Grass Tetany: When to prepare Back »

With the colder than average temperatures South Dakota has experienced this spring, normal cool season grass growth has not yet occurred. As a result, the risk of grass tetany may be delayed, but could also be a higher risk due to the cold soil temperatures. As the ambient temperature rises in the next few weeks, the cool season grasses will start to grow rapidly. With the rapid growth of these cool season grasses, specifically crested wheatgrass, and cows lactating heavily, producers should have an increased awareness of grass tetany and current risk factors.

What is Grass Tetany?

Grass tetany is a metabolic disorder associated with lush pastures due to low concentrations of blood magnesium, which results in nerve impulse failure in animals. The concern of grass tetany isn’t normally seen until May, but taking steps to prevent it now will be more effective in the long run. It is never too early to plan and ensure proper management practices are in place.

For a better understanding of grass tetany and being able to manage its risk, one needs to understand what factors play a role. These include:

  • Low magnesium (Mg) content of rapidly growing grasses and pastures
  • High potassium (K) content of rapidly growing grasses and pastures
  • High crude protein content of grasses and pastures
  • Bad weather, storms, stress, etc., that cause cattle to be “off feed” for 24-48 hours
  • Lactation: losses of Mg and calcium (Ca) in milk
  • Various combinations of the above factors resulting in low blood Mg or Ca


The key to prevention is to be proactive. Measures should be taken to minimize risks associated with cows grazing lush pastures. One long-term approach is to incorporate more legumes into pasture mixes. Legumes have higher levels of Mg and Ca than do immature grasses resulting in a better balance across the pasture. If possible, delay turn-out into lush pastures until plants are 4 to 6 inches tall. This will reduce the occurrence of tetany, in addition to giving drought-stressed pastures a little more time to rest and recover. The reality is that many producers need to utilize pastures when grasses begin to green-up and the risk of tetany is most prevalent.

If delayed grazing is not an option, other management tools should be utilized. First, always provide a high magnesium (Mg) mineral supplement or mineral mix containing at least 8-12% Mg. This should be provided two to three weeks before turn-out or before tetany is likely to occur. Palatability and adequate intake can be challenging, resulting in some of the animals consuming an inadequate amount of mineral on a daily basis. Ensure access to the mineral by all animals prior to and while grazing tetany-prone pastures, as this will help decrease the occurrence. Another potential tool is to provide hay while cattle are on lush pastures; however, cattle are not likely to eat hay unless forced to. Dry forages can act as carriers to provide the animals additional Mg and Ca at critical times. If the drinking water source can be controlled (i.e., water tanks), soluble Mg salts may be added to the water. Some examples of soluble Mg salts are magnesium acetate, magnesium chloride, and magnesium sulfate (Epsom salts). The most common form of Mg, magnesium oxide, is not soluble in water and therefore cannot be used for this purpose.

Susceptibility & Symptoms

Older, lactating cows with calves younger than 2 months of age have the greatest susceptibility to tetany; while steers, heifers, dry cows, or cows with calves older than 4 months of age are less susceptible. Mature cows are more susceptible because they are less able to mobilize Mg from bones to maintain the necessary level in their system. Also, cows within two months after calving have increased milk production and require additional Ca and Mg.  

Cattle will exhibit symptoms, but are not commonly observed as death may occur within 4 to 8 hours.  An affected animal will exhibit a series of progressive signs. These include grazing away from the herd, irritability, muscle twitching in the flank, wide-eyed and staring, muscular incoordination, staggering, collapse, thrashing, head thrown back, coma, and finally death. Affected animals should be handled calmly, since sudden death can occur if animals are stressed.

Treatment Options

There are treatment options for animals, but its effectiveness depends on the clinical stage when administered. If treatment is started one or two hours after clinical signs develop, the results are usually a quick recovery. Treatment is not effective if delayed until the coma stage.  Grass tetany can be treated with an intravenous dextrose-based commercial preparation of magnesium and calcium purchased from a local veterinarian.

Remember cattle are more susceptible to grass tetany in the spring, and certain weather conditions increase susceptibility. Consider and implement prevention practices, monitor cattle for signs of grass tetany, and treat as soon as possible according to a protocol developed with a veterinarian.

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