Grass tetany is a problem in livestock systems the world over.

One of the biological shortcomings of the cow is her lack of ability to store magnesium and the relative ease with which her metabolic pathways responsible for the mineral can fail.

She needs a constant supply and, even at that, external factors can govern whether or not she will develop grass tetany.

An American paper published in 1998 summarised an experiment in which a high-magnesium grass variety (HiMag) was tested as a potential tetany preventer.

The trial was repeated over three years and tall fescue grasses were used. Pastures were strip-grazed. Initially steers and dry cows were used and blood magnesium levels were monitored as a precautionary measure.

Then, lactating cows with calves were tested. In all, 170 animals were used.

Results

Forage from the HiMag tall fescue variety had consistently higher magnesium (22%), calcium (18%) and phosphorus (9%) concentrations.

Potassium concentrations did not differ among cultivars, which resulted in a lower tetany ratio for HiMag compared with other cultivars.

Though HiMag’s chemical analysis showed a reduced tetany risk, the majority of the other samples collected from the commercial varieties returned results in safe ranges for tetany.

This is likely a characteristic of many tall fescue grasses. Indeed, there was no incidence of grass tetany actually recorded during the trial.

Cattle in two of the five experimental comparison groups had higher blood magnesium concentrations on HiMag pastures – there were no differences detected in the other three groups. Nonetheless, the chemical composition results suggest that the use of HiMag grass could reduce the tetany risk relative to other cultivars.

Irish context

Perennial ryegrass remains the king in Ireland.

Though tall fescue can and does grow here, it is typically in wild meadow areas or old pastures and its feed value is a fraction of that of perennial ryegrass.

Best practice for tetany prevention remains to be external supplementation of magnesium and long fibre where necessary.

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