What exactly is grass tetany, and how can I best prevent my cattle from contracting it this spring?
Grass tetany—also called grass staggers, wheat pasture poisoning and hypomagnesemia—is a metabolic disease of ruminants associated with grazing lush, green pasture. The condition is caused by low blood concentrations of magnesium, which is a required mineral for cattle. There is no readily available store of magnesium in the body, so the mineral should be consumed daily. When pastures are growing rapidly in the spring, grass may not contain adequate amounts of magnesium to meet requirements, leading to grass tetany syndrome. It tends to be most common in lactating cows and ewes grazing green growing forage in spring, but it can occur in the fall when cool season grasses or cereal grains begin to grow.
High potassium and crude protein concentrations found in rapidly growing forage complicate the issue, because they can both interfere with the absorption of magnesium from the rumen; this can be further intensified by low sodium and phosphorus intakes. Magnesium requirements increase during lactation, so lactating cows have increased risk of developing grass tetany, and the risk increases as milk production increases. Low calcium intake combined with inadequate magnesium intake can result in more severe cases. Stress, storms or other conditions that result in cattle being off feed for 24 to 48 hours may decrease blood magnesium levels and cause the disease to appear in several cows in a herd at one time.
Signs of grass tetany most often include finding dead cattle with evidence they may have struggled. Symptoms in live cattle could include convulsions, weakness, disorientation or aggressive behavior. Testing for the disease can be done by collecting a blood sample in live animals, though care must be taken. Life-threatening convulsions can be caused by simply working the cattle 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.
Immediate treatment with combined calcium and magnesium solutions is called for in cows showing clinical signs. The solution should be given intravenously and slowly while monitoring the heart for slow heart rate or irregular heartbeat; it’s highly recommended that you call your veterinarian for assistance immediately if you suspect grass tetany in your herd.
Prevention is key to successfully managing grass tetany, and achieving increased calcium and magnesium consumption through supplementation is the goal. Daily intake of magnesium is important, as grass tetany can occur within 48 hours when blood magnesium concentrations are too low. This can be accomplished from salt-mineral mixes to molasses-based lick tubs, both of which are available in high-magnesium formulations at your local feed store.
Most sources of magnesium—usually magnesium oxide or other magnesium salts—are not very palatable, meaning that it’s likely necessary to supply the magnesium mixed with a more appetizing feedstuff to ensure adequate intake during the risk period.
This article was written by Rachel Endecott, Ph.D., extension beef specialist, Montana State University, and vetted by Lyle G. McNeal, a livestock specialist in the Department of Animal, Dairy and Veterinary Sciences at Utah State University.