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  • Title: Potentiometric determination of ionized and total calcium in blood plasma of Holstein and Jersey cows.
    Author: Ballantine HT, Herbein JH.
    Journal: J Dairy Sci; 1991 Feb; 74(2):446-9. PubMed ID: 2045552.
    Abstract:
    The effects of stage of lactation and breed on total and ionized calcium in blood plasma were determined for eight Holstein and eight Jersey cows fed a diet containing .25% Ca during the dry period and fed a diet with .50% Ca after calving. Blood samples were obtained by jugular venipuncture at 2-wk intervals from 4 wk before calving through wk 10 of lactation. Total and ionized Ca contents of fresh plasma were determined potentiometrically with an ion-specific electrode. Total Ca in plasma also was determined by atomic absorption spectrophotometry. Average concentrations determined by atomic absorption were approximately .7 mg/dl higher than those for the ion-specific electrode. Plasma total and ionized Ca were lower in both breeds at calving than any other time during the sampling period. Jersey cows had lower total Ca (7.47 mg/dl) and ionized Ca (4.25 mg/dl) than Holsteins (8.10 and 4.66 mg/dl) on day of calving. Ionized Ca as a percentage of total calcium increased from 55 to 57% at parturition and then decreased slowly to 53% at peak lactation. Changes in percentage of ionized Ca may indicate the contribution of bone resorption relative to dietary Ca in meeting the Ca requirement for lactation.
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