Grass tetany is a metabolic disease of cattle associated with grazing lush, green pasture. The condition is caused by low blood concentrations of magnesium, which is a required mineral for cattle. When pastures are growing rapidly in the spring, grass may not contain adequate amounts of magnesium to meet requirements. High potassium and crude protein concentrations found in rapidly growing forage complicate the grass tetany issue by interfering with the absorption of magnesium from the rumen.

Magnesium requirements increase during lactation, so lactating cows have increased risk of developing grass tetany. The risk increases as milk production increases. Low calcium intake combined with inadequate magnesium intake can result in more severe cases of grass tetany. Stress, storms, or other conditions that result in cattle being off-feed for 24 to 48 hours may decrease blood magnesium levels. Signs of grass tetany include finding dead cattle with evidence they may have struggled. Symptoms in live cattle include convulsions, weakness, disorientation, or aggressive behavior. Testing for grass tetany can be accomplished by carefully collecting a blood sample in live animals. Life-threatening convulsions can be caused by simply running the animals through the chute to collect a sample. Blood concentrations of magnesium return to normal after death, but magnesium concentrations of fluid from the eye or cerebrospinal fluid do not change near death and are good sources for testing for grass tetany in animals found dead.

Prevention of grass tetany is the key to successfully managing this condition. Achieving increased calcium and magnesium consumption through supplementation is the main goal. Daily intake of magnesium is important, as grass tetany can occur within 48 hours when blood magnesium concentrations are too low. Local feed stores have salt-mineral mixes and molasses based lick tubs available in “high-mag” formulations.

Supplying additional magnesium during the early growing season can help ranchers avoid losses from grass tetany. In addition, lush pasture results in a dilution of other nutrients in the forage. As a result, cows may not be able to meet their dry matter intake requirements with green grass alone. For example, a 1400-lb cow consuming 2.5% of her body weight on a dry matter basis would need 35 pounds of dry feed per day. If the forage the cow consumes contains 70% water, she would have to eat about 116 pounds of fresh forage in order to get 35 pounds of dry matter. In conclusion, it might not be time to put away the hay processor or bale feeders just yet.

Information for this article was taken from the Cow Sense Chronicle. The Chronicle was written by Rachel Endecott (former MSU Extension Beef Cattle Specialist). The article is available at http://animalrangeextension.montana.edu/beef/moovingminutes.html.