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The association of prepartum urine pH, and plasma total calcium at calving in Holstein dairy cows.

P. Melendez




The association of prepartum urine pH, and plasma total calcium at calving in Holstein dairy cows.
P. Melendez*1, J. Bartolome2, C. Roeschmann3, B. Soto2, A. Arevalos4, J. Moller4. 1University of Georgia Tifton, GA, 2National University of La Pampa Gral. Pico, Argentina, 3University of Chile Santiago, Chile, 4Fundo Los Laureles Chahuilco, Chile.

Hypocalcemia is a common metabolic disorder affecting dairy cows around parturition. A successful strategy to prevent clinical hypocalcemia is the use of anionic diets to induce a mild metabolic acidosis, increasing the responsiveness of receptors to PTH. The assessment of urine pH is a quick and inexpensive tool to monitor the degree of metabolic acidosis imposed by the anionic strategy. What urine pH is the ideal for the prevention of milk fever has been very controversial. The aim was to assess urine pH on prepartum Holstein cows fed anionic diets and determine its association with plasma total Ca (tCa), tMg, P, BHB at parturition. The study was conducted on a grazing dairy in Chile. At 30 d before expected parturition, cows were moved to a prepartum lot receiving 80% of their DM as a mixed ration and 20% from pasture (DCAD −109 mEq/kg DM). During Fall 2019, 345 prepartum cows were eligible for urine collection. Weekly, about 20% of the group were randomly sampled for urine pH using a portable electronic pH meter. The inclusion criteria of each sampled cow was that they had to stay at least one week in the prepartum group consuming the anionic diet. Within 6 h from calving a blood sample was obtained to collect plasma. At the end, 60 cows were tested for urine pH and blood metabolite concentrations at calving. The concentration of plasma tCa, P, tMg and BHB at d 1 pp were assessed. Metabolites were analyzed by ANOVA for polynomial regression (PROC GLM, SAS 9.4). Ten cows (16.6%) had a urine pH <6.0, 35 cows (58.3%) had a tCa concentration <2.15 mmol/L. There was a quadratic effect of urine pH on tCa. The concentration of plasma tCa was higher when the prepartum urine pH was between 6.0 and 7.5 (2.24—2.3 mmol/L), while decreased with pH <6.0 and > pH 7.5 (<2.22 mmol/L). No variables were associated with tMg at calving. There was a trend (P = 0.11) for a quadratic effect of urine pH on the concentration of BHB at parturition. BHB was higher when urine pH was <6.0 and >7.5. In conclusion, cows with prepartum urine pH <6.0 and >7.5 had a lower concentration of plasma tCa, and tended to have a higher concentration of BHB. These results indicate that anionic salts should be fed with caution, avoiding underfeeding and overfeeding. The target urine pH should be between 6.0 and 7.0.

Keywords: hypocalcemia, urine pH, anionic diets.