Effect of calcium intake and dietary cation anion difference in early lactation on bone mobilization dynamics all over lactation in dairy cows

This study aimed to evaluate the consequences of increased bone mobilization in early lactation on the dynamics of the milk Ca content during lactation and bone reconstitution. Fifteen multiparous Holstein cows were distributed among 3 treatments 5 weeks before their expected calving date. Those treatments consisted of the provision of dedicated diets through the first 10 weeks of lactation. During that period, the control treatment (NCa) consisted of a diet providing 100\% of the Ca requirements, with a dietary cation-anion difference (DCAD) of 200 mEq/kg DM. The treatments LCa (Low Ca) and LCaLD (Low Ca, Low DCAD) consisted of diets providing 70% of the Ca requirements, with a DCAD of 200 and 0 mEq/kg DM for LCa and LCaLD, respectively. After 10 weeks, all cows received the same total mixed ration which was formulated to meet 100\% of the Ca requirements. LCa and LCaLD tended to decrease the body retention of Ca at 3 weeks of lactation compared with NCa, but did not affect either the dynamics of the blood biomarkers of bone formation and resorption during the lactation or the body retention of Ca at 17 weeks of lactation. Cows almost entirely compensated for the decrease in Ca supply by increasing their apparent digestive absorption of Ca at 3 weeks of lactation, whereas the apparent digestive absorption was unaffected by the treatments at 17 weeks of lactation. Milk production tended to be lower throughout lactation with the LCa and LCaLD compared with the NCa, with a mean difference of 2 kg/d. This study indicated that measuring the dynamics of the milk Ca content during lactation cannot be considered effective for indirectly estimating the dynamics of bone mobilization of cows. The results also showed that limited Ca intake at the beginning of lactation can have deleterious effect on milk production.