Phosphorus Fertilization of Tall Fescue Pastures May Protect Beef Cows from Hypomagnesaemia and Improve Gain of Nursing Calves

Previous research has linked soil phosphorus level with forage magnesium concentration in tall fescue (Festuca arundinacea Schreb). Understanding this relationship might reduce the incidence of sub-acute hypomagnesaemia and/or grass tetany in cattle (Bos taurus). Our objective was to determine if P fertilization of tall fescue pastures affects the mineral element status of lactating, mature cows in spring and performance of their nursing calves. The treatments were: (i) tall fescue pasture fertilized to achieve 30 lb/acre of Bray I P; (ii) tall fescue pasture not fertilized with P but animals were given access to a mineral supplement that contained 12% Mg; and (iii) tall fescue pasture not fertilized with P and no supplement provided. With two exceptions, cows in all treatments showed similar serum and cerebrospinal fluid Mg concentrations. Average daily gain of nursing calves was 0.2 lb/day higher in the P-fertilized treatment compared to the other treatments. Thus, improving soil P level may alleviate symptoms of sub-acute hypomagnesaemia in cows and, more importantly, increase calf gain.