Grass Tetany in Beef Cows: Signs, Treatment and Prevention

By Dr Duncan Houston

A beef cow that suddenly becomes nervous, stiff or twitchy on lush pasture may be only minutes or hours away from a seizure.

Grass tetany can progress from subtle muscle tremors to blind galloping, collapse, violent paddling and death. Some cattle are simply found dead, with disturbed soil and flattened grass around the carcass providing the only evidence that seizures occurred.

The real concern is not only the low blood magnesium. It is that noise, chasing, transport or rough handling can trigger a fatal convulsion in a cow whose nervous system is already dangerously overactive.

Quick Answer

Grass tetany is an emergency caused by inadequate magnesium reaching the cow’s bloodstream and nervous system. It occurs most often in older, lactating beef cows grazing lush cool-season grass or cereal crops with high potassium and nitrogen concentrations.

Any cow showing nervous behaviour, muscle twitching, stiffness, staggering or seizures needs immediate veterinary treatment with calcium and magnesium. Keep the cow quiet, do not chase her, and begin preventive magnesium supplementation for the entire at-risk group immediately. (Merck Veterinary Manual)

Grass Tetany at a Glance

Question	Practical answer
What is grass tetany?	Acute hypomagnesaemia causing abnormal nerve and muscle activity
Which cattle are most at risk?	Older beef cows during early lactation
Which pastures are highest risk?	Lush cool-season grasses, wheat, oats, rye and other cereal forage
What increases pasture risk?	High potassium and nitrogen, low magnesium or calcium, cold wet weather and poor intake
What is the first sign?	Nervousness, muscle twitching, stiffness or an unsteady gait
What is the most dangerous sign?	Convulsions, collapse or a cow found dead
Can a cow have a high temperature?	Yes, intense muscular activity can raise body temperature
Is it the same as milk fever?	No, although low calcium may occur at the same time
What is the treatment?	Slow parenteral calcium and magnesium with heart monitoring
Can oral mineral treat a seizuring cow?	No
Can the condition recur?	Yes, sometimes within 36 hours
How is it prevented?	Daily magnesium intake throughout the high-risk period

Grass tetany primarily affects lactating cattle because magnesium is continuously lost in milk and the body has very limited readily available magnesium reserves. (Merck Veterinary Manual)

What Is Grass Tetany?

Grass tetany is also called:

• Hypomagnesaemic tetany

• Hypomagnesemia

• Grass staggers

• Lactation tetany

It is an acute metabolic and neurological disorder caused by insufficient available magnesium.

Magnesium is essential for:

• Stabilising nerve membranes

• Controlling muscle contraction

• Maintaining normal heart function

• Supporting energy metabolism

• Regulating calcium and parathyroid hormone activity

When magnesium concentrations fall, nerves become excessively excitable. The cow may develop twitching, exaggerated reactions to sound or touch, muscle spasms and eventually violent seizures.

Clinical disease is associated with low magnesium in the blood and cerebrospinal fluid. Low calcium is also common, particularly in cattle grazing cereal crops, which is why combined calcium-magnesium treatment is normally preferred. (Merck Veterinary Manual)

Is Grass Tetany Simply Caused by Low-Magnesium Grass?

Not always.

Some high-risk pastures contain genuinely low magnesium. In other cases, the grass may contain a seemingly reasonable amount of magnesium, but the cow cannot absorb enough of it.

High concentrations of potassium and nitrogen can impair magnesium absorption from the rumen. Low sodium and phosphorus intake may compound the problem. Rapidly growing pasture can therefore create a functional magnesium deficiency even when a basic forage report does not make the magnesium concentration look catastrophically low. (Merck Veterinary Manual)

This is why the condition is better understood as:

Too little magnesium being absorbed relative to the cow’s immediate requirement.

Why Does Lush Spring Grass Increase the Risk?

Lush pasture is not dangerous simply because it is green.

The problem is the mineral pattern that commonly develops in rapidly growing cool-season forage:

• Magnesium may be marginal or low.

• Calcium may be low.

• Potassium may be high.

• Nitrogen may be high.

• Sodium may be inadequate.

• The forage may contain large amounts of water and relatively little dry matter.

High potassium interferes with the mechanisms used to absorb magnesium across the rumen wall. High nitrogen can further reduce magnesium availability. Cows may therefore be consuming large quantities of attractive forage while failing to absorb enough magnesium to maintain normal nerve function. (Merck Veterinary Manual)

Common High-Risk Forages

Risk is particularly associated with:

• Wheat pasture

• Oat pasture

• Rye

• Triticale

• Bromegrass

• Tall fescue

• Ryegrass

• Other rapidly growing cool-season grasses

• Heavily fertilised improved pasture

• Grass receiving recent manure applications

Grass tetany can also occur in cattle fed silage indoors and in undernourished cows suddenly introduced to green cereal crops. It should not be thought of as an exclusively spring pasture disease. (Merck Veterinary Manual)

How Do Fertiliser and Manure Affect Risk?

Heavy nitrogen or potassium fertilisation can increase pasture growth while producing a mineral balance that interferes with magnesium absorption.

High-risk situations include:

• Potash fertiliser applied shortly before grazing

• Heavy nitrogen fertilisation

• Poultry manure application

• Naturally high-potassium soils

• Pastures with low soil phosphorus

• Rapid growth after rain

• Cool, cloudy or wet growing conditions

A pasture can look spectacular while being metabolically treacherous. Green does not automatically mean balanced.

What Is the Grass Tetany Ratio?

A forage laboratory may calculate the tetany ratio, also called the grass tetany index.

It evaluates potassium relative to calcium and magnesium. A ratio above approximately 2.2 is generally considered to indicate increased risk.

The calculation should be performed using the appropriate mineral-equivalent basis. Simply dividing the raw potassium percentage by the combined raw calcium and magnesium percentages may not produce the laboratory tetany ratio correctly.

The tetany ratio is a useful screening tool, but it is not a diagnosis. A cow’s risk still depends on:

• Stage of lactation

• Age

• Actual forage intake

• Mineral intake

• Weather

• Pasture availability

• Previous disease

• Individual susceptibility

A forage report may also take several days to return. Preventive magnesium should not be delayed when high-risk cows are already grazing lush cereal pasture. (extension.okstate.edu)

Practical Forage Warning Values

Recent extension guidance identifies greater concern when forage contains approximately:

• Magnesium around or below 0.15% of dry matter

• Calcium below 0.4%

• Potassium above 2.5%

These values should be interpreted as warning points rather than absolute guarantees that disease will or will not occur. High nitrogen, low sodium, poor intake and early lactation may materially change the risk. (UNL Beef)

Which Beef Cows Are Most at Risk?

Older Lactating Cows

Mature cows in early lactation are the classic high-risk group.

Their magnesium requirement rises with milk production, while older cows are less efficient at absorbing magnesium and mobilising body reserves. Cows older than approximately six years deserve especially close attention during high-risk grazing. (Merck Veterinary Manual)

High-Milking Cows

Cows producing more milk lose more magnesium through lactation.

This may include:

• High-performing maternal lines

• Dairy-influenced beef crosses

• Cows nursing twins

• Cows in excellent condition entering peak lactation

The largest, heaviest and best-milking cows may be among the first animals affected.

Cows in the First Weeks After Calving

Magnesium demand rises sharply when lactation begins. Colostrum contains a higher magnesium concentration than normal milk, making the transition around calving particularly demanding. (UNL Beef)

Cows With Poor Feed Intake

Risk rises when intake falls because of:

• Cold rain

• Snow or ice

• Transport

• Yard confinement

• Calving difficulty

• Sudden feed changes

• Limited pasture availability

• Social competition

• Illness

• Storms that interrupt grazing

Cows do not have a large readily available magnesium reserve that can safely support them through several days of inadequate intake. (Merck Veterinary Manual)

Cows With Inadequate Sodium

Sodium helps maintain the electrochemical gradient involved in magnesium absorption.

When salt is offered separately from the magnesium mineral, some cattle may satisfy their salt appetite without consuming enough of the less palatable high-magnesium product. Incorporating an appropriate amount of salt into the mineral mixture may improve reliable mineral intake. (UNL Beef)

Cows With Marginal Calcium Status

Grass tetany and hypocalcaemia frequently overlap.

Low calcium may modify the presentation, causing a cow to appear quieter or weaker than the extremely excitable animal usually associated with pure grass tetany. (Merck Veterinary Manual)

Can Grass Tetany Occur Outside Spring?

Yes.

Although the classic situation is a mature cow grazing rapidly growing spring grass, cases may also occur during:

• Autumn pasture regrowth

• A flush of growth after drought-breaking rain

• Winter cereal grazing

• Cold wet weather

• Indoor silage feeding

• Sudden movement onto green crops

• Periods of low dry-matter intake

• Transport or temporary fasting

The risk period should be based on forage growth, mineral balance and cow physiology rather than a fixed date on the calendar. (Merck Veterinary Manual)

What Are the Early Signs of Grass Tetany?

Early signs may be subtle and disappear when the cow is observed from a distance.

Watch for:

• Nervous or unusual behaviour

• Standing apart from the group

• Reduced grazing

• Facial twitching

• Ear twitching

• Muscle tremors over the shoulder or flank

• Frequent blinking

• Stiff walking

• An unsteady gait

• Exaggerated response to noise

• Exaggerated response to touch

• Repeated urination or defecation

• Difficulty rising

• Head held unusually high

• Sudden aggression or unpredictable movement

A cow may progress through these signs over two or three days, but progression can be much faster. Driving, loading, handling or moving the cow may abruptly trigger the convulsive stage. (Merck Veterinary Manual)

What Matters Most Clinically?

The combination that should concern you most is:

• Early lactation

• Lush grass or cereal pasture

• Older cow

• Twitching or nervousness

• Stiffness or incoordination

That cow should be treated as an emergency before she goes down.

What Does Advanced Grass Tetany Look Like?

In the acute form, a cow may:

• Throw her head upwards

• Bellow

• Gallop without apparent awareness

• Crash through fences

• Fall suddenly

• Paddle violently

• Chomp the jaws

• Froth at the mouth

• Develop nystagmus

• Become rigid between seizures

• Convulse repeatedly

• Stop breathing and die

The heart rate may approach 150 beats per minute, and heart sounds can become unusually loud. Body temperature may reach approximately 40.5°C or 105°F because of intense muscular activity rather than infection. (Merck Veterinary Manual)

Can the First Sign Be Sudden Death?

Yes.

Cows are commonly found dead without recognised premonitory illness.

Clues that seizures occurred before death may include:

• Flattened or churned ground

• Soil marks around the head and legs

• Broken vegetation

• Froth around the mouth

• Evidence of paddling

• Traumatic injuries from uncontrolled movement

A sudden death on high-risk pasture should trigger immediate action for the surviving cows. Do not wait for a second death before beginning magnesium supplementation and veterinary investigation. (Merck Veterinary Manual)

How Worried Should You Be?

High Pasture Risk, No Clinical Signs

The cows are:

• Older

• In early lactation

• Grazing lush cereal or cool-season grass

• Not yet receiving reliable magnesium

• Exposed to cold wet weather or recently fertilised pasture

What this means: Clinical disease may develop rapidly even though the cows currently look normal.

What to do: Begin magnesium supplementation immediately, provide hay, measure mineral consumption and inspect cows at least twice daily.

Early Clinical Grass Tetany

The cow has:

• Facial or flank twitching

• Nervousness

• Stiff gait

• Hypersensitivity

• Unusual aggression

• Mild incoordination

• Difficulty rising

What this means: The nervous system is already being affected.

What to do: Treat this as an emergency. Call the veterinarian, reduce stimulation and do not drive the cow through a yard or force her onto a trailer unless specifically directed.

Advanced Grass Tetany

The cow has:

• Blind staggering

• Rapid breathing

• Severe stiffness

• Repeated falling

• Jaw chomping

• Frothing

• Marked tachycardia

• Recumbency

What this means: Seizures and death may occur at any time.

What to do: Immediate parenteral calcium-magnesium treatment is required. Keep people clear of the cow’s legs and head.

Critical Grass Tetany

The cow is:

• Convulsing

• Paddling

• Minimally responsive

• Laterally recumbent

• Struggling to breathe

• Experiencing repeated seizures

• Found dead

What this means: The prognosis is guarded, and the rest of the herd is at immediate risk.

What to do: Obtain emergency veterinary assistance and begin herd-level preventive measures immediately.

Grass Tetany or Milk Fever?

Grass tetany and milk fever can both occur around calving and may coexist.

Feature	Grass tetany	Milk fever
Main deficiency	Magnesium	Calcium
Typical behaviour	Nervous, hypersensitive, excitable	Quiet, weak, dull
Muscle tone	Increased, stiff or spasming	Reduced and flaccid
Early movement	Stiff, twitchy, staggering	Weak, shuffling, unable to rise
Seizures	Common in advanced disease	Uncommon
Heart findings	Rapid heart rate and loud sounds	Weaker heart sounds and poorer pulse
Temperature	May rise from muscular activity	Often normal or low
Main setting	Lush high-potassium pasture	Around calving and early lactation
Main treatment	Magnesium plus calcium	Calcium
Response to treatment	May be slower	Often rapid

The distinction is not always clean. When both calcium and magnesium are low, the clinical picture depends on which abnormality predominates. This is why combined calcium-magnesium treatment is generally used for suspected grass tetany. (Merck Veterinary Manual)

What Else Can Look Like Grass Tetany?

Neurological disease in cattle should never be diagnosed from one sign alone.

Possible condition	Clues that may help distinguish it
Milk fever	Quiet, flaccid cow close to calving, cold ears, reduced gut sounds
Polioencephalomalacia	Cortical blindness, head pressing, star-gazing, opisthotonus
Lead poisoning	Blindness, jaw chomping, salivation, convulsions, access to batteries or old paint
Nervous ketosis	Abnormal licking, bellowing, circling or aggression in an early-lactation cow
Listeriosis	Circling, head tilt, facial weakness and one-sided neurological deficits
Rabies	Behaviour change, drooling, swallowing difficulty, progressive paralysis
Tetanus	Rigid gait, erect ears, third-eyelid prolapse and sustained stiffness
Nitrate or nitrite poisoning	Rapid breathing, weakness, collapse and dark chocolate-brown blood
Toxic plant ingestion	Sudden salivation, abdominal signs, tremors or violent convulsions
Organophosphate toxicity	Salivation, diarrhoea, constricted pupils, tremors and breathing difficulty
Bovine ephemeral fever	Fever, pain, stiffness and recumbency in endemic regions
Cerebral trauma	Asymmetrical signs, visible injury or recent handling accident

Some of these diseases present an additional human safety risk. Do not handle saliva or perform an unprotected oral examination when rabies remains a reasonable possibility. (Merck Veterinary Manual)

When Is This an Emergency?

Every cow showing neurological signs compatible with grass tetany should be treated as an emergency.

Call a veterinarian immediately if a cow has:

• Muscle twitching

• Nervous or aggressive behaviour

• A stiff or unsteady gait

• Hypersensitivity to noise or touch

• Difficulty rising

• Blind staggering

• Frothing

• Jaw chomping

• Seizures

• Collapse

• Severe breathing difficulty

Also seek urgent help when:

• One or more cows have been found dead

• Several cows are twitching or behaving unusually

• A cow does not respond to calcium-magnesium treatment

• A cow goes down again after initially recovering

Once neurological signs are present, waiting to see whether the cow improves after eating mineral is not appropriate.

What To Do Right Now

1. Call the Herd Veterinarian

Report:

• Cow identification

• Age and parity

• Calving date

• Current pasture

• Clinical signs

• Whether seizures have occurred

• Mineral programme

• Forage fertilisation history

• Other affected or dead cattle

• Previous treatment

2. Reduce Noise and Movement

Turn off machinery.

Keep dogs away.

Remove unnecessary people from the area.

Do not:

• Shout

• Chase the cow

• Crowd her

• Force her through a race

• Attempt to walk her to another paddock

• Use an electric prod

• Load her onto a trailer unless specifically advised

A startled cow may enter a blind gallop or convulse. This creates a serious risk to the cow and everyone nearby. (Merck Veterinary Manual)

3. Keep a Safe Distance From a Convulsing Cow

A seizuring cow has no control over her limbs.

Stay clear of:

• The head

• The neck

• The legs

• Fences or solid objects the cow may strike

Do not attempt to restrain the head manually during a convulsion.

4. Do Not Give Oral Mineral to a Down or Convulsing Cow

A cow with impaired swallowing can aspirate powder, liquid or mineral suspension into the lungs.

Clinical grass tetany requires parenteral treatment. Oral magnesium is used after stabilisation and to protect the remainder of the herd.

5. Keep the Cow From Lying Flat Where Safely Possible

Once seizures stop, a recumbent cow should be maintained in sternal position where this can be done without triggering further convulsions.

Sternal positioning reduces:

• Bloat

• Regurgitation

• Aspiration

• Pressure injury

Do not move her unnecessarily while she remains hyperexcitable.

6. Begin Herd-Level Prevention Immediately

Once one case is suspected:

• Provide high-magnesium mineral immediately.

• Supply palatable hay.

• Check every feeder.

• Calculate actual mineral intake.

• Inspect older lactating cows closely.

• Consider moving the group only when this can be done quietly and safely.

• Collect forage and mineral samples.

One case indicates that the pasture and management system may be placing the entire group at risk. (Merck Veterinary Manual)

How Is Grass Tetany Diagnosed?

Clinical History and Examination

A presumptive diagnosis is based on:

• High-risk pasture

• Early lactation

• Older cow

• Typical neurological signs

• Low magnesium intake or absorption

• Response to magnesium treatment

Blood Testing

Where practical, collect blood before treatment without delaying emergency care.

Useful measurements include:

• Total magnesium

• Ionised magnesium where available

• Calcium

• Phosphorus

• Potassium

• Sodium

• Glucose

• Kidney and muscle values

The normal total magnesium reference range for cattle is approximately 1.8 to 2.4 mg/dL. Clinical tetany commonly occurs when plasma magnesium falls below approximately 1.2 mg/dL, although clinical signs and concurrent calcium abnormalities must also be considered. (Merck Veterinary Manual)

Urine Magnesium

Urine magnesium can provide useful information about current magnesium status.

Urine magnesium may become extremely low or undetectable in hypomagnesaemic cows. This can help assess cattle that have not yet been treated. (Merck Veterinary Manual)

Why Is Blood From a Dead Cow Less Reliable?

After death and severe convulsions, magnesium may leak out of damaged muscle cells and increase the measured blood concentration.

A normal postmortem serum magnesium result therefore does not reliably exclude grass tetany.

More useful postmortem samples include:

• Cerebrospinal fluid collected within approximately 12 hours

• Vitreous humour from the eye collected within approximately 24 to 48 hours

Interpretation is most reliable when the carcass has remained below approximately 23°C. (Merck Veterinary Manual)

This is important real-world diagnostic reasoning. The easiest sample to collect is not always the sample most likely to answer the question.

Forage, Mineral and Water Analysis

A herd investigation may include testing for:

• Magnesium

• Calcium

• Potassium

• Sodium

• Phosphorus

• Sulfur

• Nitrogen or crude protein

• Dry matter

• Tetany ratio

Mineral concentration should be reported on a dry-matter basis.

Sample what the cows are actually eating, not merely the prettiest plants available at the field entrance.

How Is Grass Tetany Treated?

Immediate Parenteral Calcium and Magnesium

Clinical grass tetany requires rapid parenteral replacement of magnesium, usually alongside calcium.

Treatment is preferably given:

• Slowly by the intravenous route

• With continuous heart monitoring

• Using a product or combination selected for the cow

• With minimal stimulation

• By a veterinarian or someone trained under a veterinary protocol

The response may be slower than the dramatic response seen after calcium treatment for uncomplicated milk fever. Magnesium must move back into the cerebrospinal fluid before normal nervous-system control is restored. (Merck Veterinary Manual)

Veterinary Treatment Note

An adult cow generally requires approximately 1.5 to 2.25 g of elemental magnesium during emergency treatment.

Merck describes one common regimen using calcium borogluconate combined with magnesium sulfate, administered by slow intravenous injection, with additional subcutaneous magnesium in selected cases. Commercial product concentrations differ substantially, so the volume cannot be transferred safely from one bottle or formulation to another. (Merck Veterinary Manual)

Intravenous calcium-magnesium treatment is not a casual farm injection. Incorrect concentration, rapid administration or poor venous placement can cause cardiac arrhythmia, collapse, tissue injury or death.

Why Is Calcium Included?

Many affected cows are also hypocalcaemic.

Calcium:

• Supports heart contraction

• Improves muscle function

• Helps address concurrent milk fever

• May improve initial stability

Treating with calcium alone, however, may produce incomplete or temporary improvement if magnesium remains critically low.

Why Must the Heart Be Monitored?

Both calcium and magnesium affect cardiac conduction and contraction.

During administration, the veterinarian monitors:

• Heart rate

• Rhythm

• Pulse quality

• Breathing

• Level of consciousness

The infusion may need to be slowed or paused if the heart rhythm changes.

Do Not Stimulate the Cow During Treatment

Avoid:

• Trying to force the cow to stand

• Shouting

• Slapping

• Moving her legs repeatedly

• Driving other cattle past her

• Allowing dogs nearby

Even after treatment begins, stimulation can trigger another fatal seizure. (Merck Veterinary Manual)

What Happens After Treatment?

Leave the Cow Quiet

Allow her nervous system time to recover.

A cow with grass tetany may not jump up immediately. The slower response does not automatically mean the treatment has failed.

Move Her Off the High-Risk Pasture Once Stable

Only move the cow when:

• Seizures have stopped

• She is coordinated enough to walk

• The movement can be calm

• A safer feed source is available

Do not drive a recently recovered cow a long distance.

Begin Oral Magnesium

Following emergency treatment, cows require ongoing oral magnesium.

Merck recommends providing hay treated with approximately 60 g of magnesium oxide per cow daily during the immediate danger period. Without ongoing supplementation, recurrence can occur within approximately 36 hours. (Merck Veterinary Manual)

Magnesium oxide is poorly palatable, so it usually needs to be mixed into:

• Molasses

• Grain or concentrate

• Pellets

• A formulated mineral mixture

• Hay as a top-dressing

Recheck the Cow

Monitor for:

• Recurrent twitching

• Nervousness

• Reduced feed intake

• Stiffness

• Weakness

• Difficulty rising

• Seizures

A relapse requires immediate veterinary reassessment.

What if the Cow Does Not Respond?

Failure to improve may mean:

• The magnesium dose was inadequate.

• Treatment was delayed.

• The cow sustained severe muscle or brain injury.

• Hypocalcaemia remains significant.

• Another electrolyte abnormality is present.

• The diagnosis is incorrect.

• A toxic or infectious disease is occurring.

• The cow has concurrent trauma.

Do not repeatedly administer calcium-magnesium bottles without reassessing the heart and diagnosis.

What Is the Prognosis?

More Favourable Prognosis

The outlook is better when:

• Twitching is recognised before seizures.

• Treatment is given immediately.

• The cow remains standing.

• She responds without repeated convulsions.

• Oral magnesium follows parenteral treatment.

• The group is moved onto safer feed.

Guarded Prognosis

Concern increases when:

• The cow is recumbent.

• Several seizures have occurred.

• Treatment has been delayed.

• Body temperature is markedly increased.

• Severe trauma occurred during a seizure.

• The cow remains unable to eat or stand.

• Another metabolic or toxic disease is present.

Poor Prognosis

The prognosis is poor when:

• Convulsions continue despite treatment.

• The cow is comatose.

• Severe aspiration or bloat develops.

• There is major head, spinal or limb trauma.

• Breathing cannot be maintained.

Some cows die before treatment can be organised, which is why prevention is far more reliable than emergency rescue.

How Should the Rest of the Herd Be Managed After One Case?

One affected cow should be treated as a herd warning.

Inspect the Highest-Risk Cows

Prioritise:

• Older cows

• Cows within the first weeks of lactation

• High-milking cows

• Cows with previous tetany

• Cows with reduced intake

• Cows behaving differently

Provide Magnesium Immediately

Do not wait for forage results.

Use a veterinarian or nutritionist-approved delivery method that provides reliable daily intake.

Offer Dry Hay

Hay can:

• Increase dry-matter intake

• Dilute the mineral imbalance of lush pasture

• Reduce reliance on rapidly growing forage

• Provide a practical carrier for magnesium oxide

Review Pasture and Fertiliser History

Document:

• Nitrogen applications

• Potassium applications

• Manure applications

• Recent rainfall

• Pasture maturity

• Forage availability

• The date cattle entered the field

Monitor at Least Twice Daily

Early signs may be brief.

Observe cattle calmly from a distance before walking through the group.

How Can Grass Tetany Be Prevented?

Begin Magnesium Before the Risk Period

Start high-magnesium supplementation approximately three to four weeks before cattle enter high-risk pasture.

This allows:

• Cows to become accustomed to the bitter product

• Intake problems to be identified

• Feeders to be repositioned

• Mineral delivery to stabilise

Continue throughout the high-risk pasture period and early lactation. (extension.okstate.edu)

Use a Proper High-Magnesium Mineral

Commercial grass-tetany minerals commonly contain approximately 10 to 15% magnesium, usually supplied mainly as magnesium oxide.

A typical target intake is approximately 3 to 4 oz, or 85 to 115 g, of mineral per cow daily, depending on the formulation and the total ration.

The percentage on the label is meaningless unless cows consume the intended amount. (extension.okstate.edu)

Calculate Actual Intake

Use:

Mineral used ÷ number of cows ÷ number of days

For example, if 50 cows consume 35 lb of mineral over seven days:

• 35 lb equals 560 oz

• 560 oz ÷ 50 cows ÷ 7 days

• Average intake equals 1.6 oz per cow daily

That is well below a 3 to 4 oz target.

Do not judge mineral intake by whether the feeder occasionally looks empty. Rain, wildlife, calves and spillage may account for part of the disappearance.

Remember Individual Variation

A herd average can conceal:

• Cows consuming too little

• Dominant cows consuming more

• Newly calved cows avoiding the feeder

• Cows in distant parts of the paddock

• Cows satisfying salt appetite elsewhere

Provide enough feeder space and consider multiple locations.

Improve Palatability

Magnesium oxide is bitter.

Intake may improve by:

• Using a professionally formulated product

• Including salt within the mineral mix

• Adding approved palatable ingredients

• Mixing magnesium through a concentrate

• Using molasses or a suitable feed carrier

• Introducing the mineral before turnout

• Keeping it dry and fresh

University of Nebraska guidance suggests that a small amount of corn or dried distillers grains may be mixed into mineral where intake is poor, provided the final ration remains properly formulated. (UNL Beef)

Do Not Add Unnecessary Potassium

Check the mineral label.

A product intended to prevent grass tetany should not add substantial potassium to a ration already high in potassium.

Provide Salt Appropriately

If magnesium is delivered through fortified feed, free-choice salt may still be needed.

When a free-choice mineral is being used, including the appropriate salt within the mix may reduce the chance that cattle consume plain salt while ignoring the high-magnesium mineral. (UNL Beef)

Use Controlled Feeding When Free-Choice Intake Is Unreliable

Where risk is high and cattle will not consume enough free-choice mineral, consider:

• Magnesium mixed in a daily concentrate

• Magnesium top-dressed onto hay

• Formulated pellets or cubes

• Controlled feeding of a complete mineralised supplement

• Regionally approved slow-release rumen devices

The method should match the production system and local product availability.

Free-choice access is convenient, but convenience is not the same as dose reliability.

Should Cows Receive 60 g of Magnesium Oxide Every Day?

Merck recommends approximately 60 g of magnesium oxide per adult cow daily during the danger period, particularly for immediate prevention following a clinical case. (Merck Veterinary Manual)

Commercial mineral programmes may provide less supplemental magnesium because some magnesium is also supplied by the forage.

This is why there are two different-looking recommendations:

• A high-magnesium mineral containing 10 to 15% magnesium at 3 to 4 oz daily

• A high-risk or outbreak programme delivering around 60 g of magnesium oxide daily

The correct programme depends on:

• Forage magnesium

• Forage potassium

• Cow intake

• Stage of lactation

• Herd history

• Supplement formulation

Have the ration calculated rather than choosing whichever number is easier.

How Can Pasture Management Reduce Risk?

Provide Hay Before and During Turnout

Offering dry hay before cattle enter lush pasture may:

• Prevent rapid gorging

• Increase dry-matter intake

• Improve magnesium delivery

• Dilute the high-potassium forage

• Reduce sudden nutritional change

Avoid Turning Hungry Cows Onto High-Risk Pasture

Hungry cows consume large amounts of lush forage rapidly.

Ensure cows have eaten before turnout and continue providing adequate forage.

Avoid Heavy Potassium and Nitrogen Fertilisation

Use soil and forage testing to guide fertiliser decisions.

Do not apply potassium simply because spring growth is desirable without considering the impact on cow mineral absorption.

Use Legumes Carefully

Legumes generally contain more magnesium and calcium than grasses and can reduce grass-tetany risk when included in a mixed pasture. (Beef Research)

However, lush legumes can increase the risk of frothy bloat.

A mixed pasture needs both:

• Tetany planning

• Bloat planning

Solving one metabolic problem by creating another would be a rather poor trade.

Match Cattle Class to Pasture Risk

Where possible, use lower-risk pasture for:

• Older lactating cows

• Previous tetany cases

• High-milking cows

Higher-risk fields may be better suited to:

• Nonlactating cattle

• Younger cattle

• Steers

• Cows after peak lactation

Stocker cattle can still develop related magnesium problems, but the classic disease is much less common than in mature lactating cows.

Maintain Adequate Forage Availability

Short pasture reduces total dry-matter and magnesium intake.

A cow cannot meet magnesium requirements from pasture she cannot consume in sufficient quantity, even when the forage concentration appears reasonable. (Merck Veterinary Manual)

Protect Intake During Cold Wet Weather

Cold rain, snow and strong wind may reduce grazing while increasing maintenance requirements.

Provide:

• Shelter

• Hay

• Supplemental feed

• Reliable mineral

• Accessible water

Do not assume cattle will compensate later without consequence.

Is Magnesium Fertiliser Enough?

Applying magnesium-containing fertiliser may increase forage magnesium on some soil types, but the response is inconsistent.

It should not replace direct magnesium supplementation in high-risk cows. (Merck Veterinary Manual)

Can Magnesium Be Applied Directly to Pasture?

In some systems, magnesium oxide may be dusted onto forage or magnesium sulfate may be sprayed over pasture.

This approach is vulnerable to:

• Rain

• Uneven application

• Variable pasture intake

• Labour requirements

• Losses before cattle consume the treated forage

Merck notes that heavy rain shortly after dusting may require reapplication. Direct pasture treatment should be planned with a nutritionist or extension adviser rather than used as the only emergency strategy. (Merck Veterinary Manual)

Can Grass Tetany Be Prevented Completely?

Risk can be reduced substantially, but prevention requires consistency.

Failures often occur because:

• Magnesium was offered too late.

• Cows did not eat enough.

• The feeder was empty.

• The mineral became wet.

• Plain salt competed with the mineral.

• The pasture potassium was exceptionally high.

• Hay was removed.

• Cold weather reduced intake.

• One distant group was forgotten.

• The herd average hid individual non-consumers.

The prevention plan must be audited, not merely purchased.

Common Grass Tetany Mistakes

Waiting Until a Cow Is Down

Early twitching or unusual nervousness is already an emergency.

Chasing the Cow to the Yards

Movement and stimulation can trigger fatal seizures.

Treating With Calcium Alone

Concurrent calcium deficiency may be present, but magnesium must be replaced.

Pouring Oral Magnesium Into a Recumbent Cow

This creates a major aspiration risk and cannot correct acute neurological disease quickly enough.

Assuming Offered Mineral Equals Consumed Mineral

Measure actual disappearance and investigate spillage.

Starting Mineral on the Day of Turnout

Introduce high-magnesium mineral several weeks earlier.

Using One Feeder for a Large or Dispersed Group

Some cows may never consume the target amount.

Offering Plain Salt Separately Without Reviewing Intake

Cows may satisfy their salt appetite while avoiding the less palatable magnesium mix.

Relying Only on the Tetany Ratio

Forage testing is useful, but cow age, lactation, intake, weather and supplementation determine actual risk.

Moving the Entire Herd Abruptly After One Case

A rushed movement can trigger another cow. Make changes calmly and provide feed where the cattle already are if immediate movement is unsafe.

Sampling Only Postmortem Blood

Serum magnesium may be misleading after death. Vitreous humour or cerebrospinal fluid is more informative when collected correctly.

Assuming Spring Is the Only Risk Period

Autumn regrowth, winter cereal pasture and sudden dietary changes can also produce disease.

A Practical Grass Tetany Prevention Plan

Four Weeks Before High-Risk Grazing

1. Test the pasture or conserved forage.

2. Review magnesium, calcium, potassium and sodium.

3. Select a high-magnesium mineral.

4. Confirm that the product does not add unnecessary potassium.

5. Introduce the product.

6. Establish baseline mineral intake.

7. Identify older and previously affected cows.

One Week Before Turnout

1. Recalculate mineral intake.

2. Add feeders where access is poor.

3. Correct palatability problems.

4. Arrange hay supply.

5. Review the emergency treatment protocol.

6. Confirm veterinary contact and drug availability.

During the High-Risk Period

1. Maintain uninterrupted mineral access.

2. Calculate intake at least weekly.

3. Observe cows twice daily.

4. Increase checks after rain or cold weather.

5. Keep hay available.

6. Avoid unnecessary transport or fasting.

7. Record all abnormal behaviour.

After a Clinical Case

1. Treat the affected cow immediately.

2. Begin or intensify magnesium for the whole group.

3. Provide hay.

4. Sample forage, mineral and water.

5. Collect appropriate diagnostic samples.

6. Recheck every high-risk cow.

7. Review feeder placement and actual intake.

8. Maintain prevention for the remainder of the danger period.

Frequently Asked Questions

Can Grass Tetany Occur When the Forage Magnesium Level Looks Normal?

Yes. High potassium and nitrogen can impair magnesium absorption even when the forage magnesium concentration appears acceptable. The total mineral pattern, cow intake and stage of lactation matter. (UNL Beef)

How Quickly Can Grass Tetany Kill a Cow?

In acute cases, a cow may progress from apparently normal grazing to convulsions and death within a few hours. Sometimes the first recognised sign is a dead cow. (Merck Veterinary Manual)

How Long Should High-Magnesium Mineral Be Fed?

Begin approximately three to four weeks before high-risk grazing and continue throughout the danger period. Cattle require a reliable daily supply because they do not maintain a large readily available magnesium reserve. (extension.okstate.edu)

Is Grass Tetany the Same as Milk Fever?

No. Grass tetany is primarily caused by low magnesium and usually produces nervousness, stiffness and seizures. Milk fever is caused by low calcium and more commonly produces quiet weakness and flaccid recumbency. Both abnormalities can occur together. (Merck Veterinary Manual)

Can a Cow Recover After Having Seizures?

Yes, but the prognosis becomes more guarded with repeated convulsions, delayed treatment, severe overheating, trauma or prolonged recumbency. Treatment must be immediate and stimulation kept to a minimum.

Final Thoughts

Grass tetany is one of the fastest-moving metabolic emergencies in beef cattle.

The most important points are:

1. Grass tetany is caused by inadequate available magnesium.

2. Mature lactating beef cows are at greatest risk.

3. High potassium and nitrogen can prevent adequate magnesium absorption.

4. Lush cereal crops and cool-season grasses are common high-risk forages.

5. Early signs include twitching, nervousness, stiffness and incoordination.

6. Driving or stressing an affected cow can trigger a fatal seizure.

7. Some cattle are found dead without previously recognised illness.

8. Grass tetany and milk fever may occur together.

9. Clinical cases require emergency parenteral calcium and magnesium.

10. Intravenous treatment must be given slowly with cardiac monitoring.

11. Do not force oral mineral into a down or seizuring cow.

12. Recovery may be slower than after treatment for uncomplicated milk fever.

13. Recurrence may occur within approximately 36 hours without oral magnesium support.

14. One affected cow means the rest of the group is at risk.

15. Magnesium must be consumed daily throughout the danger period.

16. High-magnesium mineral is ineffective when cattle do not eat the target amount.

17. Begin supplementation several weeks before high-risk turnout.

18. Forage testing, hay, feeder management and fertiliser planning all contribute to prevention.

The mistake that causes the most harm is trying to move the cow before treating her. With grass tetany, quiet is not merely good stockmanship. It can be the difference between recovery and a fatal convulsion.

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ASK A VET™ can help organise forage analyses, mineral labels, pasture history, intake records and high-risk cow lists while producers work directly with their attending livestock veterinarian and nutritionist. A cow showing twitching, staggering, aggression, seizures, collapse or breathing difficulty still requires immediate hands-on veterinary treatment.