As spring approaches and grass begins to grow, grazing livestock may experience a forage-related problem known as grass tetany, grass staggers, lactation tetany, or hypomagnesemia. Grass tetany is a metabolic disorder caused by reduced magnesium (Mg) levels in the animal’s blood. In cattle, it generally affects older, lactating cows but can also be seen in dry cows, young cows, and in rare cases, growing calves. Symptoms often observed include nervousness, lack of coordination, muscular spasms, staggering, convulsions, coma, milk yield decrease, and death. If you suspect cattle are stricken with grass tetany, a veterinarian should be contacted immediately as early treatment can save animals. 

Young cool-season grasses and small grains are commonly associated with this disorder. Grass tetany occurs most frequently in the spring, but may occur in the fall and winter when these forages start growing rapidly again or when cereal grain forages are grazed High levels of nitrogen (N) and potassium (K) in the soil can increase the risk of grass tetany because they reduce the availability of magnesium to the animal. Farmers should refrain from placing cattle in a field that has been recently fertilized or has resulted in the disease before. Pastures where a significant amount of manure has been applied often have excessive potassium fertility increasing the risk to grass tetany. A farmer can also increase the legume content in his/her pastures with clover or alfalfa since they have higher magnesium levels to compensate for the lack of it in the new lush grass.

Feeding high magnesium or high “mag” mineral supplements is the preferred method to reduce the occurrence of grass tetany. High “mag” mineral mixes are available at most feed stores and contain higher inclusions of magnesium oxide than other complete mineral mixes. Cattle should begin consuming this high “mag” mineral during the late winter months and into early spring when new plant growth is starting. In late spring once temperatures are consistently above 60ºF, a producer can quit feeding the high “mag” mixtures. High mag mineral does not need to be fed year round, but is not problematic if it is. Free-choice high mag mineral should contain 12 to 15% magnesium from magnesium oxide. Cattle need to consume four ounces of the mineral supplement daily. Magnesium oxide is unpalatable, which can result in low mineral intake. Co-product feedstuffs such as dried distillers grains, molasses or a flavoring agent is added to the mineral mix to increase palatability.

If free-choice mineral is not a viable option, producers can also mix their own supplement by adding the appropriate amount of magnesium oxide to another palatable feedstuff, i.e. feeding in or with 1 to 2 lbs. of corn or other by-product that provides 20-25 grams of magnesium. For dairy cows, magnesium oxide can be added to the grain mix to provide an intake of 20 g of magnesium per cow per day. Magnesium oxide may be routinely used as a buffer in these grain mixes for dairy cows, so producers should check with their nutritionist to make sure adequate amounts and proper sources are being used to prevent grass tetany.

Besides magnesium oxide, another source of Mg is magnesium sulfate, which is more palatable than magnesium oxide. The downside to feeding magnesium sulfate is it can be an issue where cattle are consuming high sulfate water or other feedstuffs high in sulfur. Producers that are feeding corn co-products (distiller’s grains or corn gluten feed), adding additional sulfur to the diet in the form of magnesium sulfate, or have high sulfur water could create a sulfur toxicity.

Grass tetany blocks provide magnesium similar to that of a mineral supplement. The major disadvantage of this method is that all the animals may not consume an adequate amount of the block. Multiple blocks should be available with one block per ten cows.

The season for grass tetany will be developing as temperatures rise and grasses begin to grow. To reduce health problems and livestock death to this disorder, it is important to provide a quality high “mag” mineral or supplement containing Magnesium oxide.