The Effect of Oral Calcium Boluses and Subcutaneous Calcium Injection on Blood Metabolites and Milk Yield in Holstein Cattle Fed Anionic Diets

BACKGROUND: Clinical hypocalcaemia is a metabolic condition that remains prevalent even with preventive measures. Calcium supplementation at calving could complement preventive strategies in mature cows. OBJECTIVES: To compare the effects of oral calcium boluses and subcutaneous injections on total (tCa) and ionized Ca (iCa), total Mg, P, beta-hydroxybutyrate (BHB) and milk yield in adult Holstein cows fed prepartum anionic diets. METHODS: The study was conducted between September 2022 and January 2023. A total of 56 cows were initially randomized into three groups, but nine cows were excluded due to protocol deviations, and two additional cows were removed due to clinical hypocalcaemia. Thus, 45 cows completed the sampling protocol: Oral boluses (OB, n = 16; two oral boluses with calcium chloride and calcareous algae), subcutaneous injection (SC, n = 14; subcutaneous administration of calcium gluconate) and control no treatment (CON, n = 15). Treatments were applied within 6 h postpartum (pp). Lactation distribution included second (n = 16), third (n = 9) and fourth or higher (n = 20) lactations. One cow exited the herd during the 30-day disease monitoring period. A blood sample was collected on Days 0 (before any intervention), 1, 2, 3, 4 and 7 pp. Blood metabolites and daily milk yield were analysed by mixed model ANOVA for repeated measures. Due to reduced sample size, statistical power was recalculated for each outcome. RESULTS: levels were significantly higher over time in the CON group compared to the treated groups (p ≤ 0.05). Blood BHB (mmol/L) was significantly lower in the OB (1.17 ± 0.1) and SC (1.02 ± 0.1) than CON (1.58 ± 0.1) on Day 2 pp (p ≤ 0.05), and OB (1.10 ± 0.1) and SC (0.93 ± 0.1) than CON (1.45 ± 0.1) on Day 3 pp (p ≤ 0.05). No significant differences were found for P, Mg or albumin. Disease incidence showed no significant treatment effects (p > 0.05). Subclinical ketosis was observed in six OB, five SC and seven CON cows. Other conditions included retained foetal membranes, metritis and displaced abomasum, with low frequencies across groups. No significant differences in milk yield were detected during the first 30 days pp (p > 0.05). Reproductive data from 36 cows showed no significant differences in pregnancy rates or days to conception among groups (p > 0.05). The variables analysed demonstrated low statistical power, with varying degrees of impact across measures. Consequently, the results should be interpreted with caution. CONCLUSIONS: Calcium supplementation via oral bolus or subcutaneous injection modestly improved early pp iCa and reduced BHB concentrations, suggesting potential moderated metabolic benefits. However, these treatments did not significantly affect disease incidence, milk production or reproductive performance within the study period (low power calculated). Further research with larger sample sizes may be warranted to confirm these findings.