Hypomagnesemic Tetany (Grass Tetany) in Ox: Seizures, Stiffness, and Sudden Death Risk

Hypomagnesemic tetany, often called grass tetany, grass staggers, or lactation tetany, is an acute neurologic and metabolic emergency caused by dangerously low magnesium in the blood. In cattle, it is most often seen in adult lactating animals grazing lush pasture or green cereal crops, but it can also happen in cattle fed low-magnesium forage indoors.

Magnesium is essential for normal nerve and muscle function. When blood magnesium drops too low, the nervous system becomes overreactive. That can lead to twitching, stiffness, startling easily, collapse, paddling seizures, breathing distress, and sudden death. Some cattle are found dead before anyone notices earlier signs.

This condition moves fast. A cow may seem normal, then become excitable, stiff, and uncoordinated, and progress to convulsions within a short time. Because the body does not keep a large readily available magnesium reserve, at-risk cattle need a dependable daily magnesium intake during danger periods.

For pet parents and producers, the most important point is that grass tetany is both an individual emergency and a herd-management problem. One sick animal often means others on the same pasture may also be at risk, so your vet may recommend both immediate treatment and rapid prevention steps for the rest of the group.

Grass tetany happens when magnesium absorbed from the diet cannot meet the animal's needs. Adult lactating cattle are the classic high-risk group because they lose magnesium in milk and cannot quickly mobilize enough from body stores. Risk rises when they graze short, lush, rapidly growing pasture with low magnesium content, especially in cool spring conditions.

Pasture chemistry matters too. High potassium and high nitrogen in forage can reduce magnesium absorption from the rumen. Low sodium and low phosphorus can add to the problem. Fields heavily fertilized with potash or nitrogen, or soils with mineral imbalance, may increase risk. Low forage availability can also contribute because cattle eat less total magnesium.

Stressors can tip a borderline animal into crisis. Inclement weather, transport, fatigue, calving, early lactation, and sudden diet changes may all reduce intake or increase demand. Some cattle also have concurrent low calcium, which can make clinical signs more likely and more severe.

In practical terms, grass tetany is usually not caused by one single mistake. It is more often the result of animal factors like age and lactation, forage factors like lush low-magnesium grass, and management factors like inadequate daily magnesium intake during a known risk period.

Your vet will usually start with a presumptive diagnosis based on the history and exam findings. Important clues include sudden neurologic signs in a grazing adult cow or ox, recent turnout onto lush pasture or green cereal crops, frequent urination, stiffness, hypersensitivity, seizures, or sudden death in the group.

Because this disease can kill quickly, treatment often starts before every test result is back. A strong improvement after magnesium treatment supports the diagnosis. Your vet may collect blood before treatment to check serum magnesium and calcium. In cattle, total serum magnesium below the normal reference range supports the diagnosis, and tetany often occurs when plasma magnesium is very low.

Urine magnesium can also help. In affected cattle, urine magnesium may be very low or undetectable. If an animal dies, postmortem testing may include cerebrospinal fluid or vitreous humor from the eye, because blood magnesium after death can be misleading.

Your vet may also consider other emergencies that can look similar, such as lead toxicity, polioencephalomalacia, rabies, severe hypocalcemia, toxic plant exposure, or other causes of seizures and sudden death. That is why a calm exam, herd history, pasture review, and targeted testing all matter.

Prevention depends on one key principle: at-risk cattle need dependable daily magnesium intake during danger periods. Because the body has limited readily available magnesium reserves, cattle cannot safely "catch up" after missing intake for a day or two. Your vet may recommend high-magnesium loose mineral, top-dressed magnesium oxide, fortified feed, or another practical delivery method for your herd.

Pasture management also matters. Risk is often highest in early spring on lush, rapidly growing cool-season grass or green cereal crops. Feeding hay, avoiding turnout onto very short hungry pasture, and reducing sudden diet changes can help. Cornell guidance also notes that pasture mixes with more legumes can lower risk, and that supplementation may need to begin several weeks before calving in high-risk herds.

Herd selection and grouping can reduce exposure. Older lactating cows, especially around peak milk production, are more susceptible than steers, heifers, and many dry cows. If possible, your vet may suggest grazing less susceptible animals on the highest-risk fields and managing known repeat cases separately.

Longer term, ask your vet and nutrition team about forage and soil testing, fertilizer strategy, mineral palatability, feeder access, and competition within the group. Prevention fails when cattle do not consistently consume the supplement. A practical, well-monitored plan usually works better than a perfect plan that animals will not eat.