Effect of prepartum dietary cation-anion difference strategy and level of dietary calcium on postpartum blood calcium status and milk production of multiparous Holstein cows

Hypocalcemia in dairy cows can be mitigated by manipulating prepartum DCAD. What remains unclear is the degree to which DCAD diets should be implemented and the appropriate level of dietary Ca to be fed in conjunction with negative DCAD. This study examined 2 prepartum DCAD levels, 2 prepartum dietary Ca levels, and their interactions on postpartum Ca metabolism, DMI, and milk performance of Holstein cows. In a randomized block design, data from parous Holstein cows (n = 98) enrolled in the study from 32 d before expected calving through 63 DIM were used for analysis. At 26 d before expected calving, cows were assigned to 1 of 4 treatments in a 2 × 2 factorial arrangement with main effects of DCAD (-2.61 mEq/100 g, partial acidification [PAS] vs. -10.26 mEq/100 g, full acidification [FAS]) and Ca (1.5% DM, high Ca [HCa] vs. 0.7% DM, low Ca [LCa]). Cows fed FAS and PAS diets targeted urine pH of 5.5 to 6.0 and 6.5 to 7.0, respectively. Cows fed FAS had lower urine pH and greater prepartum urinary ammonium excretion than cows fed PAS (FAS 5.64 vs. PAS 6.71 ± 0.10 pH). Urine was collected for mineral analysis once per week upon enrollment until parturition and at 1, 2, and 3 DIM. Blood samples were collected once per week from enrollment until 1 wk before calving then at 0.5, 1, 1.5, 2, and 3 DIM for analysis of Ca, Mg, and P. Dry matter intake and milk production were recorded daily, and BW data were collected weekly. Statistical analyses were conducted using SAS software (v. 9.4) using linear mixed models with covariates and repeated measures when appropriate. Differences in pre- and postpartum DMI were not detected between FAS- and PAS-fed cows. Prepartum DMI was not affected by dietary Ca level. Postpartum DMI and ECM were greater for cows fed HCa than LCa prepartum. Circulating postpartum total Ca was not different by prepartum Ca nor prepartum DCAD. Cows fed FAS excreted more urinary Ca than cows fed PAS prepartum and cows fed LCa excreted more urinary Ca than their HCa fed counterparts. In the conditions of this study, milk production and postpartum intake were not affected by magnitude of prepartum acidification. Feeding greater dietary Ca in the prepartum period improved postpartum DMI and ECM yield, independently of prepartum acidification level.