Effect of serpentine rock and its acidulated products as magnesium fertilisers for pasture, compared with magnesium oxide and Epsom salts, on a Pumice Soil. 1. Dry matter yield and magnesium uptake

Low pasture magnesium (Mg) intakes by grazing dairy and beef cows is a main cause of hypomagnesaemia in these animals soon after calving. Magnesium fertiliser application to pasture can increase pasture mixed-herbage (hereafter herbage) Mg concentration and reduce the risk of this disorder. However, the effectiveness of Mg fertilisers is variable depending on fertiliser type and soil properties. A 32-month Mg fertiliser field trial was conducted on dairy pasture on an Immature Orthic Pumice Soil (near Taupo, New Zealand; exchangeable Mg 0.4 cmol(c) kg(-1), pasture Mg 0.13%) to compare the effectiveness of a range of Mg fertilisers in elevating herbage Mg concentration. The fertilisers used were ground serpentine rock, three acidulated serpentine rock products, Epsom salts, and E-mag (magnesium oxide). All fertilisers were applied at 100 kg Mg ha(-1) at the start of the trial. The trial also had two treatments where Epsom salts was applied at 25 and 50 kg Mg ha(-1) and a treatment where serpentine rock was reapplied during the second and third year at 100 kg Mg ha(-1). Magnesium fertiliser treatments did not affect pasture dry matter yield significantly at any harvest, however, the effect of treatments on herbage Mg concentration was highly significant (P < 0.001) at all harvests. The single application of serpentine significantly increased herbage Mg concentration at 10 harvests, with five of these harvests having Mg concentrations >= 0.20%, which is generally considered to be the minimum threshold level for stock health. In comparison, the Mg concentration in pasture not receiving Mg fertiliser remained below the minimum threshold level throughout the trial. Application of serpentine rock annually further increased herbage Mg concentration and accumulation. The herbage Mg concentrations for this treatment were similar to those of the Epsom salts (50 kg Mg ha(-1)) treatment at the majority of the harvests. These results indicate that the annual application of serpentine rock may have potential as an alternative strategy to the application of more soluble Mg sources for raising herbage Mg concentration to levels considered adequate to meet stock health requirements. The more soluble Mg fertilisers-Epsom salts, the two serpentine-superphosphate products, and an acidulated serpentine rock plus reactive phosphate rock product-had the greatest effect on increasing herbage Mg concentration and accumulation. These treatments increased herbage Mg concentration to >= 0.20% within 3-6 months after application and for the majority of harvests over the rest of the trial period. The effect of E-mag on herbage Mg concentration was similar to that of serpentine rock during the first year, but the subsequent performance of E-mag was similar to the more soluble Mg fertilisers.