A really wet week across the northern half of the country has meant that early weaning is taking place on some farms. The combination of stress and lower grass supplies could bring on tetany cases so it’s important to have control measures in place

Grass tetany, or hypomagnesaemia, is caused by a deficiency of magnesium (Mg) in the blood. It is a fatal condition in beef and dairy cows and a common cause of sudden death. Cows are generally found dead without having shown any signs of sickness.

Suckler cows are poor at storing magnesium in their bodies and need a daily supply to prevent deficiency. It is common with suckler cows when they are grazing very bare as well as lush pastures.

It can also occur after a very wet night were animals haven’t grazed properly and therefore have insufficient levels of magnesium in their blood.

This grass is frequently low in Mg due to quick growth and heavy slurry spreading, which is high in potassium and can have a negative effect on Mg uptake. It can be associated with stress like transport, wet weather, cows in heat or changes in diet.

In terms of treatment, it’s vital that veterinary attention is sought immediately and a mix of calcium and magnesium can be given with some sedation to calm the animal. Brain damage can occur very quickly if not treated in time.

A single case in a herd of cows could mean that there are a number of other cows on the brink taking tetany with lower magnesium levels in the herd likely.

If you do have a case it’s important to halt any weaning or other management tasks that might cause stress to the cows and increase the risk of tetany. Ways of controlling or preventing grass tetany include:

  • Feeding high Mg concentrates.
  • The addition of Mg to the drinking water (this can be questionable in periods of wet weather).
  • Buffer feeding with hay or straw prior to going out on to lush pasture can help prevent grass tetany by reducing the rate of passage of food, allowing time for Mg absorption.
  • Avoiding pastures that have had heavy slurry applications or the application of high-potassium fertilisers.
  • Regular liming of grazing land to maintain the correct soil pH and improve Mg uptake.
  • Giving free access to high Mg minerals, either by way of powder mineral or mineral licks.
  • The use of magnesium bullets — at least two bullets/boluses should be used per cow, which will release Mg at a controlled rate each day for four to six weeks.

    • All these preventative strategies will reduce the risk of grass tetany, but no one measure will give guaranteed protection, especially if the cows are exposed to a significant stressor such as very cold, wet weather.