet. It was concluded that availability of Ca from forage diets may be lower than previously anticipated.
7. Faecal endogenous loss ranged from 16 to 40 mg Ca/kg body-weight per d, and was similar on both diets.
8. These and other findings are used to discuss more fully the subject of Ca nutrition in sheep, in particular, the implications of the strong homeostatic control of Ca absorption and the influence of protein status on the relative contribution of the diet and the skeleton in meeting the net Ca requirement of the ewe during lactation.