Adequate Soil Phosphorus Decreases the Grass Tetany Potential of Tall Fescue Pasture

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 (Pfertilized) 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, coolseason 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 Fig. 1. (A) Tall fescue (Festuca arundinaceae Schreb.) beginning spring growth. (B) Mixtures of cereal rye (Secale cereale L.) and annual ryegrass (Lolium multiflorum Lam.) can contain too low Mg levels for grazing beef cows in lactation. Grass tetany is not always controlled by Mg supplementation (19). The disease is also difficult to diagnose in its early stages because symptoms are not always evident (10). Recent work to increase the amount of dietary Mg to grazing cattle has focused on boosting the early spring Mg concentration of grasses such as tall fescue (Festuca arundinacea Schreb.) (4,17,18). By increasing herbage Mg concentration, cattle might consume appropriate levels of Mg while grazing. Thus, producers would be less dependent upon unreliable supplements to provide cattle with enough dietary Mg. Recent research indicates that the Mg concentration of tall fescue leaves is dependant on soil P level (17,18). In these experiments, tall fescue was grown on plots that had between 8 and 97 lb/acre available P and received 0 or 25 lb/acre triple-super phosphate in early spring. When available soil P was 8 or 15 lb/acre and no P-fertilization occurred, leaf Mg concentration was 0.21% DM. Fertilization of these low P soils with 25 lb/acre P increased leaf Mg concentration to over 0.25% DM. In these experiments, 15 lb/acre Mg fertilization with MgCl2 only increased tall fescue leaf Mg concentration when 25 lb/acre P was also applied or when soil P was greater than 26 lb/acre (18). The impact of soil P on tall fescue forage Mg concentration could be an important consideration across the southeastern quarter of the United States. It is reported that half of the soils in this region test medium, low, or very low for soil P (5). In addition, more than 24 million acres of tall fescue are grown in this area (1) where grass tetany is a common disease. To determine if P-fertilization of tall fescue pastures increases the amount of dietary Mg available to cows, experiments are needed to determine if the Mg uptake of tall fescue is enhanced under grazing conditions. Our objective was to compare, under grazing conditions, the Mg, K, Ca, and P concentration of tall fescue forage when soil P was considered either adequate (30 lb/acre) or low (6 lb/acre). Grazing Experiment Testing Tall Fescue Mineral Concentration The grazing portion of the experiment was conducted from 15 February to 11 April, 2000, and from 6 March to 1 May, 2001. The location was the University of Missouri Southwest Research and Education Center near Mt. Vernon, MO (37°04’N, 93°53’W; elevation 1150 ft). The soil series was a Creldon silty clay loam (fine, mixed, mesic Mollic Fragiudalf). An area low in soil P was divided into three blocks, each with two 2.0-acre pastures of established ‘Kentucky 31’ tall fescue that received one of the following treatments: • Cows grazing tall fescue grown on a soil with an adequate level of P (30 lb/acre) (P-fertilized); • Cows grazing tall fescue grown on a soil low in P (6 lb/acre) (Control). A B Crop Management 16 August 2002 Pastures in the P-fertilized treatment were broadcast fertilized in early February and late August with calcium orthophosphate (triple-super phosphate; 46% P2O5 by mass) beginning in February, 1999 and ending in February, 2001. The total amount of P2O5 applied ranged between 286 and 313 lb/acre. These applications raised soil P to the level considered adequate for tall fescue forage production (13). All pastures were treated with 120 lb N/acre as ammonium nitrate and 300 lb K2O/acre as potassium chloride 14 days prior to the start of grazing each year. In addition, ammonium nitrate was broadcast in July, 1999 and 2000 at 60 lb N/acre to promote fall growth after pastures were cut for hay. Table 1 shows the soil pH, K, Ca, Mg, and P levels of the pastures during the experiment. Table 1. Soil pH, K, Ca, Mg, and P levels of P-fertilized and Control tall fescue pastures. Data were pooled across two years from samples taken before and after the experimental period each year. Pasture Management Grazing occurred during the typical grass tetany season, which begins approximately three weeks before new growth starts and ends six to eight weeks later (10). Three lactating cows with calves grazed each pasture to simulate a typical grazing system in the early spring. The cows grazed stockpiled tall fescue early in the study and actively-growing grass after spring growth began. Equal amounts of forage were allocated to animals grazing the pastures using the putand-take method so that forage availability was equal for both treatments. Forage Sampling and Analyses Forage samples were harvested with a sickle-bar mower. They were collected at the start of grazing and at 14 day intervals thereafter. Six 1.0-ft-x-15.0-ft strips were cut from each pasture to a residual height of 2 in. Forage from the six strips was combined and sub-samples were collected. Sub-samples were dried at 122° F for four days in a forced-air oven for dry matter determination. The subsamples were ground through a 0.04 in screen using a cyclone mill. Plant tissue was analyzed for K by flame emission; Mg and Ca by atomic absorption; and P by colorimetry (15). Statistical Analyses All data were pooled across years because error variance was homogeneous. The model used to analyze the data was a split-plot in time (9). Main plots were two pasture treatments and split plots were five harvest dates. All hypotheses were tested at the 0.10 alpha level. Means for all data were separated using least significant difference. Means are presented by treatment and date because treatment by date interactions occurred. Magnesium Concentration of Tall Fescue Forage Forage from both treatments showed low but equal Mg concentrations for the first 14 days of the experiment (Fig. 2). Research from West Virginia has shown that stockpiled tall fescue has a seasonal low in Mg concentration during mid-February (3). The lack of a treatment difference early in our study was likely a result of Mg leaching from the leaves over winter and little grass growth. However, by day 28, the Mg concentration of the Control was 0.13% DM, which was 15% less than at day zero. In contrast, the P-fertilized treatment did not Treatment pH K Ca Mg P