Spring Pastures May Bring Grass Tetany

Submitted by Eileen A. Coite, Livestock Agent for Wayne County

Now that we’ve had some off and on warm days and a good dose of rain, I suspect our winter annuals will take off with spring growth.  With that in mind, it’s always a good time to remember and be cautious of grass tetany, because this is the time we may see symptoms.  Grass tetany is a disorder affecting ruminant animals, specifically cattle, sheep, and goats.  It is also commonly known as magnesium tetany or grass staggers, and most often occurs in nursing females (cows, ewes, or does) during early lactation, especially during late winter and early spring on tall fescue or small grain pastures.  As the name "magnesium tetany" implies, the disorder is a result of low levels of the magnesium mineral in rapidly growing forages, as well as an interference with the absorption of magnesium due to various factors.  Grass tetany is known to be one of the most substantial nutritional problems in grazing livestock, with estimated annual losses of up to $150 million in the US.  

The causes of grass tetany:

Late winter and early spring is time for calving (lambing, or kidding) in many herds.  Because of this, the lactating female requires additional magnesium to offset losses of the mineral through her milk production and urine. Even though the disorder is most often seen in lactating females, stocker cattle may also be affected, especially when grazing small grains. Magnesium requirements are generally greater for lactating animals, and greater for older than for younger animals.   In addition, the pastures many of these animals depend on this time of the year are low in magnesium due to many factors.  Wet soils that are low in oxygen may prevent plants from taking up sufficient levels of magnesium (Mg) regardless of levels in the soil.  Soils that are low in phosphorous, but high in potassium and nitrogen are also a problem, because this tends to counteract the uptake of magnesium.  Generally, forages containing less than .2% magnesium are more likely to cause problems. 

Grass staggers is another name for grass tetany, and for a good reason. Females that show signs of the disorder generally will have a reduced appetite, dull appearance, and stagger when moving.  Other signs include nervousness, frequent urination and defecation, muscle tremors, and excitability, followed by collapse, paddling of the legs and feet, coma, and death. The first sign will often be a dead cow that appeared healthy the last time she was checked. 

Grass tetany can be prevented by feeding ruminants supplemental hay or grain, by fertilizing pastures with dolomitic limestone, which contains magnesium, or by providing a mineral mix or supplement which contains magnesium oxide.  The supplement during these risk periods is the most practical method of prevention.  Supplementing one ounce per day greatly reduces the incidence of the disease.   This can be provided in a home-mixed mineral with 30% magnesium oxide, a commercial "high mag" mineral of 10-14% Mg, or mixed at 6% of a grain mix, fed at one pound/head/day (increase to 2 pounds/head if following an outbreak) If a high mag mineral is used, it should be the only source of salt available in the pasture.  In addition, it's important to monitor the herd and make sure that everyone is getting one ounce/day of the mineral.  Pastures containing legume forages will normally be less affected, because legumes tend to be high in Mg.  Unfortunately, legume growth is often limited in winter months, so this usually won't fix the problem.

To be safe, as mentioned above, it's a good idea to provide additional sources of Mg to your herds in the late winter/early spring when grass tetany can be a problem.  The only way to know for sure if forages are low in Mg is to have them analyzed, which can be done at the NCDA forage testing lab in Raleigh.  Grass tetany isn't hard to prevent, but sure can be devastating to a herd once affected.