The mineral element content of spring pasture in relation to the occurrence of grass tetany and hypomagnesaemia in dairy cows

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Abstract
1. A survey was carried out on dairy farms in the south of Scotland during the period 1958–59 to obtain information on: (a) the mineral element composition of spring pastures associated with grass tetany or staggers and hypomagnesaemia, and (b) the influence of magnesium supplements on the incidence of these conditions and on serum magnesium levels.2. Pastures associated with grass tetany had a significantly lower average magnesium and sodium content and a higher potassium content than those which were classed as normal. These differences were magnified in the values of the ratios K/(Ca + Mg) and (K + Ca + P)/(Mg + Na) which were considerably higher in tetany pastures. The incidence of tetany was positively correlated with the value of the former ratio. These results are similar to those obtained in Holland.3. The analytical results for pastures associated with hypomagnesaemia in clinically normal cows were not significantly different from those for normal pastures and there was no evidence of any association between the composition of the pasture and the incidence or severity of the hypomagnesaemia or the serum magnesium levels.4. The magnesium and sodium contents of the majority of the pastures examined would be judged as inadequate in the light of available information on the requirement of these elements.5. There was no evidence of any correlation between the magnesium content of the pasture and that of any of the other elements studied.6. Cases of grass tetany occurred in 25% of the 108 herds studied, the mean incidence in affected herds being 4·5 ± 0·65%. The overall incidence was 1·1 ± 0·25% and was significantly lower when a magnesium supplement was given as a prophylactic measure.7. Hypomagnesaemia was detected in clinically normal cows after turning out in 41·2% of the herds, the overall incidence being 8·7 ± 1·0%. On tetany free farms the feeding of calcined magnesite or magnesium-rich mineral mixtures did not appear to reduce the incidence of hypomagnesaemia or the fall in serum magnesium levels which occurred after turning out.8. These results emphasize the wide individual variation in the susceptibility of different cows to the factors causing hypomagnesaemia and grass tetany and also the differences between the two conditions in their dietary relationships. It is suggested that dietary factors may be responsible for producing the clinical signs of grass tetany in hypo-magnesaemic cows.