© 2002 Plant Management Network. Accepted for publication 24 July 2002. Published 9 August 2002. Adequate Soil Phosphorus Decreases the Grass Tetany Potential of Tall Fescue Pasture Thomas R. Lock, Robert L. Kallenbach, Dale G. Blevins, Timothy M. Reinbott, and Gregory J. Bishop-Hurley, Department of Agronomy, University of Missouri, Columbia, 65211; and Richard J. Crawford and Matt D. Massie, University of Missouri Southwest Research and Education Center, Mt. Vernon, 65712 Corresponding author: Thomas R. Lock. trl13b@mizzou.edu Lock, T. R., Kallenbach, R. L., Blevins, D. G., Reinbott, T. M., Bishop-Hurley, G. J., Crawford R. J., and Massie, M. D. 2002. Adequate soil phosphorus decreases the grass tetany potential of tall fescue pasture. Online. Crop Management doi:10.1094/CM-2002- 0809-01-RS. Abstract Grass tetany is a nutritional disease of ruminants caused by low dietary Mg. Previous research has shown that early spring P-fertilization increases the leaf Mg concentration of tall fescue (Festuca arundinacea Schreb.) hay. However, little is known about how P-fertilization alters the mineral concentration of tall fescue under grazing. Our objective was to compare, under grazing, the Mg, K, Ca, and P concentration of tall fescue when soil P was considered either adequate or low. The treatments were tall fescue grown on soil fertilized to achieve 30 lb/acre P (P- fertilized) or left unfertilized at 6 lb/acre P (Control). Three cow/calf pairs grazed each pasture from 15 February to 11 April, 2000, and 6 March to 1 May, 2001. Forage samples were collected at the start of grazing and at 14 day intervals thereafter. Under grazing, fertilization with P increased tall fescue forage Mg, K, and Ca concentration once spring growth started. However, the ratio of K/ (Ca+Mg) never approached the critical level thought to induce grass tetany. These results suggest that fertilizing tall fescue pastures to an adequate soil P level improves the amount of dietary Mg available to ruminants during early spring and decreases the grass tetany potential of the forage. Grass tetany is a nutritional disease that kills or affects approximately 350000 beef cows in the USA each year (6). Grass tetany typically occurs when cattle are moved from a winter diet of hay or stockpiled forage to lush, cool- season or winter annual grass pastures in early spring (11) (Fig. 1). Most research indicates that grass tetany is caused by a dietary Mg deficiency. When forage Mg concentration is below 0.20% dry matter (DM), the balance between Mg, Ca, and K in the animal is upset and grass tetany can occur (7). In addition to the low forage Mg concentration, the high water content in spring growth of cool-season or winter annual grass pastures makes it difficult for grazing cattle to consume adequate amounts of Mg (2). Other forage mineral concentrations have been implicated with the disease in addition to Mg. These include a forage Ca concentration less than 0.4% and/or a forage K level above 3.0% DM (20). Crop Management 16 August 2002