Magnesium Concentration in Agropyron desertorum Fertilized with Mg and 1st

Magnesium Concentration in Agropyron desertorum Fertilized with Mg and 1st H. F. Mayland and D. L. Grunes2 of humid pastures with N and K often increases the i id of tetany 3 4 nc ence ( , 4). 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 q 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 MgS0,.71-1,0, having split plots with 0 and 150 kg N/ha applied as NH,N0,. 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%). Ap. plying 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. Additional index words: Crested wheatgrass, Grass tetany, Hypomagnesemia, Aridisols, Range forage. F ORAGE containing 0.2% Mg normally provides sufficient Mg to protect ruminants against grass tetany (3, 4). This is not a fixed dietary level, but it reflects the Mg concentration above which tetany seldom occurs unless the forage is unusually high in N or K. The apparent Mg availability to ruminants is about 28% for cured legume hay, but values as low as 12% have been reported for fresh grass (9). It follows, then, that most cases of tetany are reported for all grass or predominantly grass pastures. Magnesium deficiency in ruminants is reported most frequently for humid and subhumid grass pastures growing on coarse-textured soils low in available Mg. Under these conditions, soil fertilization or forage dusting with Mg minerals is commonly used to minimize the tetany problem (3, 4). However, fertilization Contribution from the Western Region, Agricultural Research Service, USDA, Idaho Agricultural Experiment Station cooperating. Received Jan. 22, 1973. Soil Scientists, Snake River Conservation Research Center, Kimberly, ID 83341, and U.S. Plant, Soil and Nutrition Laboratory, Ithaca, NY 14850, respectively. The authors wish to thank Mrs. Arlene Florence, Harold Waggoner, D. A. Hewes, and P. K. Jon for their assistance during the study. B . Morphological and classification details were provided by Mr. Warren W. Rasmussen, Agricultural Research Service, Kimberly, Idaho, and the Nevada State Office, Soil Conservation Service, Reno, respectively. Grass tetany also occurs in cattle grazing the semiarid rangelands of the western United States. Livestock losses in Idaho and Nevada alone were estimated to average 4,000 head annually during the period 1965 through 1971 (H. F. Mayland, unpublished data). The magnitude of these losses varied from very few in some years to 3% of grazing animals in other years. Production losses caused by chronic Mg deficiencies in ruminant animals would further increase the economic loss attributed to grass tetany. The objectives of this study were: 1) to determine the Mg fertilizer level required on these calcareous soils to produce grass forage containing at least 0.2% Mg, and 2) to determine the effect of N fertilization on forage Mg concentrations. MATERIALS AND METHODS