Grass Tetany Potential of Tall Fescue as Affected by Soil O21

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Abstract
Grass tetany, a metabolic disorder in cattle on pasture, has been related to low Mg and high K levels in tall rescue (Festuca arundinacea Schreb.) and other forages. These mineral imbalances in forages and most cases of tetany usually occur in late winter and early spring when soil O2 is low because of frequent precipitation and low evaporation rates.In a greenhouse experiment, gas mixtures were used to study the effect of soil O2 levels on Mg and K in forage of four tall rescue entries. Magnesium content of these entries was 0.25% or less at 2% soil O2 but ranged from 0.35 to 0.44% at 21% soil O2. Entries differed in Mg content at 21% but not at 2% soil O2. Potassium content ranged from 2.1 to 2.7% at 2% soil O2 and from 2.9 to 3.7% at 21% soil O2. Root length was greatly reduced by low soil O2 levels and was the only root characteristic we studied related to Mg and K uptake.A replicated field experiment was used to study the effect of soil drainage on forage Mg content of ‘Kentucky 31’ tall rescue. Forage from poorly drained areas had Mg contents of 0.18 to 0.25%, whereas nearby well‐drained areas produced forage with 0.27 to 0.40% Mg.Our results showed that low soil O2 levels and, therefore, poor soil drainage may be important factors in production of tetanogenic tall fescue forage. Draining wet areas, removing animals from poorly drained pastures, and maintaining legumes on these problem areas should be helpful practices for preventing tetany.