Beware of grass tetany in cattle

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By David Harrison

Michelle Arnold, UK Extension Veterinarian encourages cow/calf producers to start feeding high magnesium mineral. Grass will be greening up soon so it is best to be proactive to prevent grass tetany.
Grass tetany is caused by abnormally low magnesium in the bloodstream of cattle. Pregnant cows should be fed supplemental magnesium from 60 days before calving until the beginning of the breeding season to help prevent it.
Grass tetany occurs most often in cows grazing lush spring forages, especially small grains and cool season grasses. It is the more common in spring calving cows that are high producers in their third to fifth lactation.
Several factors contribute to the increased incidence of tetany. The magnesium requirement of cows doubles from late gestation to early lactation. When this rapid increase in magnesium requirement combines with lowered magnesium in the plant, along with certain components which lower the availability of magnesium (such as high application of nitrogen and potassium fertilizer), tetany may develop.
Weather may also have an effect. The greatest threat is when temperatures are between 40 and 60 degrees. Temperatures over 60 degrees markedly decrease the incidence of tetany. When all of these risk factors are combined, the risk can be very high.
Cattle affected with grass tetany may isolate themselves and stagger. As the diseases progresses, they may exhibit extreme nervousness, rapid breathing and muscle trembling. They may become aggressive and charge. In the most severe state, the animal collapses to the ground with muscular spasms. Treatment must be given rapidly as death can occur within an hour after the onset of convulsions.
Producers should be concerned with preventing tetany. About two ounces of magnesium oxide is recommended to meet the magnesium needs of lactating beef cows. Cows grazing spring grass pasture should have magnesium in the mineral mixture or, in high risk situations, supplement.