Evaluation of supplemental mixtures containing magnesium for prevention of grass tetany in beef cattle

A study was conducted at The University of Tennessee, Highland Rim Experiment Station, Springfield, Tennessee, to evaluate the effects of free-choice salt-CSM-dry molasses-magnesium oxide and Mg Cl₂-molasses mixtures on blood serum Mg levels and the incidence of grass tetany in pregnant beef cows. The cows grazed either non-nitrated or nitrated-fescue pastures and were supplemented with medium quality grass hay when the pastures were insufficient to support them. Blood and forage were collected monthly and analyzed for Mg, K and Ca. Soil samples were collected annually. During the winter of 1972-73, the Mg blood serum level of the cows fed the MgO-salt mixture (1.61 mg/ml) was higher (P<.05) than that of the non-supplemented control cows (1.51 mg/ml). The mean serum Mg level of cows fed the MgO-salt (1.65) and Mg Cl₂-molasses mixture were both higher (P<.01) than that of the control cows in 1973-74. Hay and supplemental Mg consumption was lower for cows grazing nitrated fescue pastures than for those grazing non-nitrated pastures. Mg supplementation, MgO-salt and Mg Clg₂-molasses, partially pre-vented the severe drop in blood Mg level at calving which was observed in the control cows. In addition, the blood serum Mg level increased more rapidly following calving in the Mg supplemented groups. There appeared to be a positive relationship between winter serum Mg level of the cows and the subsequent performance of her calf. These results suggest that the problem of grass tetany can be reduced by supplemental Mg fed in the form of MgO-salt or Mg CI₂- molasses mixtures. Variation among cows within a pasture with respect to serum Mg level tended to be a problem and probably reflected varia-tion in individual consumption. These data suggest that early initiation of Mg supplementation and continuing measurement of con-sumption is vital to successful prevention of grass tetany by this means