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OSU Extension

College of Food, Agricultural, and Environmental Sciences

April 15, 2018 - 3:17pm -- lewandowski.11@...

          Pastures are beginning to green up and the grazing season is on the horizon for pasture-based livestock enterprises.  Grass tetany, sometimes called grass staggers, is potentially an issue early in the grazing season.  Low magnesium levels in the blood cause grass tetany.  Magnesium is one of the macro minerals required by animals and it is involved in crucial metabolic functions such as the transmission of nerve impulses and muscle contraction.  About 70% of the total body content of magnesium is stored in bones and teeth, so adequate blood levels of magnesium are dependent upon daily magnesium intake.  Older cows and cows in the first two months of lactation are most at risk for grass tetany.  Older cows are not able to mobilize magnesium from bone storage as readily as younger animals.  Milk production requires a high amount of magnesium that needs to come from the diet.
      The chapter on grass tetany in the Beef Cattle Handbook says that there is a relationship between potassium (K), calcium (Ca), magnesium (Mg) and nitrogen (N).  If the ratio of K to Ca + Mg is greater than 2.2 in forage then that forage is termed tetany prone.  Cool season grasses and small grains such as wheat and rye grazed in the early spring present the greatest risk for grass tetany problems.  These forages are most often low in magnesium and calcium and high in potassium.   Vegetative stage small grains can commonly have K concentrations of 3 to 5 percent with low levels of Ca and Mg that result in a ratio greater than 2.2.  Cool soil temperatures in the spring combined with soils that have high K levels result in cool season grasses luxury consuming potassium, in preference to magnesium. High plant nitrogen levels following a fertilizer nitrogen application in the spring can also limit magnesium availability.  Therefore, the highest risk grass and small grain pastures for grazing livestock are those that have high soil potassium levels and/or have recently received a nitrogen or potassium fertilizer application. Legume or legume/grass pastures offer a much lower risk of grass tetany because they naturally contain higher levels of calcium and magnesium.
       The first signs of grass tetany in the animal are restlessness, nervousness and flighty behavior.  There may be twitching of the skin and muscles that progress to muscle spasms and convulsions.  The affected animal may exhibit loss of coordination and stagger around.  Eventually the animal will collapse, lie on her side and paddle with her front legs.  Respiratory failure during a seizure results in death.  The interval between the first symptoms and death can be as short as 4 to 8 hours so often the first indication of a problem is a dead animal.  For those animals found in time, treatment is in the form of a solution of magnesium and calcium administered intravenously.
       Prevention, rather than curative treatment, is the best way to deal with the risk of grass tetany.  Provide high-risk animals grazing lush, early spring pastures with supplemental magnesium via a mineral mix.  Free-choice high magnesium mineral should contain 12 to 15% magnesium from magnesium oxide.  Cattle need to consume four ounces of the mineral supplement daily. Magnesium oxide is unpalatable, which can result in low mineral intake.  Intake can be encouraged by mixing the mineral with grain or a flavoring agent like molasses.  Provide a high magnesium concentration mineral mix at least two weeks before the early spring grazing period and continue through late spring until forages are more mature and temperatures are consistently warmer typically with daytime highs above 60 F.
     Other management options that can decrease the risk of grass tetany include: Feed small amounts of hay and/or grain to cattle on lush pasture during susceptible periods or limit grazing to 2-3 hours per day and graze the less susceptible or non-lactating animals (heifers, dry cows, stocker cattle) on the higher risk pastures.