Forage-Related Disorders in Cattle-Hypomagnesemic Tetany or “Grass Tetany”

Stockmen should monitor beef cows for grass tetany,looking out for the tell-tale signs of low magnesium, says a University of Kentucky, Large Ruminant Veterinarian.
calendar icon 17 December 2013
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What is “Grass Tetany” and when are cattle susceptible: Grass tetany, also known as spring tetany, grass staggers, wheat pasture poisoning, winter tetany or lactation tetany, is a condition caused by an abnormally low level of magnesium (Mg) in the blood.

Maintenance of normal blood magnesium is completely dependent on absorption of magnesium obtained from the diet, writes Dr. Michelle Arnold, Large Ruminant Extension Veterinarian, University of Kentucky.

Deficiencies occur most often in beef and dairy cows in early lactation grazing lush pastures high in potassium (K+) and nitrogen (N+) and low in magnesium (Mg++) and sodium (Na+). Affected cattle are often found to have low blood calcium concurrently.

Typically grass tetany occurs when grazing ryegrass, small grains (i.e. wheat, rye) and cool season perennial grasses in late winter and early spring (Feb-April) although it can occur in fall calving cows. Fast-growing spring grass is usually high in potassium and crude protein, and low in sodium and magnesium.

Cause of Grass Tetany: A number of factors contribute to the ability of magnesium to be absorbed through the rumen (stomach) wall. To be absorbed, magnesium must be “in solution” (dissolved) and this is largely dependent on the pH of the rumen fluid. When the pH is high (or more alkaline or “basic”), this decreases the available Mg because its solubility declines at a higher pH.

Lush spring grass with its high level of crude protein combined with the use of nitrogen fertilizers in the soil, causes an increase in ammonia in rumen fluid and a corresponding increase in pH. Grass plants also do not take up magnesium as well when the weather is cool and the soils are water-logged. Several known magnesium binders can also exist within forages that form insoluble Mg salts in the rumen, preventing the passage of Mg into the blood.

In order to move magnesium out of the rumen, there is a “pump” mechanism that actively transports the dissolved Mg across the rumen wall to the bloodstream. The active transport of Mg across the rumen wall is compromised by high dietary potassium and low sodium because this changes the electrical potential necessary for the pump to work. Adding salt to the ration will improve Mg transport when forage sodium is low but too much sodium will ultimately cause more problems by loss of magnesium in the urine.

Research has shown that the negative effects of high potassium cannot be overcome by the addition of large quantities of salt. However, a high rumen magnesium level, achieved by feeding high magnesium mineral mixes, will allow magnesium to passively flow into the bloodstream of the cow without the need for the active transport pump.

Clinical Signs: Grass tetany is characterized by hyperexcitability (nervousness), tetany (constant contraction of muscles or muscle stiffness and rigidity), convulsions, and death. The earliest signs begin when blood magnesium levels fall below 1.1 mg/dL and include twitching of the facial muscles, shoulder, and flank. As the fall in blood magnesium progresses, tetanic spasms of the muscles (muscles stay contracted so legs are stiff and rigid) become more common, eventually causing the cow to stagger and fall. Rapid convulsions or seizures quickly follow, with chomping of the jaws and frothy salivation.

The low concentration of magnesium in the cerebrospinal fluid (the fluid present around the spinal cord or “CSF”) is responsible for the convulsions seen in grass tetany. Affected animals lie with the head arched back and the legs paddling. The heart rate may reach 150 beats per minute (normal is 60 to 80) and can often be heard without the use of a stethoscope. Respiratory rates of 60 breaths per minute and a rectal temperature as high as 105°F may result from the excessive muscle activity. Animals may stand up and repeat these convulsive episodes several times before they finally die. A moderate form of hypomagnesemia with blood Mg levels of 1.1 to 1.8 mg/dL can occur with milder signs of reduced feed intake, nervousness, and reduced milk production.

Diagnosis is made based on history, clinical signs, and low magnesium level in the blood or CSF. Blood samples are not always an accurate measure of Mg levels because muscle damage may cause leakage of Mg from within cells, giving an artificially elevated reading. After death, samples of CSF or vitreous humor (fluid within the eye) that test below 1 mg/dL of magnesium are reliable indicators of grass tetany for approximately 24-48 hours.

Treatment: Animals exhibiting grass tetany are in need of immediate veterinary treatment; preferably 1.5 to 2.25 grams of magnesium administered intravenously for an adult cow. Tranquilization by the veterinarian may be needed to reduce the risk of injury to both the animal and the doctor during treatment. 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 concentrations in CSF to return to normal. Many of these cows will relapse and require additional treatment within 12 hours. Administering oral magnesium gel once the animal has regained good swallowing reflexes or drenching with magnesium oxide or magnesium sulfate 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 members of the herd to prevent further cases.

Prevention: Prevention is based on providing a high concentration of soluble magnesium in the rumen during times when conditions for grass tetany exist. 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 from working such as high potassium level in the forage, the second or “backup” pathway is to increase the amount of magnesium in the diet, for example 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 per cent 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 14 per cent  magnesium in the complete mineral mix and all from magnesium oxide (no dolomitic limestone or magnesium mica). At least a third of the magnesium oxide should be in the prilled form to increase palatability. In addition to supplying supplemental magnesium, several management factors may 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.

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