Association between prepartum urine pH and blood calcium concentration at calving in Holstein cows: a systematic review and meta-analysis.

Anionic salts are commonly used during the prepartum period to prevent hypocalcaemia. However, the level of anionic salts for preventing clinical and subclinical hypocalcaemia remains controversial, with the prevailing suggestion of maintaining a very low urine pH. The objective of this meta-analysis was to determine the association between urine pH within 8 days prior to parturition and blood total calcium within 48 h after parturition in Holstein multiparous cows, aiming to identify the optimal urine pH for preventing hypocalcaemia, regardless of dietary strategy. A systematic review was conducted in accordance with the preferred reporting items for systematic reviews and meta-analyses guidelines. PubMed, CABI, and Scopus databases were searched for studies meeting the predefined eligibility criteria. A random-effects meta-regression analysis was performed on the quantitative data extracted from eligible study groups to estimate the relationship between urine pH and blood calcium levels. From the initial search, 480 studies were identified across the three databases. Following the screening process, 19 studies (with 51 study groups) were included in the meta-analysis, involving 752 Holstein cows (with only 10 cows being primiparous). The model revealed a statistically significant linear negative association between prepartum urine pH and postpartum blood total calcium (coefficient = -0.0698; P = 0.023). This indicates that lowering the average urine pH within 8 days of parturition could improve the postpartum blood total calcium at the herd level. Regular monitoring of prepartum urine pH within a moderate acidification range (i.e., ∼6.2-6.8) or at herd-specific target levels could help maintain blood calcium levels and prevent both clinical and subclinical hypocalcaemia in the herd. The heterogeneity observed in our study also suggests that other unmeasured factors may still influence the relationship between urine pH and blood calcium levels. Nevertheless, the lack of information on other potential factors that may explain variation in blood calcium in the manuscripts considered in this meta-analysis prevented the evaluation of potential interactions between urine pH and variables such as parity, dietary calcium content, or dietary cation-anion difference level. In conclusion, the findings of this association study provide a practical approach to refine and establish herd-specific baselines for prepartum urine pH and postpartum blood calcium. This helps to support hypocalcaemia prevention and to reduce the risk of uncompensated metabolic acidosis, which can negatively affect feed intake. Complementary strategies to maximise postpartum feed intake should also be implemented to further prevent hypocalcaemia and ensure a smooth transition period.