Abstract
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.
Highlights
Dairy cows excrete a large amount of Ca during lactation due to the high Ca content in milk [1], and this Ca flow suddenly and notably increases later during lactation [2]
The first response is an increase in bone resorption, mediated by the secretion of 2 hormones, namely, parathyroid hormone (PTH) and PTH-related peptide (PTHrP), which allow other organs to use the Ca contained in the mineralized matrix of bone [3]
VanHouten et al [27] showed that a decrease in Ca intake in mice induced lower Ca secretion in milk and higher bone resorption mediated by PTHrP secretion, with both being mediated by the Ca-sensing receptor (CaSR) in the mammary gland. These results suggest that the monitoring of milk Ca content during lactation could be an inexpensive means of indirectly estimating the dynamics of bone resorption
Summary
Dairy cows excrete a large amount of Ca during lactation due to the high Ca content in milk [1], and this Ca flow suddenly and notably increases later during lactation [2]. Current recommendations are based on the principle that daily excretions of Ca and P allow a certain production level to be maintained with minimal fecal and urinary losses and with replacement by an equivalent amount of daily intake of those elements [15,16,17] This principle does not consider bone mobilization during early lactation and reconstitution during late lactation that could constitute either an extra supply or a specific requirement, which may facilitate devising supplementation strategies at the scale of the whole lactation-gestation cycle by taking bone reserves into account
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