by Tracy Brewer, Research Assistant Professor of Range Science, Joe Skeen Institute for Rangeland Restoration, Dept. of Animal and Range Sciences, Montana State University


"The combination of unseasonably warm temperatures, early green-up of cool season grasses and the recent, lingering drought have increased the risk of grass tetany in livestock."

It is the first week of March and, already, I have talked to ranchers in our state that have lost cattle to grass tetany. What is wrong with this picture—it’s only early March! The combination of unseasonably warm temperatures, early green-up of coolseason grasses and the recent, lingering drought have increased the risk of grass tetany in livestock in Montana. It’s not too early to brush up on what grass tetany is, what animals are prone to it, prevention measures, and possible treatment methods.

What is grass tetany and when does is occur? 

Grass tetany is a nutritional or metabolic condition in beef cattle and sheep triggered by low amounts of magnesium (Mg) in the blood serum. This disorder occurs most frequently in the spring when livestock graze young, succulent, cool season grasses. It intensifies in warm periods, five to 10 days after a cool, wet period when grass is growing rapidly. Although it is less prominent, grass tetany can also occur in the fall when regrowth of cool season grasses occurs.

What causes grass tetany? 

Factors that can increase the occurrence of grass tetany in livestock include stress, drought, diets low in Mg and phosphorous (P), diets with nutrient imbalances that interfere with Mg metabolism, high levels of nitrogen (N) or potassium (K) in feed or soils, increased Mg demand during lactation, minimal availability of standing dead forage, and forages with a “tetany ratio” {K/(Calcium + Mg)} of greater than 2:2. Because it is a function of soils, plant species, harvested feed, environmental factors and animals, the severity of this disorder varies between states, counties, ranches and pastures.

What are the symptoms? 

Early symptoms of grass tetany include uncoordinated gait, staggering, nervousness, excitability and muscle spasms. In many cases, animal mortality is the first sign of grass tetany identified, due to a short, four- to eight-hour time interval between the onset of early symptoms and death.

Which animals may it affect? 

Female animals have been shown to be more prone to grass tetany than males. Older animals, animals that have recently given birth, those nursing young less than eight weeks of age, high milk producers and fat animals are also more susceptible to grass tetany than other classes of animals. However, it has also been seen in young or dry cows and in growing calves in extreme conditions. Older animals have a diminished ability to absorb Mg, Mg requirements increase greatly directly post-partum and during heavy lactation periods, and fat cows have less Mg available for absorption in their body fluids than lean cows.

Grass tetany prevention

The following measures can be taken to decrease the likelihood of grass tetany in your herd or flock:

  • Graze animals that are less susceptible to grass tetany on tetany-prone pastures (i.e., steers, heifers, stocker calves, cows with calves older than four months).
  • Delay spring grazing until grass is 4 to 6 inches tall.
  • Graze pastures that have residual standing dead forage
    present before grazing pastures without.
  • Feed animals roughage for 10 to 14 days at the beginning
    of the grazing season before turning them out on lush, green pastures.
  • Feed legumes, which are higher in Mg than grasses, or graze legume pastures first in the spring.
  • Do not fertilize pastures with nitrogen (N) or potassium
    (K) before spring grazing.
  • Check animals grazing lush, succulent pastures periodically
    throughout the day for symptoms of grass tetany.
  • Supplement animals with a legume hay.
  • Supplement animals with a mineral or protein-energy supplement, using a form that will ensure that every animal will receive 1 to 2 ounces of magnesium oxide per day, for 30 days before spring grazing and for 30 days after the initiation of spring grazing.
  • Supplement animals by treating controlled water sources with soluble Mg salt.
  • Cull or develop alternative management strategies for animals that develop grass tetany because they will be prone to develop it in the future.


The window of opportunity for treatment is generally small once grass tetany symptoms are identified. However, treatment options do exist. Remember that animals suffering from early stages of grass tetany are highly excitable and must be handled gently.

  • A sterile solution containing Mg and Ca (e.g., 200-500 ml calcium magnesium gluconate) can be given to the animal intravenously. This solution should be administered
    carefully and slowly to avoid a toxic reaction to the minerals.
  • Magnesium enemas have been used successfully, are less expensive, and are less dangerous than an I.V. This enema consists of 60 grams of magnesium chloride dissolved in 200 ml of water, placed in a collapsible, plastic bottle. The solution is administered through a plastic tube attached to the bottle.

The effects of grass tetany move very rapidly. Therefore, taking preventative measures is the most effective method for keeping your livestock alive this spring where tetany conditions exist. Watch for identifiable symptoms and contact your local veterinarian immediately if you suspect grass tetany in your herd or flock.

This information is for educational purposes only. Reference to commercial products or trade names does not imply discrimination or endorsement by the Montana State University Extension Service.