Abstract
SUMMARY Changes in the packed cell volume, the concentrations of plasma calcium, magnesium, potassium and sodium, and the concentrations of red blood cell potassium and sodium which occur when dairy cattle are transferred from conditions of stall feeding to those of grazing pastures under different manurial treatments, have been studied. The plasma magnesium concentrations fell significantly during the first 2 weeks at pasture and then rose again, but the recovery could not be associated with any change in the magnesium content of the herbage. The magnitude of the fall in plasma magnesium concentration for individual animals grazing pastures which had received the same manurial treatment varied to such an extent that no effect of manurial treatment on this reaction could be demonstrated. Plasma sodium concentrations fell over the first 2 weeks of grazing and then rose again, whilst red blood cell sodium concentrations rose immediately after the animals were turned out to graze and thereafter gradually returned towards the pre-grazing concentrations. No significant changes in the plasma calcium and potassium concentrations or the red blood cell potassium concentrations were found. The packed cell volumes fell in the second and third weeks and rose again slightly in the last week. Concentrations of plasma magnesium as low as 0.6 mg/100 ml. were found, in the presence of normal plasma calcium, potassium and sodium concentrations, without the manifestation of any clinical signs of tetany. Analysis of blood samples taken from 3 field cases of grass tetany revealed reduced plasma concentrations of both calcium and magnesium. In spite of the fact that the manurial treatments produced pastures which had extremely high protein and potassium contents, no clinical cases of grass tetany occurred amongst the experimental animals, although in certain animals pronounced hypomagnesaemia occurred. It was concluded that high potassium and protein contents of the herbage do not invariably lead to the development of grass tetany.
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