Grass tetany is primarily a disease of early-lactation beef cows characterized by low blood magnesium levels. As the name implies, grass tetany occurs mostly in cattle consuming lush, early spring, grass pastures. While the earliest growth pastures are potential high risk, the highest risk of having clinical signs seems to be five to 10 days after a period of cold, wet weather.

Many times, the first indication of a herd problem is finding a dead cow in the pasture with marks of struggling on the ground around her legs and head. Cows in the early stage of the disease may appear as “down cows” similar to a dairy cow with milk fever, but more likely they are nervous, trembling, overly excited or staggering.

Cows on primarily grass or fall-seeded small grain (oats, rye, wheat, rye grass) pastures are at the highest risk because the plants accumulate potassium and tend not to accumulate magnesium. Although pastures with a legume component are less likely to cause problems, you can see grass tetany in these pastures. This is due to the fact that the grass grows more quickly than the legume early in the spring, and that cows actually prefer the taste of grass versus legume.

A high level of potassium in the forage has one of the highest correlations to the incidence of grass tetany. Heavy fertilization of pastures in the spring with nitrogen and potassium (potash) significantly increases the risk of grass tetany. Previously, many studies pointed to too little magnesium in the soil as the primary cause of grass tetany. We now know that the key is excessive potassium.

It is truly an emergency situation when a cow is suspected of having grass tetany. Your herd health veterinarian should be called immediately to confirm the diagnosis and initiate treatment of the suspected cow with an intravenous solution of calcium, and magnesium.

While use of magnesium-containing mineral mix is one of the commonly mentioned preventive strategies, it shouldn't be the only one. Studies of cattle consuming free-choice mineral show that while the average intake for the group may be exactly as desired, some cows actually eat none while some eat two to three times more than needed.

About half the cows I've treated for grass tetany have been on an adequately formulated, free-choice mixture. The simplest formulation is equal parts of magnesium oxide, trace mineral salt, dicalcium phosphate and ground corn. Magnesium is not palatable and, thus, when mixing ground corn with the salt:mineral mix is desirable.

Place this mixture in numerous mineral feeders in the pasture. Cows should consume 4 oz./head/day to maintain adequate magnesium blood levels.

Be sure cows aren't being supplemented with a high level of potassium in the mineral mix. While sodium in the form of salt is included in all complete cattle salt:mineral mixes, recent work has demonstrated a positive effect of sodium in aiding in the prevention of grass tetany. As always, check with your herd-health veterinarian, Extension specialist or nutritionist for the best salt:mineral mix for your area.

There are also areas of the country where grass tetany is rarely seen. In these instances, the normal salt:mineral mix should be fed. I have also encountered clients who feed a high magnesium mix year-round “just in case.” This simply amounts to a case of wasting hard-earned money on unnecessary products. If an animal has a sufficient supply of a given nutrient, anything in excess is simply wasted.

Other preventive strategies for grass tetany include:

  • Soil test pastures in the fall and apply appropriate amounts of nutrients at that time (don't over-fertilize with nitrogen; especially not with potassium).

  • Add lime if needed.

  • Add legumes to grass pastures.

  • Continue to feed dry hay daily when cows are on new, lush pasture.

  • Graze “low-risk” animals (stocker calves, yearling heifers, cows with calves more than four months) on pastures at high risk for grass tetany before cows with younger calves.

Mike Apley, DVM, PhD, is an associate professor of beef production medicine at Iowa State University in Ames. W. Mark Hilton, DVM, is a clinical assistant professor of beef production medicine at Purdue University in West Lafayette, IN.

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