Losing a healthy cow overnight is something no cattle producer forgets. Grass tetany in cattle is one of the few conditions that can take down a seemingly normal animal between morning chores and evening feeding. I have talked with ranchers who walked a pasture at dawn and found a cow they had just checked the night before, dead with no warning beyond a few drag marks in the mud.
The condition is fast, it is unforgiving, and it targets some of your most productive animals. The good news is that grass tetany is also one of the most preventable metabolic disorders in a beef herd, provided you understand what triggers it and act before symptoms appear.
In this guide, I will walk you through exactly what grass tetany is, why it happens, how to spot the earliest signs, what to do in an emergency, and the management steps that keep your herd safe. Everything here comes from university extension research, veterinary sources like the Merck Veterinary Manual, and real-world experience shared by working cattlemen.
Whether you run five cows or five hundred, the information below can save animals this spring.
Table of Contents
What Is Grass Tetany in Cattle?
Grass tetany, also called grass staggers or hypomagnesemia, is a rapidly progressing metabolic disorder caused by abnormally low levels of magnesium in the blood. The condition almost always strikes lactating cows grazing lush, rapidly growing pasture, and it can become fatal within hours of the first visible symptom.
To understand why this happens, it helps to know how cattle handle magnesium differently from other minerals. Cattle do not store magnesium in their bones the way they store calcium or phosphorus. Instead, they rely on a steady daily intake from feed to maintain the magnesium their nervous system needs. When that intake drops or absorption is blocked, blood magnesium falls fast.
Magnesium plays a direct role in nerve impulse transmission and muscle contraction. When blood magnesium drops below a critical threshold, the nervous system becomes hyperexcitable. That overstimulation is what produces the staggering gait, muscle tremors, and violent seizures that give the disease its “staggers” nickname.
Normal blood magnesium in a healthy cow runs around 1.8 to 3.0 mg per deciliter. Grass tetany typically appears when that level falls below 1.2 mg per deciliter. The drop can happen in as little as 24 to 48 hours when a cow transitions from dry hay to lush spring grass.
This is not a rare or exotic condition. University extension services across the country, including Missouri, Penn State, and Oklahoma State, report losses every spring in beef herds that were not supplemented with magnesium. The risk window is narrow, but it catches producers off guard year after year.
What Causes Grass Tetany in Cattle?
The short answer is low blood magnesium. The real story is more complicated, because several overlapping factors combine to create the perfect conditions for a tetany outbreak. Understanding these causes is what separates producers who prevent losses from those who experience them.
Young, fast-growing cool-season grasses like fescue, ryegrass, and small grains are naturally low in magnesium. When these grasses explode with growth in early spring, their magnesium content has not caught up to their overall mass. A cow eating her fill of this forage may consume large volumes of feed but still take in too little magnesium to meet her needs.
Lactation adds enormous pressure. A beef cow producing milk at the peak of her lactation can lose 2 to 3 grams of magnesium per day through milk alone. If her dietary intake barely covers maintenance, milk production drains her reserves and blood magnesium crashes.
But low magnesium in the grass is only part of the problem. Two other minerals, potassium and sodium, play a major role in whether the cow can actually absorb the magnesium she does eat.
Potassium: The Hidden Culprit Behind Spring Losses
Potassium is the mineral most producers overlook when troubleshooting grass tetany. High potassium levels in forage directly interfere with magnesium absorption in the rumen. When a cow eats grass with elevated potassium, her rumen wall transports less magnesium into the bloodstream, even if the grass contains adequate amounts.
Why is spring grass so high in potassium? Several factors converge at once. Cool-season grasses take up potassium aggressively during rapid spring growth. Fields fertilized with potash or manure carry even higher potassium loads. And the wet, cool conditions of early spring promote potassium uptake from the soil at rates that outpace magnesium uptake.
Forage tests on spring pasture frequently show potassium levels above 3 percent on a dry matter basis. When potassium exceeds 2.5 to 3 percent, magnesium absorption in the cow drops sharply. This is why grass tetany can strike even when soil magnesium appears adequate and even when cows have access to mineral supplements.
The lesson here is simple but often missed: the potassium-to-magnesium ratio in your forage matters more than the absolute magnesium level. If you test your forage, look at both numbers.
Sodium’s Overlooked Role in Magnesium Absorption
Of all the factors behind grass tetany, sodium may be the least understood by cattle producers. University of Missouri Extension researchers have highlighted that adequate sodium in the diet actually helps the rumen absorb magnesium more efficiently. When sodium is low, magnesium absorption drops.
Spring grass is typically very low in sodium. Cattle on lush pasture without access to a salt source can develop a sodium deficiency within days. That sodium shortfall compounds the magnesium problem, because now the cow absorbs even less of an already limited magnesium supply.
This is why loose salt or salt-based mineral mixes are so important during the tetany season. Providing adequate sodium does not just satisfy a general mineral need, it directly supports magnesium uptake at the rumen level. It is one of the cheapest and most effective tools in your prevention toolkit.
Seasonal Timing and High-Risk Cattle Categories
Grass tetany is overwhelmingly a spring disease, though it is not limited to spring. The classic outbreak pattern follows the first flush of cool-season grass growth, typically February through April in the southern United States and March through May in the northern plains and Midwest.
Not every cow faces the same risk. The animals most vulnerable to grass tetany are older, lactating beef cows, especially those over 6 years of age. Older cows absorb magnesium less efficiently than younger animals. Combine that with the demands of peak lactation and the transition to lush pasture, and you have a high-risk profile.
Dairy cattle transitioning from a grain and hay ration to lush pasture are also at significant risk. Beef cows calving in late winter or early spring are at peak danger because their lactation curve overlaps exactly with the spring grass flush.
A less common but still dangerous variant is winter tetany. This form occurs when cattle eat winter annuals like wheat or rye pasture, which share the same low-magnesium, high-potassium profile as spring cool-season grasses. The same prevention principles apply.
Older dry cows and bulls are rarely affected. Calves under 4 months old are generally protected because they get magnesium from milk, though calves fed exclusively on milk replacer can develop a different form of hypomagnesemia.
Recognizing the Symptoms of Grass Tetany
The challenge with grass tetany is that symptoms escalate rapidly and the window for intervention is short. A cow that looks slightly off in the morning can be down and seizing by afternoon. Knowing the progression helps you act in time.
Early Warning Signs
The earliest signs are subtle and easy to miss if you are not looking for them. The cow may appear nervous, restless, or unusually alert. She may stop grazing and separate herself from the herd. Producers often describe the cow as “watchy” or “jumpy.”
Physical signs at this stage include muscle twitching, particularly around the face and flanks. The ears may flick erratically. Some cows begin to chew or grind their teeth. You might notice a stiff or cautious gait, as if the cow is walking on uncertain footing.
Excessive salivation is another early indicator. If you approach the cow, she may overreact to your presence, bolting or balking at movements that normally would not bother her.
Advanced Symptoms
As magnesium drops further, the nervous system loses control of muscle function. The classic staggering gait emerges. The cow walks with an uncoordinated, weaving motion and may stumble or fall. Muscle tremors become visible across the shoulders and hindquarters.
The cow becomes increasingly excitable and may react violently to sudden sounds or touch. Teeth grinding intensifies. The animal may throw her head, paddle her legs while standing, or lurch forward uncontrollably.
In the next stage, the cow goes down and cannot rise. She may lie on her side with her legs paddling in a running motion. Full-body seizures develop. The animal may bellow, thrash violently, and chew involuntantly. Body temperature rises from the muscle activity and seizure activity.
At this point, death is imminent without immediate treatment. The cow may progress from staggering to dead in as little as 2 to 6 hours.
How Quickly Does Grass Tetany Progress?
The speed of progression is what makes grass tetany so deadly. From the first visible symptom to death, the entire course can unfold in 6 to 12 hours. Some cases progress even faster. I have read accounts from forum members on Cattle forums who checked cows at 6 AM, saw nothing unusual, and found a dead animal by 10 AM.
This rapid timeline is why prevention matters more than treatment. By the time you see symptoms, you are already behind. The best strategy is to assume risk is present during the tetany season and supplement before symptoms ever appear.
Grass Tetany vs. Milk Fever vs. Ketosis
One of the biggest challenges forum users report is telling grass tetany apart from two other metabolic disorders that strike cows around calving: milk fever and ketosis. All three can cause a cow to go down, but the causes and treatments are very different.
Milk fever, or hypocalcemia, is caused by low blood calcium rather than low magnesium. It typically strikes within 48 hours of calving. The cow goes down quietly and seems depressed rather than excited. Her muscles are flaccid, her head is turned back along her flank, and she feels cold to the touch. Treatment requires intravenous calcium, not magnesium.
Grass tetany is the opposite in presentation. The cow is hyperexcitable, not depressed. She twitches, staggers, and reacts violently. Her muscles are rigid, not flaccid. Treatment requires intravenous magnesium, often combined with calcium because the two conditions can overlap.
Ketosis develops over days or weeks rather than hours. The cow loses appetite, loses body condition, and becomes lethargic. Her breath may smell sweet or like acetone. Ketosis responds to energy supplementation and glucose therapy, not mineral injection.
If you see a lactating cow that suddenly appears nervous, staggering, or seizing during spring grazing, assume grass tetany first. The speed of onset is the biggest clue. Milk fever and ketosis develop more slowly, while grass tetany can kill within hours.
When in doubt, call your veterinarian immediately. Giving the wrong treatment wastes the narrow window you have to save the animal.
Emergency Treatment and Response
If you suspect grass tetany, you are racing against the clock. Treatment must begin immediately, and every minute counts. The goal is to raise blood magnesium fast enough to stop seizure activity before brain damage or exhaustion kills the animal.
What to Give a Cow with Grass Tetany
The standard treatment is an intravenous injection of a magnesium solution, most commonly magnesium sulfate. Because grass tetany and milk fever often occur together, veterinarians frequently use a combined solution of calcium borogluconate and magnesium. These solutions come in commercial IV bags designed for cattle and are available from veterinary supply companies.
Intravenous magnesium must be administered slowly, over 5 to 10 minutes, because rapid injection can cause fatal heart arrhythmias. This is critical. Pouring the solution in too fast can kill the cow you are trying to save.
After the IV injection, your veterinarian will typically follow up with a subcutaneous injection of a magnesium solution. This slower-release deposit keeps blood magnesium elevated for several hours while the cow begins to recover normal intake.
Some producers keep oral magnesium gels or pastes on hand for early-stage cases where the cow is still standing and can swallow. These are not a substitute for IV treatment in a down cow, but they can stabilize an animal showing early signs while you wait for the vet.
When to Call the Vet and What to Do First
Call your veterinarian the moment you suspect grass tetany. Do not wait to see if symptoms worsen, because they will. The timeline from staggering to death can be just a few hours, and IV treatment requires professional skill to administer safely.
While you wait for the vet, handle the affected cow as quietly and gently as possible. Any excitement, loud noise, or physical stimulation can trigger a seizure in a cow already on the edge. Move other cattle away, keep dogs and vehicles at a distance, and avoid sudden movements.
If the cow is down and seizuring, do not try to restrain her forcefully. Protect her from injury by removing nearby obstacles, but let the seizure run its course. Trying to hold a seizing 1,200-pound cow is dangerous for both of you.
One important note from forum discussions: many rural areas do not have a veterinarian available at all hours. If you operate in a remote area, talk to your vet before tetany season about keeping emergency treatment supplies on hand and learning proper IV administration. Some producers in isolated areas learn to administer calcium-magnesium solutions themselves as a last resort.
After treatment, keep the recovered cow on a magnesium supplement and dry hay for several days before returning her to lush pasture. A cow that has had grass tetany once is at high risk of recurrence.
How to Prevent Grass Tetany?
Prevention is far easier and cheaper than treatment, and it is the only reliable way to protect your herd during tetany season. The strategy comes down to three pillars: magnesium supplementation, adequate sodium intake, and smart pasture management.
Magnesium Supplementation That Cows Actually Eat
The single most effective prevention tool is offering a high-magnesium mineral mix starting two to three weeks before the risk period begins and continuing until grass growth slows. Most extension services recommend a mineral mix containing 12 to 15 percent magnesium, provided as magnesium oxide.
Target intake is about 1 to 2 ounces per cow per day, delivering roughly 1 to 2 grams of magnesium. A 50-pound bag of high-mag mineral typically lasts a 30-cow herd about 2 to 3 weeks at proper intake rates.
The problem producers run into is palatability. Magnesium oxide is bitter, and cows will avoid it if given the chance. I have seen forum posts from ranchers who set out high-mag mineral only to find cows refusing to touch it. There are several ways to solve this problem.
Mix the magnesium oxide with something cattle find palatable. Many producers blend it with salt, which cattle will seek out naturally. Adding dried molasses improves taste and drives consumption. Some commercial high-mag mineral products already include flavoring and intake enhancers, which takes the guesswork out of mixing.
Monitor intake closely. Check mineral feeders every few days and track consumption per cow. If intake is too low, adjust the mix. If intake is too high, add more salt to slow consumption. The goal is consistent, steady intake across the herd, not a few cows overeating while others get nothing.
Provide mineral in enough feeders for the whole herd. Dominant cows will crowd a single feeder and prevent timid animals from getting their share. One feeder per 20 to 30 cows is a good starting point.
Pasture, Soil, and Herd Management
Beyond supplementation, several management practices reduce the risk of grass tetany. These address the root causes rather than just supplementing around them.
Test your forage and soil. A forage test tells you the actual potassium-to-magnesium ratio your cows are eating, which is more useful than soil tests alone. If potassium is very high, consider adjusting your fertilizer program. Avoid applying potash in the weeks before spring turnout, since it drives potassium levels even higher in the grass.
Delay turnout on lush spring pasture when possible. Waiting until grass is 6 to 8 inches tall allows magnesium levels to catch up somewhat with overall growth. Feeding dry hay for an hour or two before turning cows onto spring pasture also dilutes the low-magnesium intake with some hay-based magnesium.
Consider planting legumes in your pasture mix. Legumes like clover and alfalfa contain significantly more magnesium than grasses. A pasture with 30 percent or more legumes carries a much lower tetany risk than a pure grass stand.
Feed loose salt alongside your mineral program. Remember that sodium supports magnesium absorption at the rumen level. Providing loose salt, not just salt blocks, makes it easy for cows to consume enough sodium during the critical spring window.
Identify and monitor your highest-risk animals. Older lactating cows deserve extra attention during tetany season. Check them daily for early signs. A cow that separates from the herd, acts nervous, or shows any gait abnormality during spring grazing warrants immediate concern.
Frequently Asked Questions
What are the first signs of grass tetany in cattle?
The first signs of grass tetany include nervousness, restlessness, and a watchful or jumpy demeanor. The cow may stop grazing, separate from the herd, and show muscle twitching around the face and flanks. Excessive salivation, teeth grinding, and a stiff or uncoordinated gait are also early warning signs.
What do you give a cow with grass tetany?
A cow with grass tetany needs an immediate intravenous injection of magnesium sulfate, often combined with calcium borogluconate since milk fever can occur simultaneously. The IV solution must be administered slowly over 5 to 10 minutes. A follow-up subcutaneous magnesium injection helps maintain blood levels during recovery. Always call a veterinarian immediately.
How long can a cow live with grass tetany?
Without treatment, a cow showing clinical signs of grass tetany may die within 6 to 12 hours. Some cases progress even faster, from first symptoms to death in as little as 2 to 4 hours. This rapid timeline is why prevention through magnesium supplementation is far more effective than waiting to treat.
What time of year is grass tetany most common?
Grass tetany is most common in early spring when cool-season grasses like fescue and ryegrass produce rapid, lush growth. The risk window runs from February through April in southern states and March through May in northern regions. A similar condition called winter tetany can occur on wheat or rye pasture during winter months.
Final Thoughts on Grass Tetany
Grass tetany in cattle is one of the most preventable causes of sudden death in a beef herd, yet it claims animals every single spring. The pattern is always the same: lush grass, high potassium, low magnesium, low sodium, and older lactating cows that cannot absorb what little magnesium is available.
The prevention formula is straightforward. Start a high-magnesium mineral program two to three weeks before spring turnout. Provide loose salt to support magnesium absorption. Monitor your highest-risk cows daily during the danger window. Test your forage so you know the potassium-to-magnesium ratio your herd is actually eating.
Treatment is a last resort, not a plan. By the time symptoms appear, you have hours, not days, to act. Build your prevention strategy now, before the grass starts growing. Your cows depend on the decisions you make before spring arrives.