5/6/03

Contacts: Nolan Hartwig, Extension veterinarian, 515-294-0711, nhartwig@iastate.edu

Wendy Miller, Iowa Beef Center, 515-294-9124

Grass Tetany: A Quick and Silent Killer

AMES, Iowa – Hypomagnesemia, more commonly referred to as grass tetany, occurs when cows do not receive enough magnesium in their diets, usually in early spring when forages are not yet established. Often, clinical symptoms are not observed and the only evidence is a dead cow in the pasture. “The trouble with tetany is that the symptoms, when noticed, can vary widely and often resemble other disorders such as milk fever,” said Nolan Hartwig, Extension veterinarian at Iowa State University. “Prevention is key.”

Tetany is most likely to occur in unsupplemented beef cowherds in April or May; once nighttime temperatures begin rising to 55 degrees or higher, the incidence of tetany falls because grasses are more able to draw magnesium from the soil at warmer temperatures. Pastures with high levels of potassium in the soil often have more cases of tetany, however, it can occur in pastures of all soil types. Heavy milking cows nursing calves less than 2 months old seem to be most at risk.

Symptoms of grass tetany can be:

Excitability

Wild stare

Appearance of blindness

Lack of coordination

Trembling muscles

Grinding teeth

Violent convulsions

Deep coma

“If you suspect tetany, it’s probably a good idea to obtain blood samples from several similar animals to get an idea of herd status,” Hartwig said. “The kidneys begin conserving magnesium when the blood serum level drops too low, therefore, low urinary magnesium is also a good indicator.”

Early treatment is very important. Cows down for more than 12 to 24 hours are prone to suffer significant muscle damage and will rarely recover. Cows showing early signs of tetany should be handled very carefully – any handling technique producing excitement is liable to cause sudden death. The animals can usually be treated with a calcium solution containing magnesium if the condition is caught early enough.

“In the case of tetany, the old adage ‘an ounce of prevention is worth a pound of cure’ rings true,” Hartwig commented. “There are some relatively simple management practices producers can incorporate that will significantly reduce the risk of tetany to their herds.”

1. Keep plenty of magnesium oxide available from October until May. Make sure each cow gets at least 2 ounces of magnesium during peak risk times. Magnesium alone is not palatable to cattle; the best technique is to offer it with other a salt and mineral mix well before calving begins so the cows will become accustomed to it; increase the amount of magnesium in the mix as tetany season approaches.
2. Don’t start grazing pastures too early; wait until growth is 8 to 10 inches as magnesium is more available in mature plants as opposed to younger ones.
3. Incorporate a mix of legumes in early-season pastures. Legumes can contain twice the magnesium as other grasses grown on the same soil.
4. If soil magnesium is low, develop a long-term plan to increase this nutrient using dolomitic limestone.
5. Supplement cows with:

• Good legume hay
• Mineral mixtures: Salt (with iodine and cobalt), 30; bone meal, 30; magnesium oxide, 30; dried molasses (for palatability), 10. You can also use salt, 75 and magnesium oxide, 25, but cattle may not find this palatable. Some mineral mixes have inadequate percentages of magnesium to be effective – be sure to read the label. At least 10 percent magnesium is desirable.
• Concentrates. For self-feeding (with adequate water available) and forage containing 10 percent crude protein. Ground grain (corn, oats, milo, etc.). 65; magnesium oxide, 20; salt, 15.
• Watch out for mineral mixes that contain very low levels of magnesium, even though they claim to be effective as a grass tetany preventative.

For more information on grass tetany, please contact the Iowa Beef Center at 515-294-BEEF, your area Extension livestock field specialist or your local veterinarian.

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