Grass tetany

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Abstract
The effect on the plasma levels of magnesium, calcium, and phosphorus of grazing pure Huia white clover and Ariki or Ruanui ryegrass pastures containing different levels of magnesium, calcium, and phosphorus was studied using 7 sets of identical twin cows and 30 yearling Romney wethers. Grazing either the white clover or the ryegrasses had no influence on the plasma levels of calcium and phosphorus. In cattle grazing Huia white clover, magnesium levels were significantly lower (P < 0.01) than those of animals grazing Ruanui ryegrass, even though the clover contained a higher level of magnesium. In contrast, sheep grazing Huia white clover had significantly higher (P < 0.05) plasma magnesium levels than sheep on Ariki ryegrass on the majority of occasions that the animals were sampled. A significant (P << 0.05) pasture species grazed x time of sampling interaction was observed for plasma magnesium levels in grazing sheep. The observed differences in plasma magnesium levels between the white clover and the ryegrasses could not be explained in terms of current knowledge on the interrelationships between the level of nitrogen and potassium in the herbage and the availability and/or metabolism of dietary magnesium.