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Be Vigilant for Grass Tetany this Spring

January 19, 2012

                Cattle producers across the state have been dealing with a short forage supply for nearly two years in some areas.  To compensate for poor hay yields and inadequate fall grazing conditions, many have turned to wheat, rye, and ryegrass for a hopeful abundance of spring forage.  The quality of spring forage is hard to surpass and easily exceeds the protein and energy needs of mature beef cows; however, some cattle producers may realize unpleasant results if adequate precautions are not taken before grazing. 

During periods of rapid spring forage growth, forages may take up an excessive amount of potassium from the soil.  During this time, cattle become susceptible to a condition termed grass tetany – a condition associated with inadequate magnesium in the diet as excess potassium blocks magnesium uptake by plants and magnesium absorption in the cow.   Mature lactating cows are most susceptible because not only is dietary magnesium limited but these cattle are also loosing magnesium through the milk they produce. 

Cattle with grass tetany become excitable, develop muscle tremors, and have difficulty breathing and in the worst case, death.  As a result, care must be taken when handling and treating cattle that are exhibiting symptoms to avoid exacerbating the situation. 

The most common method of preventing grass tetany is to supplement the herd with magnesium beginning at least 1 month prior to spring grazing.  Mineral supplements that contain 10 to 12% magnesium as magnesium oxide, called High Mag minerals in lay terms, are commonly used.  At 3 to 4 oz intake, such minerals will provide 40 to 50% of a cow’s daily magnesium requirement.   Some producers will opt to blend higher rates of magnesium into their mineral and add cottonseed meal or similar feedstuff to increase supplement palatability and magnesium consumption.   Magnesium oxide is commonly used for supplemental magnesium because of its high magnesium concentration (55-60%).  Magnesium sulfate (Epsom salts) may also be considered; however, the magnesium content of Epsom salt is below 20%, and magnesium sulfate in excess of 140 g per day will have a laxative effect.  Magnesium sulfates and chlorides are water soluble allowing for supplemental magnesium to be delivered through a water tank; however, managing magnesium supplementation through water may be more difficult than supplementing through feed.

Cattle diagnosed with grass tetany should be treated by slowly administering a calcium/magnesium solution intravenously, given additional magnesium subcutaneously, and removed from the susceptible pasture.  A veterinarian should be consulted for diagnosis and treatment.  If deaths have occurred, your veterinarian should be consulted to properly establish cause of death.

While well fertilized spring pasture poses the greatest risk for grass tetany, when these forages are harvested at an immature stage for hay, winter tetany may develop – grass tetany on a hay based diet.  When forages are managed under the conditions described, cattle producers should include magnesium supplementation during hay feeding. 

To assess the risk for grass tetany, forages may be tested for major mineral content (calcium, phosphorus, magnesium, potassium, sodium, and sulfur) to calculate a “tetany ratio”.  Major mineral analysis costs $6 per sample at the University of Arkansas, Agricultural Diagnostics Service Laboratory. The ratio examines the quantity of potassium to the quantity of magnesium and calcium, and ratios greater than 2.2 are likely to result in grass tetany.  Unfortunately for growing forages, there is usually a one week lag between stage of growth and receiving forage test results from the lab.  For properly cured hays, mineral content is stable and the forage analysis will be current.

For more information on mineral supplementation, preventing grass tetany, or forage testing, visit your local county Extension agent.

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