It’s that time of year again, when cattle producers should be concerned about grass tetany.  Grass tetany is a common ailment in the spring and fall of the year when cattle are grazing lush grasses in large quantities.  The cause of this ailment is a low level of magnesium (Mg) in the plant, which leads to a magnesium deficiency to develop in the bovine consuming the forage.  Grass tetany, also known as hypomagnesemia, may only affect 1-2% of grazing animals.

There are many reasons why a forage grass may be low in Mg.  High fertilization of grasses with potassium (K), also known as potash, depresses Mg uptake in the plant.  High amounts of nitrogen (N) fertilization can also increase the plant’s fatty acids which may also depress Mg availabilty.  Low soil temperatures and high soil moisture content that leads to low soil oxygen content, in the spring and fall, may also decrease Mg uptake by the plant.

An animal’s susceptibility to grass tetany increases five days after the mean daily air temperature exceeds 57 degrees Farenheit (following a period below this temperature).  Symptoms of Mg deficiency include nervousness and hypersensitivity, muscle twitching, staggering, and convulsions.  A change in the blood serum Mg level will also be seen.  Mg values < 1.0 mg/100 ml are considered critical in cattle.  Normal levels are between 1.5-3.5 mg/100ml.

There’s an old saying that goes, “An ounce of prevention is worth a pound of cure.”  That’s definitely the case with grass tetany.  Producers can do several things to ensure that this problem does not occur in their cattle herd.  Providing a magnesium supplement, whether free choice or included in a feed mix is one great option.  Practical fertilizer management of pastures or hay fields is also key.  Avoid excessive N fertilization in the spring and if applying manure, do so in a responsible manner, to ensure that available K levels in the soil are not in excess.

There’s is always that 1-2% chance that a cow may still become hypomagnesemic.  In that situation, producers need to have intravenous calcium and magnesium salts on hand.  Immediate treatment by intravenous injection of Ca and Mg salts will help to alleviate the symptoms of grass tetany and hopefully save the animal.

To read more about grass tetany check out these extension publications:

• Grass Tetany in Cattle, University of Florida, UFAS Extension
• Look Out for Grass Tetany, Montana NRCS