Changes in blood composition of pregnant cows during the onset of hypomagnesaemia

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Abstract
Mg, Ca, phosphate, total protein and free fatty-acid concentrations, and a lipase activity using p-nitrophenylpalmitate (PNP-C16) as a substrate, were determined in the serum of 15 pregnant Aberdeen Angus cows grazing a Phalaris tuberosa pasture with a tendency to induce tetany. A gradual decrease in Mg concentrations, but no definite change in the other parameters, was observed when the results were plotted directly against the time of the experiment. When the results were plotted as a function of the stage of pregnancy or lactation of the animals, Mg concentrations showed a more steady decrease which was associated with increases in Ca concentrations and, at the lowest Mg concentrations, with increases in phosphate concentrations. Total proteins and free fatty acids remained fairly constant and the highest lipase activity was associated with parturition. The lowest Mg concentrations in serum did not correspond with high levels of either lipase or free fatty acids. The hypomagnesemia observed in this experiment was attributed to the high K:Ca and magnesium ratio in the pasture and to the physiological condition (pregnant and lactating) of the animals. None of the parameters evaluated would be more sensitive than serum Mg levels for the early detection of the conditions that predispose grazing cattle to grass tetany.