We previously reported the possibility of using the electrocardiogram variable to estimate blood calcium (Ca) concentration in dairy cows based on the strong positive correlation between the blood Ca concentration and the inverse of the corrected ST peak interval (STc −1 ).To improve the accuracy of the estimation of blood Ca concentration, we investigated the relationship between blood Ca concentration and STc −1 for each postpartum day and available variables other than STc −1 . We measured multiple variables (milk yield, calving number, age, body temperature, etc.), including serum total Ca concentration (tCa), blood ionized Ca concentration (iCa) and STc −1 in 462 Holstein cows on days 0, 1, 2, 3, 5, and 7 postpartum. A very high correlation was observed between iCa and tCa. The association between tCa and STc −1 for each postpartum day had a high coefficient of determination of 0.61-0.79 postpartum 0-2 days but decreased after the third day. In the investigation using the data from postpartum days 0-2, STc −1 , heart rate interval, calving number, and age were highly correlated with tCa.In addition, a multiple regression equation was obtained with tCa as the objective variable and STc −1 and calving number as explanatory variables. The estimation accuracy was improved as compared with the simple regression equation using only STc −1 as the explanatory variable. This multiple regression equation was used for 11 cows suspected of having hypocalcemia, and it was able to correctly detect cows requiring early treatment, except for one cow.

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