Mineral composition of herbage in relation to the development of hypomagnesaemia in grazing cattle

In studies of the effect of fertiliser treatment on the incidence and severity of hypo‐magnesaemia in grazing cows, determinations of inorganic constituents were made of herbage samples from the swards grazed. From the values obtained, some of the mineral indices of Brouwer and t'Hart & Kemp were calculated for the individual experimental plots and their relationship to the observed incidence of hypomagnesaemia examined. For a given sward in a given season, the severity of hypomagnesaemia in cows grazing plots given different fertiliser treatments, generally increased as the alkaline earth alkalinity (Ca + Mg — P, expressed as mequiv./100 g. silica‐free herbage dry matter) of the herbage decreased. None of the other mineral indices considered showed a consistent relationship with the severity of hypomagnesaemia. Although the mineral composition of herbage may influence the development of hypomagnesaemia in grazing cattle, the effects of the palatability of herbage on dry matter (and hence magnesium) intake and of food factors other than minerals on the availability of herbage magnesium, preclude the use of mineral composition of herbage to assess the likelihood of the occurrence of hypomagnesaemia in grazing cattle.