Dr. Michelle Arnold, Ruminant Extension Veterinarian, University of Kentucky Veterinary Diagnostic Lab; A special thanks to Dr. Jeff Lehmkuhler for his contributions to this article.

What is “Grass Tetany” and when are cattle most likely to have it? Grass tetany, also known as spring tetany, grass staggers, wheat pasture poisoning, winter tetany or lactation tetany, is a condition resulting from a low level of magnesium (Mg) in the blood. Maintenance of blood magnesium depends on the amount obtained from the daily diet since the magnesium present in teeth and bones and is not easily mobilized in times of need. Magnesium is required for proper nerve and muscle function so low levels in the blood result in “tetanic spasms” where muscles contract uncontrollably. The disorder in an adult cow begins with separation from the herd and going off feed. The ears are often erect and twitching and the cow is alert, hyperexcitable and may be aggressive. The symptoms quickly progress to muscle spasms, convulsions, difficulty breathing, and death. Often the affected animal is found dead with evidence of thrashing and struggle on the ground around her. Deficiencies occur most often in beef cows when they are nursing a calf and grazing young, green grass in early spring. Fast-growing spring pastures are high in potassium (K+) and nitrogen (N+) and low in magnesium (Mg++) and sodium (Na+) ions. Affected cattle often have low blood calcium concurrently. Fall calving cows may also experience grass tetany during the winter months.

Will Feeding Plain White Salt to Cows Prevent Grass Tetany? This claim is shared every spring and, indeed, there are producers who do not have grass tetany that only feed salt. How can that be? Simply put, for some producers, the minerals available in their soils and forages are enough to meet the nutritional needs of their cows. Regional soil types, soil fertility, diverse forage species and differing cattle requirements based on age and stage of lactation result in different mineral needs for grazing livestock on every farm. A blanket recommendation to just feed salt ignores these factors and oversimplifies a very complex situation. Trace minerals such as copper, selenium, and zinc are all essential nutrients vital for proper growth, production, and immune system function. Trace mineral deficiencies are very common and predispose animals to serious and sometimes fatal disease conditions. Commercial trace mineral mixes are formulated to meet the needs of cattle, including their daily need for salt. Because interactions occur between all the various metals, minerals, and other elements in the diet, optimal amounts of all elements are essential for proper nutrition.

Several complex factors are in play for magnesium to be absorbed through the rumen (stomach) wall and into the blood. Primarily there is a “pump” mechanism that actively moves the dissolved “soluble” form of Mg across the rumen wall to the bloodstream. If potassium in the rumen is high and sodium is low, this setup changes the electrical potential needed to drive the pump. Research has shown that the negative effects of high potassium in early spring grass cannot be overcome by simply adding more sodium in the form of salt. In fact, too much salt will increase urination and cause magnesium to be lost in urine. Salt, as with any substance, can be dangerous and even fatal at high levels. Fortunately, a second, “passive transport” system for Mg exists which is not influenced by potassium. This transport system only works when soluble Mg in the rumen fluid is high and Mg will then flow into the bloodstream without having to be pumped. High magnesium mineral mixes prevent grass tetany by increasing the amount of dietary magnesium concentration in the rumen, allowing this passive movement of Mg to take place.

Does Grass Tetany Only Occur in the Spring? No! “Winter tetany” in beef cattle is caused by consumption of a diet low in energy and an insufficient intake of magnesium, usually over winter. It may also be observed when feeding wheat or rye baleage since these forages are often high in potassium and nitrogen but low in magnesium. Affected cattle have borderline low blood magnesium concentration then clinical signs of grass tetany are triggered by a stressor such as a severe cold snap.

Hypomagnesemia is often referred to as an “iceberg” disease because only a few clinical cases occur but there are many unobserved or subclinical cases that may become problems after a stressful event such as a weather change.

How Can Grass Tetany Be Prevented? Prevention is based on providing magnesium in the diet during times when conditions are right for grass tetany. If the active transport pump is driving magnesium across the rumen wall, grass tetany problems should not develop. However, when factors prevent this pump from working (for example, high levels of K+ in lush spring grass), the second or “backup” pathway depends on increasing levels of magnesium in the diet. Supplementation with high magnesium mineral should begin at least 30 days prior to calving. Cows require magnesium daily or 4 ounces per day of a 12% magnesium mineral mix, especially during the late winter and early spring if pregnant or lactating. The keys to using a free-choice trace mineral product are to ensure cattle have access to mineral 100% of the time, use a palatable, quality product and make sure they are consuming it
at the expected level. Remember a 50-pound bag of hi-mag mineral to be fed at 4 ounces per head per day will only last 4 days in a 50 cow herd. If the cows have calves that also eat mineral, a bag may only last 3 days. Mineral feeders should not be allowed to be empty because consistent intake is important for clinical disease prevention. Provide adequate access for cows and calves, for example 1 mineral feeder per 15 cow/calf pairs. Do not offer additional loose salt, salt blocks, or sources of salt at the same time! High magnesium mineral may be discontinued in late spring once the grass is more mature, the water content of the forage is decreased, and daily temperatures reach at or above 60°F.

Does the form of magnesium used in the mineral matter? Absolutely. The feed industry utilizes magnesium oxide (MgO) to supply magnesium but there is tremendous variation in quality and bioavailability. Magnesium oxide is bitter and unpalatable to beef cattle. Recently the UK Beef IRM mineral recommendations were updated to reflect current market conditions. The more palatable form of magnesium known as “prilled MagOx” has been removed from the Beef IRM mineral guidelines because it is unavailable at the present time.The granular or powder magnesium oxide has a greater surface area resulting in the potential for a decrease in palatability, therefore the magnesium oxide level was reduced to 12% from the previous recommendation of 14%. UK Beef Integrated Resource Management (IRM) mineral recommendations for free choice supplements for grazing beef cattle now include 15% salt and 12% magnesium in the complete mineral mix and all magnesium from magnesium oxide (no dolomitic limestone or magnesium mica). These complete mineral mixtures also supply the necessary sodium in the form of salt to aid in combatting high potassium intakes. Consumption should be monitored because cattle will not eat enough trace mineral if using poor quality products or if any additional free-choice salt is available. Only put out 1-2 weeks’ worth of mineral at a time. If feeding grain to cattle, MagOx can be added to grain to ensure magnesium consumption. For example, with approximately 60% Mg in MagOx and if feeding 2 lbs grain / cow, then adding 50 lbs MagOx / ton of feed will provide about 14 g Mg to the cow.

Are there management changes that reduce the risk of grass tetany? Yes. These include: 1) Soil test and apply fertilizer based on soil test results and use no more potassium than recommended since grasses are “luxury” consumers of potassium; 2) Legumes are high in magnesium and will help offset the problem although their growth is slow in late winter; 3) Offer hay to cattle on lush pasture during susceptible periods or limit grazing time to 2-3 hours per day to slow the rate of passage through the digestive tract and allow more time for magnesium absorption; 4) Graze the less susceptible or non-lactating animals (heifers, dry cows, stocker cattle) on the highest risk pastures. Be aware that the use of poultry litter as a feed supplement or fertilizer has frequently been associated with an increased incidence of grass tetany.

In summary, increasing magnesium intake by providing a free choice, high magnesium trace mineral mix and no alternative forms of salt, and meeting energy needs with good quality forage or supplemental feed are necessary to prevent development of grass tetany. Both are exceptionally important when mov-ing from winter rations to young spring grass pasture, especially in early lactation cows. Grass tetany is considered a true veterinary emergency requiring prompt treatment with magnesium to prevent death. Response to therapy is not always good and depends largely on the length of time between onset of symptoms and treatment. Cattle that do recover take at least an hour which is the time it takes for magnesium levels to return to normal. Many of these cows will relapse and require more treatment within 12 hours. Administering oral magnesium gel once the animal has regained good swallowing reflexes, drenching with magnesium oxide or magnesium sulfate, or administering a Mg enema will reduce the rate of relapse. If grass tetany has occurred within a herd, an effort should be made to immediately increase the intake of magnesium to other herd members to prevent further losses.