A disease associated with lush early-season grass is grass tetany. This disease is also called grass staggers or hypomagnesemia, and is caused when blood levels of the mineral magnesium become low due to a combination of plant and animal factors. 

Although young, growing animals can occasionally be diagnosed with grass tetany (particularly if grazing grasses associated with grain production such as wheat, rye or barley), lactating cows are the most commonly affected. 

Milk contains relatively high levels of magnesium, and cows in late pregnancy and early lactating cows have a high dietary requirement for the mineral. In addition, deficiencies of calcium or phosphorus, which are also required in high amounts in lactating cows, will increase the risk for grass tetany. 

In cases of grass tetany, soil levels of magnesium are typically not deficient. But soil temperature affects magnesium uptake into the plants, and grass tetany is associated with early spring before soil temperatures are consistently warm. Cool-season grass pastures, particularly if fertilized with nitrogen or a spring application of potash, are most commonly associated with grass tetany. However, even cows on unfertilized native pastures can have the disease if heavy spring rains promote rapid early forage growth. Forages prone to causing grass tetany are deficient in magnesium and sodium and have an excess of potassium. Potassium, which can be very high in lush, early-growth forage, interferes with magnesium absorption from the gut, further decreasing the amount of available magnesium. 

Signs, treatment and prevention

The first signs of grass tetany are nervousness, appearing overly alert and a stumbling gait. These signs progress to restlessness and aggressiveness. An affected cow’s gait may vary from being stiff-legged to high-stepping to staggering. 

An animal that has fallen and is unable to rise and has convulsions must be treated soon to prevent death. In fact, because of the rapid course of the disease, finding dead cows with no previous signs of illness is commonly the first indication of the problem.

If cattle are identified early in the disease, treatment with a solution containing magnesium given into the vein will likely prevent death. If a cow has been down several hours, the outlook even with treatment is not as positive. A cow that was down or staggering but was treated successfully can appear better for a few hours, and then the condition can return. To avoid these relapses, additional magnesium is usually given by another route such as an oral paste, an enema with a magnesium source or subcutaneous administration of a high-magnesium solution. 

As a preventive strategy, late-pregnancy and early-lactation cows grazing lush early-growth cool season grasses should be fed a high-magnesium mineral supplement starting about 30-days prior to turnout and until soil temperatures remain consistently high and forage growth has slowed. 

It is important cattle have a daily supply of high-magnesium mineral during this time, so check mineral feeders frequently. Cattle don’t like the taste of magnesium sources and it must be mixed with molasses, grain, soybean meal or other palatable feed. 

Grass tetany is a serious disease of cattle that can result in rapid death. It is important to know the situations, forage types and fertilization strategies most likely to be associated with grass tetany and to have a plan to prevent problems with appropriate mineral supplementation.  

Editor’s note: Robert L. Larson is a professor of production medicine and executive director of Veterinary Medicine Continuing Education at Kansas State University in Manhattan, Kan.