– Michelle Arnold, DVM, University of Kentucky Ruminant Veterinarian
Grass tetany, also known as spring tetany, grass staggers, wheat pasture poisoning, winter tetany or lactation tetany, is due to a low level of magnesium (Mg) in the blood. The amount of magnesium in the blood is completely dependent on the amount obtained from the daily diet. 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+). 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 those few lucky producers, the minerals available in their soils and forages are enough to meet the needs of their cows. A number of complex factors contribute to the ability of magnesium to be absorbed through the rumen (stomach) wall. Primarily there is a “pump” mechanism that actively transports the dissolved form of Mg across the rumen wall to the bloodstream. This pump doesn’t work when potassium in the rumen is high and sodium is low because this changes the electrical potential necessary to drive it. Adding salt to the ration will improve Mg transport only when sodium is low in the overall diet. 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.
Research has shown that the negative effects of high potassium in early spring grass cannot be overcome by the addition of large quantities of salt. However, high magnesium mineral mixes prevent grass tetany by allowing magnesium to passively flow into the bloodstream of the cow without the need for the active transport pump.
Has Limited Amounts of Salt in Mineral Mixes led to an Overconsumption of Minerals?
Regional soil types, soil fertility and different forage species result in different mineral intakes for grazing livestock on every farm. A blanket statement disregarding these factors is oversimplifying 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 extremely common in Kentucky and can predispose animals to serious and sometimes fatal disease conditions. Interactions occur between all of the various metals, minerals, and other elements in the diet, and optimal amounts of all elements are essential for proper nutrition. Trace mineral mixes are formulated to meet the needs of cattle, including the need for salt. The keys to using a free-choice product are to ensure cattle have access to mineral 100% of the time, use a palatable 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 3 ounces per head per day will only last 5 days in a 50 cow herd. If the cows have calves that eat mineral too, a bag may only last 3 days.
Does Grass Tetany Only Occur in the Spring?
“Winter tetany” in beef cattle is caused by a diet low in energy and an insufficient intake of magnesium. It may also be observed when feeding wheat or rye silage during the winter since these are often high in potassium and nitrogen but low in magnesium. Affected cattle are borderline low in 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 soluble magnesium in the rumen during times when conditions are right for grass tetany. As long as the active transport pump for magnesium is working well and driving magnesium across the rumen wall, problems should not develop. However, when factors prevent this pump from working (such as high potassium level in lush spring grass), the second or “backup” pathway is to increase the amount of magnesium in the diet with a high magnesium mineral mix. A high rumen magnesium level will allow magnesium to passively flow into the bloodstream of the cow without the need for the active transport pump. Supplementation with high magnesium mineral should begin at least 30 days prior to calving. Cows require 20 grams of magnesium daily or 4 ounces per day of a 15% magnesium mineral mix during the late winter and early spring. Mineral feeders should not be allowed to be empty because consistent intake is important for clinical disease prevention. UK Beef IRM mineral recommendations for free choice supplements for grazing beef cattle include 15% salt and 14% magnesium in the complete mineral mix and all magnesium from magnesium oxide (no dolomitic limestone or magnesium mica). These complete mineral mixtures supply the necessary sodium in the form of salt to aid in combatting high potassium intakes. Consumption should be moni¬tored because they seldom eat enough trace mineral if using poor quality products. Feeding ionophores (monen¬sin, lasalocid) has been shown to improve magnesium absorption efficiency. High magnesium mineral may be discontin¬ued 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.
In addition to supplying supplemental magnesium, several management factors should decrease the risk of grass tetany. 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 often limited in late winter; 3) 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; 4) Graze the less susceptible or non-lactating animals (heifers, dry cows, stocker cattle) on the higher risk pastures.
In summary, increasing magnesium intake by supplementing with magne¬sium oxide, offering adequate salt to prevent sodium deficiency, and increas¬ing total energy intake with good quality forage or supplemental feed are all effective tools in preventing grass tetany. These are exceptionally important when mov¬ing from winter rations to young spring grass pasture, especially in heavily milking cows. Grass tetany is considered a true veterinary emergency requiring prompt treatment with magnesium to prevent death.
