RESUMO
At the initiation of lactation, Ca homeostatic mechanisms have to react to a tremendous increase in demand for Ca. Mobilization of Ca from bone and increased absorption from the gastrointestinal tract are required to re-establish homeostasis. It has been shown that dietary anions play an important role in the prevention of milk fever by mobilizing Ca from bone and by increasing Ca absorption in the GI tract. The purpose of the present study was to investigate the influence of different Ca contents in diets supplemented with anionic salts on bone metabolism of dairy cows. Twenty-four holstein cows (housed inside, second to fourth lactation) without a milk fever history were divided into four groups (A, B, C, D). Each group was fed a different diet which was given from day 263 of gestation till the day of parturition. Group A and B received a low calcium diet (4 g/kg DM) whereas group C and D received a high Ca diet (8 g/kg DM). In addition group B and D received anionic salts. The DCAD was calculated with the formula: DCAD (mEq/kg DM)=(0.2 Ca2++0.16 Mg2++Na++K+)-(Cl-+0.6 S2-+0.65 P3-). Blood and urine samples were collected on days 256, 270 and 277 of gestation, on the day of parturition as well as the following 5 days and on days 9, 14 and 19 after parturition. Serum Ca, P, Mg, ICTP, OC, VITD, PTH and urinary pH were analysed. The bone resorption marker ICTP showed a significant increase after parturition in all the groups. On the contrary, the bone formation marker OC decreased after parturition in all the groups. The VITD concentrations in group D and the urinary pH in group B were significantly lower compared to the other groups (p<0.05). The Ca concentrations tended to be higher in group B around parturition than in all the other groups. No significant influence of the four different diets on all the other parameters could be shown. In conclusion, this data showed that the addition of anions and the different Ca contents had no significant influence on bone resorption and bone formation markers. This may be because of the fact that the dietary cation-anion balance was not low enough (DCAD-group A: 181 mEq/kg DM, group B: -48 mEq/kg DM, group C: 210 mEq/kg DM and group D: 28 mEq/kg DM) to induce a metabolic acidosis with all its positive effects on calcium metabolism.