Grass Tetany in Grazing Animals

— Written By and last updated by Patricia Burch

With warming temperatures and the flush of new forage, livestock producers are no doubt feeding less hay and turning animals back onto pastures. During this time of rapid forage growth the plants are not able to take up nutrients from the soil at the same rate. Animals grazing these pastures are at risk of grass tetany. Furthermore, forages subjected to abundant moisture and poorly drained soils are more likely to increase the incidences of tetany.

Grass tetany is caused by a low level of magnesium (Mg) in the blood stream of affected animals. It is commonly seen in early lactation cattle, sheep, and goats grazing cool season forages such as fescue, ryegrass, and small grains. Since magnesium is a component of milk, the highest producing, heaviest milking animals in the herd are usually the ones affected. Animals in other stages of production, suck as growing animals, dry females, and females with older offspring, can be affected as well.

Most cases occur in the early spring but grass tetany can strike in the middle of winter or summer on farms where soil fertility is high. This is especially true where poultry litter has been used to fertilize cool season forages. High levels of nitrogen and potassium tend to inhibit forage magnesium uptake. Small grain forages (rye, wheat, oats, and triticale) are probably the most dangerous plants in their probability of causing tetany. It is interesting to note that animals grazing legume or grass/legume mixtures seldom develop grass tetany.

The first sign of a possible tetany problem will usually be a dead cow that was healthy when last checked. Symptoms of magnesium deficiency include nervousness, staring eyes, staggering gait, grinding teeth, and elevated respiration and heart rates. As the condition progresses, the animal goes down with muscle spasms and convulsions. If not treated, the animal will die.

Treatment is possible by getting magnesium into the blood stream. The best “treatment” is prevention. This is done by supplementing the herd with magnesium oxide during risk periods. Magnesium oxide is the most commonly used supplement due to good bioavailabilty, economics, and high magnesium (50%) content. Magnesium from dolomitic limestone has poor bioavailability and impedes carbohydrate digestion and, therefore, should be avoided. Magnesium oxide may be provided in a home mix consisting of 30% magnesium oxide, 30% trace mineral salt, 30% dicalcium phosphate, and 10% dried molasses. It can also be provided in a commercial high-mag (10%-14% Mg) mineral mix. For convenience and to make sure they have extra magnesium out when needed, some producers choose to offer this high-mag mineral year round.

Daily intake is crucial because there is little or no storage of magnesium by older animals. Most magnesium provided to the animal in feed is lost within 20-30 hours of ingestion. Mineral supplements should be located in cattle lounging areas so that intake is adequate. Do not place minerals too close to the water supply as this will lead to overconsumption. When using a high-mag mineral mix, monitor intake to insure animals are consuming the proper amount, usually 3 to 4 ounces per day for cattle. If intake is above or below target levels, relocate the supplement accordingly. If intake is extremely high, some companies offer a mix with higher salt levels to reduce consumption but will still meet the animal’s needs. The mineral mix should be the only source of salt available to the animals as well.