Magnesium Concentration in Agropyron desertorum Fertilized with Mg and N

Grass tetany, a Mg deficiency in ruminants, is responsible for large economic losses throughout temperate regions. Significant livestock losses occur in the semiarid western United States primarily when livestock are grazing the spring growth of Agropyron desertorum, an introduced grass species which provides much needed early spring forage. The objective of this study was to increase forage Mg to about 0.2% by Mg and N fertilization and thus meet animal needs and reduce losses from death. The field study was located on A. desertorum grassland which had previously produced grass tetany. Two calcareous aridisols were each fertilized with 0, 90, 200, and 600 kg Mg/ ha as MgSO?•7H?O, having split plots with 0 and 150 kg N/ha applied as NH?NO?. Forage was harvested at regular periods intended to bracket the spring occurrence of tetany for three seasons following fertilization. Fertilization with 600 kg Mg/ha was necessary to increase forage Mg to the recommended level (0.2%). Applying 150 kg N/ha increased forage Mg concentration as much as did 200 kg Mg/ha. The N and Mg fertilizers were additive in increasing forage Mg concentrations. Forage Mg concentrations decreased with increasing age of vegetatively growing grass, and the benefits from fertilization were less with each successive season following fertilization. Little residual effect of 600 kg Mg/ha or 150 kg N/ha fertilization on plant Mg concentration would be expected after 5 years. Rapid decreases in water-soluble soil Mg with the resulting formation of some unknown insoluble phase, as well as high investment costs, preclude Mg fertilization of these ranges to meet Mg requirements of grazing animals