Grass tetany is a metabolic disorder caused by a magnesium (Mg) deficiency in blood serum. The most common names are grass tetany, grass staggers, or hypomagnesemia. The disorder is also known as winter tetany, lactation tetany, hypomagnesemic tetany, or wheat pasture poisoning.
As the name implies, it is most likely to occur after animals are turned out to pasture. The problem is usually confined to lactating cows, although any cattle can develop the disorder.
Grass tetany is apt to appear under conditions of nutritional stress. Placing cattle on winter pasture directly from frosted or other low quality pasture may cause such a nutritional stress.
In Florida, grass tetany is more severe when cattle are grazing young forage–small grain or ryegrass pastures–particularly the first flush of growth during December and January. Rapidly growing lush pastures are the most dangerous. Once the forage becomes more mature, the likelihood of problems is reduced.
The disorder is more common during cool, cloudy, and rainy weather and often occurs when cool weather is followed by a warm period. Animals–particularly lactating cows–get grass tetany most often when grazing cool-season grasses or small grain pastures in spring and fall.
Types of Grasses
Grass tetany has occurred on perennial ryegrass, crested wheatgrass, and all small grain pastures such as oats, wheat and rye, but It is not common on legume pastures. It occurs most frequently on pastures grown on soils low in available magnesium and high in available potassium. Heavy applications of broiler house litter or other high-nitrogen and potassium manures may increase the hazard of grass tetany.
Forages containing less than 0.2% magnesium and more than 3% potassium and 4% nitrogen (25% protein) are likely to cause grass tetany under the right conditions. Forages that are high in potassium and nitrogen should also contain at least 0.25% magnesium on a dry matter basis.
Grass tetany quickly leads to death if not treated. Many cows demonstrate clinical symptoms almost indistinguishable from milk fever or ketosis. When the disease is suspected, immediately call a veterinarian to diagnose and initiate treatment.
The complete sequence of visible symptoms often takes place in only six to ten hours. Symptoms occur in the following order:
- excitement and nervousness
- lack of coordination
- muscle twitching
- grinding of teeth
- frequent urination
- staggering and falling
- labored breathing
- tetanic contraction of muscles
In beef herds, the herdsman does not always have the opportunity to observe the signs of the disease, and affected cattle may be found dead in the pasture.
Treatment of hypomagnesemia can be successful if given early and without excessive handling of the affected animals. Several commercial preparations are available. Contact your veterinarian or county Extension agent for more information.
USDA leaflet No. 561 reported that 200 ml of a fifty percent solution of magnesium sulfate injected under the skin increased the level of magnesium in the blood in fifteen minutes. The USDA recommends that an intravenous injection of chloral hydrate or magnesium sulfate should be used to calm excited animals before treating them with a calcium-magnesium gluconate solution. Such intravenous injections must be administered slowly. If given too rapidly, a danger of heart failure is always possible.
Heart and respiratory rates should be closely monitored. If they increase rapidly, treatment should be suspended or continued at a slower rate. Failure to slow or suspend treatment may end in death due to cardiac failure.
Immediately remove animals from pastures where problems are encountered, and provide them with rations containing higher levels of magnesium.
Check the calcium (C) to phosphorus (P) ratio, and energy intake of the animals. Ideally, cattle should have a 2:1 calcium to phosphorus ratio and at least maintenance energy intake. Grass tetany may be less likely to occur when these factors are near optimum.
Factors associated with this disease include low levels of magnesium and high protein and potassium levels in the forage. Several preventive methods are available.
Use dolomitic limestone–which contains magnesium–to increase forage magnesium levels if the level of soil magnesium is low and a soil pH increase is needed. If no lime is needed on high-pH level soils, magnesium oxide (MgO) can be included with fertilizer materials. Applying magnesium fertilizer to the soil can help increase the level of magnesium in the plants.
Excess nitrogen and potassium fertilization tends to reduce the magnesium level in most forage plants. Consequently, these fertilizer elements should not be applied in excess on temporary winter pastures. Follow recommendations based on soil test results.
The likelihood of grass tetany may be reduced by feeding cattle supplemental magnesium, such as a meal mixture consisting of magnesium oxide, salt, dry molasses, and cottonseed. Supplementation should begin a few weeks before the cattle are placed on questionable pastures.
Commercial mineral mixtures containing ten to fifteen percent magnesium are available for feeding during periods of increased grass tetany probability. Each cow needs to consume six to twelve ounces of this mineral per day. When grazing such pastures, dairymen using a supplemental grain feeding program are encouraged to add five to ten pounds of magnesium oxide per ton.
Place mineral feeders placed at convenient locations around the pasture. If mineral consumption is low, move mineral feeders closer to watering and resting areas.
Compensation for Previous Problems
In herds that have had previous grass tetany problems, increase the supplementation to ½ – 1 oz of magnesium oxide per cow per day, beginning two weeks before grazing winter pasture or the start of calving until winter pastures are more mature and the grass tetany risk is reduced. The magnesium oxide may be included in grain mixtures or magnesium fed in the mineral feeders.
In herds where there are clinical cases of grass tetany, increase the magnesium intake to 1 – 2 oz per head daily, and continue this amount until the high-risk pasture grazing period is past.
Remove animals from pasture, or allow access to hay or dry pasture. Limit grazing during periods of rapid growth. Additionally, limit grazing of the temporary winter pastures when moving cattle directly from poor quality frosted grass pastures, since a rapid change in feed can cause digestive upsets and nutritional stress.
Adapted and excerpted from:
Y.C. Newman, M.J. Hersom, and W. E. Kunkle, “Grass Tetany in Cattle” (SS-AGR-64), UF/IFAS Agronomy Department (rev. 07/2013).
B. Harris and J. K. Shearer, “Nitrate, Prussic Acid (HCN) and Grass Tetany Problems in Cattle Feeding” (DS6), UF/IFAS Animal Science Department (rev. 6/2003).