The influence of a low and a high prepartum calcium (Ca) intake on Ca mobilization rate around parturition was studied in 44 dairy cows fed a ration sufficient for 1.90 times maintenance requirements during the dry period. The plasma Ca level declined on the day of parturition in the group fed the low Ca intake (LCa: 26.4 g/d) as well as in the group fed the high Ca intake (HCa: 87.2 g/d). Plasma Ca levels of the HCa group were lower at parturition, and in this group 1 cow had milk fever after parturition. In the HCa group 6 cows had a plasma Ca level less than or equal to 2.0 mmol/l at parturition and/or 10 h post-partum (pp), versus 1 cow in the LCa group. Na2EDTA was intravenously infused at 10 h pp to induce hypocalcaemia to a level of plasma Ca not bound to EDTA of approximately 1.0 mmol/l. The LCa groups tended to require more Na2EDTA than the HCa groups, however the difference was significant only in the younger cows. After the Na2EDTA infusion 7 cows of the HCa group and 1 of the LCa group did not recover spontaneously and needed to be treated. The mean plasma PTH levels of the LCa group ante-partum were slightly higher than those of the HCa group. The Ca level of the prepartum ration did not influence urinary hydroxyproline excretion, which suggests that the Ca intake of 26.4 g/d was too high to stimulate bone turnover. Comparison of the present results with those of an earlier experiment in which the prepartum Ca intake at a low feeding level (1.12 times maintenance) was studied, led to the conclusion that higher prepartum feed intake has a clear negative influence on Ca homeostasis around parturition.

The rate of osteogenesis was studied in 8 non-pregnant, non-lactating Friesian dairy sheep, 3-6 years old, by means of a treatment with 3 different bone seeking agents. Four sheep were fed a low calcium ration (LCa:1.8 g Ca/d) and four other sheep a high calcium ration (HCa:12.7 g Ca/d). The bone markers, oxytetracycline-HCl, alizarine-complexion and demeclocycline-HCl, were administered at intervals of 6 weeks, and the sheep were killed 1 week after administration of the last marker. In undecalcified cross sections from the middle of ribs 2, 10 and 12, and from the proximal and distal parts of rib 10, the numbers of labelled osteons and the number of osteons with 1, 2 or 3 markers were counted under fluorescent microscopy. In the ribs of sheep from the LCa group, the number of labelled osteons and the quantity of labels per osteon tended to be higher than those of sheep from the HCa group. When osteogenic activity was compared in the different sites of ribs analysed, lowest osteogenic activity was observed in the proximal part of the 10th rib. The use of fluorescing markers offers the possibility of studying osteogenic activity over a certain period of time in adult sheep.