This study investigated the consequences of a low supply of dietary Ca with or without a low dietary cation-anion difference (DCAD) during early lactation on bone mobilization and reconstitution during lactation and on the dynamics of milk Ca content. Fifteen multiparous Holstein cows were distributed among 3 treatments 5 weeks before their expected calving date. These treatments differed based on the provision of diets through the first 10 weeks of lactation. During this period, the control treatment (NCa) consisted of a diet providing 100% of the Ca requirement, with a DCAD of 200 mEq/kg dry matter (DM). The LCa (low Ca) and LCaLD (low Ca, low DCAD) treatments consisted of diets providing 70% of the Ca requirement, with a DCAD of 200 and 0 mEq/kg DM, respectively. After 10 weeks, all cows received the same total mixed ration, which was formulated to meet 100% of the Ca requirement. LCa and LCaLD tended to decrease the body retention of Ca at 3 weeks of lactation compared with NCa but affected neither the dynamics of the blood biomarkers of bone formation and resorption during lactation nor the body retention of Ca at 17 weeks of lactation. Cows almost entirely compensated for the decrease in Ca supply caused by LCa and LCaLD by increasing their apparent digestive absorption of Ca at 3 weeks of lactation, whereas their apparent digestive absorption was unaffected by the treatments at 17 weeks of lactation. Milk production tended to be lower throughout lactation with LCa and LCaLD compared with NCa, with a mean difference of 2 kg/d. The results of this study also indicated that measuring the dynamics of milk Ca content during lactation cannot be considered effective for indirectly estimating the dynamics of bone mobilization in cows. The results also suggested that limited Ca intake at the beginning of lactation may have deleterious effects on milk production.

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.