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Grass Tetany at Spring Turn-Out

09:35AM May 02, 2014

BT Rotator Unloading Stocker Cattle( Wyatt Bechtel )

Cattlemen should be on the watch for grass tetany in cattle that will be turned out on spring pasture.
By: Adele Harty, Cow/Calf Field Specialist, SDSU Extension

Grass tetany is a metabolic disorder associated with lush pastures due to low levels of blood magnesium concentration, which results in nerve impulse failure in animals. The moisture received across much of South Dakota last fall and through the winter, along with the increasing temperatures is resulting in rapid growth of cool season grasses, which can lead to grass tetany. Ranchers should evaluate the steps they are taking to prevent grass tetany in their herds. Even though it may seem early, there have been some confirmed cases of grass tetany in western South Dakota.

Multiple factors play a role in causing grass tetany, including:

  • 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 the 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 risk associated with cows grazing lush pastures. One long term measure is to incorporate more legumes into pasture mixes, as 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. 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 needs to be provided two to three weeks before turn-out or before tetany is likely to occur. Palatability and consumption can be challenging, resulting in some of the animals consuming an inadequate amount of the mineral on a daily basis. Be sure all animals have access to the mineral while they are 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 them unless forced. Dry forages can act as carriers to provide additional Mg and Ca to the animals at a critical time. If the drinking water source can be controlled (i.e., water tanks), soluble Mg salt 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.

Older, lactating cows with calves younger than 2 months of age are the most susceptible 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 of Mg in their system. Also, cows within two months after calving have increased milk production and require additional Ca and Mg.

Cattle will exhibit symptoms of grass tetany, but they may not be 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 death. Affected animals should be handled in a quiet manner, since sudden death can occur if animals are stressed.

There are treatment options for animals, but the effectiveness of treatment depends on the clinical stage when treatment is 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 that cattle are more susceptible to grass tetany in the spring of the year, and certain weather conditions increase the susceptibility. Consider and implement prevention practices, monitor cattle for signs of grass tetany, and treat them as soon as possible according to a treatment plan developed with a veterinarian.