The effect of lα-hydroxycholecalciferol on the concentrations of calcium, magnesium and inorganic phosphate in the plasma of parturient cows

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Abstract
Summary: 135 Cows which were expected to produce a second or subsequent calf within 4–11 d received an intramuscular injection of 500 μg lα-hydroxycholecalciferol (lα-OHD3). In those animals in which the interval between injection and calving was at least 7 d there was, at 7 d after injection of lα-OHD3, a significant increase (P < 0.001) in the blood plasma concentrations of Ca and Pi and a significant reduction (P < 0.05) in that of Mg, compared with those in control animals. Within 24 h of calving there was a significant increase (P < 0.001) in the concentration of Pi but no change in Ca or Mg compared with pre-injection levels, whilst the control animals showed a significant fall (P < 0.001) in the concentration of Ca and a rise in those of Pi (P < 0.001) and Mg (P < 0.05). There was a significant reduction (P < 0.001) in the number of animals with hypocalcaemia (defined as a plasma Ca concentration 3; this reduction was particularly marked when the interval between injection and calving was between 1 and 8 d. The previously reported reduction in the incidence of clinical milk fever is considered to be a consequence of the effect of lα-OHD3 on the concentration of Ca in plasma.