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

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 leafMg concentration of tall fescue (Festuca arundinacea Schreb.) hay.However, little is known about how P‐fertilization alters the mineralconcentration of tall fescue under grazing. Our objective was to compare, undergrazing, the Mg, K, Ca, and P concentration of tall fescue when soil P wasconsidered either adequate or low. The treatments were tall fescue grown on soilfertilized 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 11April, 2000, and 6 March to 1 May, 2001. Forage samples were collected at thestart of grazing and at 14 day intervals thereafter. Under grazing, fertilization with P increased tall fescue forage Mg, K, and Ca concentrationonce spring growth started. However, the ratio of K/(Ca+Mg) never approached thecritical level thought to induce grass tetany. These results suggest thatfertilizing tall fescue pastures to an adequate soil P level improves the amountof dietary Mg available to ruminants during early spring and decreases the grasstetany potential of the forage.

Phosphorus Fertilization of Tall Fescue Pastures May Protect Beef Cows from Hypomagnesaemia and Improve Gain of Nursing Calves

“…At this sampling, cows in the P‐fertilized and supplement treatments had serum Mg levels that were 20% greater than those from the cows in the control treatment. This increase in serum Mg for P‐fertilized and supplement‐treated cows loosely mirrored dietary Mg (8).…”

“…Calf gain during the first 90 days after birth depends almost entirely on cow milk production (1). In almost all measures, forage nutritive value after spring growth began (after Day 28) was greater for the P‐fertilized treatment than for the two low‐soil‐P treatments (7,8). This increase in forage nutritive value could conceivably drive greater milk production and calf gains.…”

“…4). Concentrations of Ca and K in serum did change slightly over time, typically mirroring changes of these elements in the forage (8). It seems likely that cows mobilized Ca from storage tissues to replace serum Ca lost due to lactation.…”

“…It seems likely that cows mobilized Ca from storage tissues to replace serum Ca lost due to lactation. It is somewhat surprising that no treatment differences existed for serum P concentrations even though forage P concentration of the P‐fertilized treatment was 30 to 50% greater than the control and supplement treatments at each date (8). The change in serum K concentration of cows over time may be due to cows coping with increased dietary K (13) as their diet changed from stockpiled forage (or hay) to new spring growth (8).…”

“…Recent Missouri research showed that raising soil P levels in the plow layer from below 16 to at least 25 lb/acre (Bray I P) increased the concentration of Mg in tall fescue during early-spring when compared to unfertilized pastures (8,12). Thus, cattle may consume their Mg requirement while grazing, eliminating the need for Mg supplementation.…”