Grass Tetany in Beef Cows: Signs, Emergency 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 cows are simply found dead, with churned soil and flattened grass around the body providing the only evidence that convulsions occurred.

What matters most is recognising the early neurological changes and avoiding unnecessary stimulation. Chasing, loading or aggressively restraining an affected cow can trigger a fatal seizure. (Merck Veterinary Manual)

Quick Answer

Grass tetany is an acute neurological emergency caused by inadequate available magnesium, often combined with low calcium. It most commonly affects older beef cows in early lactation while grazing lush cool-season grass or cereal crops.

A cow showing twitching, nervousness, stiffness, staggering or seizures needs immediate veterinary treatment with parenteral magnesium and calcium. Keep her quiet, do not force her to walk, and begin preventive magnesium supplementation for the rest of the at-risk herd 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, lactating beef cows
Which pastures carry the greatest risk?	Lush cool-season grasses, wheat, oats, rye and other green cereal crops
What increases the risk?	High potassium and nitrogen, low magnesium or calcium, low feed intake and inadequate sodium
What are the earliest signs?	Nervousness, muscle twitching, stiffness and an unsteady gait
What is the most dangerous sign?	Convulsions, collapse or sudden death
Can body temperature rise?	Yes, intense muscular activity may raise it to approximately 40.5°C
Is it the same as milk fever?	No, although calcium and magnesium may both be low
What is the emergency treatment?	Parenteral calcium and magnesium, preferably given slowly IV with cardiac monitoring
Can oral mineral treat a seizuring cow?	No
Can it recur after treatment?	Yes, particularly without continued oral magnesium
How is it prevented?	Reliable daily magnesium intake throughout the risk period

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

What Is Grass Tetany?

Grass tetany is also called:

Hypomagnesaemic tetany
Hypomagnesaemia
Grass staggers
Lactation tetany

It develops when the magnesium absorbed from the digestive tract cannot meet the cow’s maintenance and milk-production requirements.

Magnesium is essential for normal nerve stability, muscle function, cardiac activity and energy metabolism. As magnesium concentrations fall, nerves become increasingly excitable. The cow may develop muscle twitching, exaggerated reactions to sound or touch, stiffness, spasms and eventually violent seizures. (Merck Veterinary Manual)

Clinical tetany is usually associated with marked hypomagnesaemia in the blood and cerebrospinal fluid. Many affected cows are also hypocalcaemic, particularly when grazing cereal crops. This overlap is why combined calcium-magnesium treatment is usually preferred rather than magnesium or calcium alone. (Merck Veterinary Manual)

Is Grass Tetany Simply Caused by Low-Magnesium Grass?

Not always.

Some pastures genuinely contain too little magnesium. In other situations, the forage may contain a seemingly adequate amount, but high potassium and nitrogen interfere with magnesium absorption from the rumen.

Low sodium, low phosphorus, poor forage availability and reduced feed intake can further increase the risk. The practical problem is therefore not simply the magnesium concentration printed on a forage report. It is whether the cow absorbs enough magnesium to meet her immediate needs. (Merck Veterinary Manual)

Why Does Lush Spring Grass Increase the Risk?

Rapidly growing cool-season pasture commonly has a mineral pattern that favours grass tetany:

Magnesium may be low or marginal.
Calcium may be low.
Potassium may be very high.
Nitrogen may be high.
Sodium may be low.
The forage may contain a great deal of water and less dry matter than expected.

High potassium interferes with magnesium absorption across the rumen wall. High nitrogen may further reduce magnesium availability. A cow can therefore consume large amounts of lush forage while still failing to absorb enough magnesium. (Merck Veterinary Manual)

Common High-Risk Forages

Risk is particularly associated with:

Wheat pasture
Oat pasture
Cereal rye
Triticale
Bromegrass
Ryegrass
Tall fescue
Other rapidly growing cool-season grasses
Heavily nitrogen- or potassium-fertilised pasture

Grass tetany can also occur in lactating cows fed conserved forage or silage indoors. It is not exclusively a spring grazing disorder. (Merck Veterinary Manual)

How Do Fertiliser and Manure Increase Risk?

Potash fertiliser, heavy nitrogen applications and some manure sources can raise forage potassium or nitrogen concentrations.

Soils that are naturally high in potassium also create concern. Poultry manure is specifically recognised as a potential source of both potassium and nitrogen loading.

A pasture may look exceptionally productive while having a mineral balance that sharply reduces magnesium absorption. The real objective is not simply growing more forage. It is growing forage that cattle can use safely. (Merck Veterinary Manual)

Which Beef Cows Are Most at Risk?

Older Lactating Cows

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

Their magnesium requirements increase at calving because of colostrum and milk production. Older cows also appear less capable of mobilising magnesium from bone than younger cattle, making them more vulnerable when dietary absorption falls. (UNL Beef)

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 at peak lactation

The biggest and apparently strongest cows may therefore be among the first animals affected.

Cows With Reduced Feed Intake

Risk rises when forage or supplement intake falls because of:

Cold rain
Snow or ice
Strong wind
Transport
Yard confinement
Calving difficulty
Sudden ration changes
Limited forage availability
Illness
Social competition

Cattle have no large readily available magnesium reserve to protect them through several days of inadequate intake. (Merck Veterinary Manual)

Cows With Low Sodium Intake

Sodium contributes to the electrochemical gradient involved in magnesium absorption.

If plain salt and a less-palatable high-magnesium mineral are offered separately, some cattle may satisfy their salt appetite without consuming enough of the magnesium product. Including an appropriate amount of salt within a formulated mineral can improve more reliable intake. (UNL Beef)

Cows With Low Calcium

Grass tetany and hypocalcaemia frequently occur together.

A cow with both abnormalities may appear less violently excitable and more weak or recumbent. The clinical picture depends on which deficiency predominates. (Merck Veterinary Manual)

Can Grass Tetany Occur Outside Spring?

Yes.

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
Feeding low-magnesium hay
Sudden movement onto green crops
Periods of interrupted intake or undernutrition

The risk period should be based on forage growth, mineral balance, stage of lactation and actual consumption rather than a fixed date on the calendar. (Merck Veterinary Manual)

What Is the Grass Tetany Ratio?

A forage laboratory may calculate a tetany ratio comparing potassium with calcium and magnesium.

A lab-calculated ratio above approximately 2.2 indicates increased risk. The ratio should be calculated on the correct mineral-equivalent basis rather than by simply dividing the raw percentages printed on the report. (extension.okstate.edu)

The tetany ratio is a useful risk indicator, but it is not a guarantee that disease will or will not occur.

The final risk still depends on:

Cow age
Lactation stage
Actual dry-matter intake
Magnesium-supplement intake
Weather
Pasture availability
Sodium status
Individual susceptibility

Forage-analysis results may also take several days to return. During a known high-risk period, preventive magnesium supplementation should not be delayed while waiting for the report. (extension.okstate.edu)

Practical Forage Warning Values

Risk is higher when forage contains approximately:

Magnesium around or below 0.15% of dry matter
Calcium below 0.4%
Potassium above 2.5%

These should be treated as warning points rather than absolute safety cut-offs. High nitrogen, low sodium, poor intake and early lactation can alter the risk substantially. (UNL Beef)

What Are the Early Signs of Grass Tetany?

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

Watch for:

Nervous or unusual behaviour
Standing away 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 sound
Exaggerated response to touch
Repeated urination
Difficulty rising
Head held unusually high
Sudden aggression or unpredictable movement

A cow may remain in this stage for up to two or three days, but transport, driving or other stimulation can abruptly trigger severe convulsions. (Merck Veterinary Manual)

The Clinical Combination That Matters Most

The pattern that should concern producers most is:

Older cow
Early lactation
Lush cereal or cool-season grass pasture
Twitching or nervous behaviour
Stiffness or incoordination

That cow should be treated as an emergency before she becomes recumbent.

What Does Advanced Grass Tetany Look Like?

In acute grass tetany, a cow may:

Throw her head upwards
Bellow
Gallop without apparent awareness
Crash through fencing
Fall suddenly
Paddle violently
Chomp her jaws
Froth at the mouth
Develop rapid involuntary eye movements
Become rigid between seizures
Convulse repeatedly
Stop breathing and die

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

Can the First Sign Be Sudden Death?

Yes.

Cows are frequently found dead without recognised premonitory signs.

Evidence of pre-death seizures may include:

Flattened or churned ground
Soil marks around the head and limbs
Broken vegetation
Froth around the mouth
Evidence of paddling
Traumatic injuries

One sudden death on high-risk pasture should trigger immediate preventive action for the rest of the herd. Do not wait for a second death before providing magnesium and beginning the investigation. (Merck Veterinary Manual)

How Worried Should You Be?

High Pasture Risk, No Clinical Signs

The cows are:

Mature
In early lactation
Grazing lush cereal or cool-season grass
Not receiving reliable magnesium
Exposed to recently fertilised pasture or cold, wet weather

What it means: Clinical disease may develop rapidly even though the group currently appears normal.

What to do: Begin magnesium supplementation immediately, provide palatable hay, measure mineral consumption and observe the herd closely 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 it means: The nervous system is already being affected.

What to do: Contact the veterinarian immediately. Reduce noise and stimulation. Do not drive her through yards or force her onto a trailer.

Advanced Grass Tetany

The cow has:

Blind staggering
Rapid or laboured breathing
Severe stiffness
Repeated falling
Jaw chomping
Frothing
Marked tachycardia
Recumbency

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

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

Critical Grass Tetany

The cow is:

Convulsing
Paddling
Laterally recumbent
Minimally responsive
Struggling to breathe
Experiencing repeated seizures
Already dead

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

What to do: Obtain emergency veterinary care and begin herd-level magnesium supplementation 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 and excitable	Quiet, weak and dull
Muscle tone	Increased, stiff or spasming	Reduced and flaccid
Early movement	Stiff, twitchy and staggering	Weak, shuffling and 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 because of muscular activity	Often normal or low
Typical setting	Lush high-potassium pasture	Calving and the first days of lactation
Main treatment	Magnesium plus calcium	Primarily calcium
Response to treatment	May be slower	Often rapid

Where calcium and magnesium are both low, the cow may show a mixed picture. This is another reason treatment and diagnosis should not be based only on whether the cow appears excited or quiet. (Merck Veterinary Manual)

What Else Can Look Like Grass Tetany?

Possible condition	Important clues
Milk fever	Quiet, flaccid cow near calving, cold ears and reduced gut movement
Polioencephalomalacia	Blindness, head pressing, star-gazing and opisthotonus
Lead poisoning	Blindness, jaw chomping, salivation, seizures and access to batteries or old paint
Nervous ketosis	Abnormal licking, bellowing, circling or aggression in early lactation
Listeriosis	Circling, head tilt, facial weakness and one-sided neurological deficits
Rabies	Behavioural change, drooling, swallowing difficulty and progressive paralysis
Tetanus	Rigid gait, erect ears, third-eyelid prolapse and sustained stiffness
Nitrate or nitrite poisoning	Rapid breathing, weakness, collapse and chocolate-brown blood
Toxic plant ingestion	Neurological or gastrointestinal signs following specific pasture exposure
Organophosphate toxicity	Salivation, diarrhoea, constricted pupils, tremors and breathing difficulty
Bovine ephemeral fever	Fever, pain, stiffness and recumbency in endemic regions
Head or spinal trauma	Asymmetrical deficits, visible injury or recent handling accident

Do not perform an unprotected oral examination or handle saliva when rabies remains a reasonable differential diagnosis.

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 develops:

Muscle twitching
Nervous or aggressive behaviour
Stiffness
An unsteady gait
Hypersensitivity to noise or touch
Difficulty rising
Blind staggering
Frothing or jaw chomping
Seizures
Collapse
Severe breathing difficulty

Also seek urgent assistance when:

One or more cows are found dead
Several cows are twitching or behaving abnormally
A cow fails to respond to calcium-magnesium treatment
A cow goes down again after initial recovery

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

What To Do Right Now

1. Contact the Herd Veterinarian

Report:

Cow identification
Age and parity
Calving date
Pasture or ration
Clinical signs
Whether seizures have occurred
Current mineral programme
Fertilisation or manure history
Other affected or dead cattle
Any treatment already administered

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
Use an electric prod
Force her through a race
Make her walk to another paddock
Load her unless specifically directed

Transport or forced movement can shorten the period between subtle signs and severe convulsions. (Merck Veterinary Manual)

3. Keep Clear of a Convulsing Cow

A seizuring cow has no control over her limbs.

Stay away from:

The head
The neck
The legs
Fences or structures she may strike

Do not attempt to hold the head manually during a seizure.

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

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

Oral magnesium is too slow to correct acute neurological disease. It is used after stabilisation and to protect the remaining herd.

5. Maintain Sternal Recumbency Once Seizures Stop

Where this can be done without triggering another convulsion, position the cow chest-down rather than flat on her side.

Sternal positioning reduces:

Bloat
Regurgitation
Aspiration
Pressure injury

Do not repeatedly reposition or stimulate her while she remains hyperexcitable.

6. Start Herd-Level Prevention Immediately

Once one case is suspected:

Provide a high-magnesium mineral immediately.
Supply palatable dry hay.
Check every feeder.
Calculate actual mineral consumption.
Inspect older lactating cows closely.
Prevent access to untreated alternative salt sources where these are reducing mineral intake.
Collect forage and mineral samples.

One affected cow indicates that the 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 or feed
Early lactation
Older cow
Typical neurological signs
Inadequate magnesium intake or absorption
Response to treatment

Blood and Urine Testing

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

The normal total magnesium reference range in 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 concurrent hypocalcaemia and the clinical presentation must also be considered.

Urine magnesium is often extremely low or undetectable in hypomagnesaemic cows and can provide useful information about current magnesium status. (Merck Veterinary Manual)

Useful blood tests may include:

Total or ionised magnesium
Calcium
Phosphorus
Potassium
Sodium
Glucose
Kidney and muscle values

Why Can Blood From a Dead Cow Be Misleading?

After death and severe muscle injury, magnesium may leak from damaged muscle cells into the blood. A normal postmortem serum magnesium concentration therefore does not reliably exclude grass tetany.

Forage, Mineral and Water Analysis

A herd investigation may include:

Magnesium
Calcium
Potassium
Sodium
Phosphorus
Sulfur
Nitrogen or crude protein
Dry matter
The tetany ratio

Sample the material cattle are actually consuming, including different paddock areas where pasture growth or fertilisation varies.

How Is Grass Tetany Treated?

Immediate Parenteral Calcium and Magnesium

Clinical grass tetany requires rapid parenteral magnesium, usually combined with calcium.

Treatment is preferably:

Given slowly by the intravenous route
Administered with continuous cardiac monitoring
Selected according to the formulation available
Performed with minimal stimulation
Given by a veterinarian or someone specifically trained under a veterinary protocol

Recovery may be slower than after treatment of uncomplicated milk fever because magnesium must be restored within the cerebrospinal fluid before normal neurological control returns. (Merck Veterinary Manual)

Veterinary Treatment Reference

Merck states that an adult cow requires approximately 1.5 to 2.25 g of elemental magnesium during emergency treatment. Commercial calcium-magnesium formulations differ substantially in magnesium source and concentration, so injection volumes must be calculated from the exact product rather than copied between bottles. (Merck Veterinary Manual)

Additional magnesium may be administered subcutaneously in selected cases after the initial IV treatment. Strict asepsis and correct product selection remain essential.

Rapid IV calcium-magnesium administration can cause cardiac arrhythmias, collapse or death. This is not a treatment to learn during an active seizure from an online dosing table.

Why Is Calcium Included?

Many affected cows are also hypocalcaemic.

Calcium supports:

Cardiac contraction
Skeletal muscle function
Concurrent milk-fever treatment
Initial circulatory stability

Calcium alone may produce only partial or temporary improvement when magnesium remains critically low.

Why Must the Heart Be Monitored?

Both magnesium and calcium affect cardiac conduction.

During administration, the veterinarian monitors:

Heart rate
Rhythm
Pulse quality
Breathing
Level of consciousness

The infusion may need to be slowed or stopped temporarily if the rhythm changes.

Leave the Cow Quiet After Treatment

Do not immediately force her to stand.

Avoid:

Shouting
Slapping
Pulling on the head
Repeatedly moving the limbs
Driving other cattle past her
Allowing dogs near her

Stimulation during treatment or early recovery can trigger another fatal seizure. (Merck Veterinary Manual)

What Happens After Emergency Treatment?

Move Her Away From the High-Risk Feed Once Stable

Only move the cow when:

Seizures have stopped
She is coordinated enough to walk
Movement can occur calmly
A safer feed source is ready

Do not force a recently recovered cow to walk a long distance.

Begin Ongoing Oral Magnesium

Emergency treatment corrects the immediate blood deficit but does not remove the underlying pasture risk.

Merck recommends providing approximately 60 g of magnesium oxide per adult cow daily during the immediate danger period after a clinical case. This can be mixed with hay, molasses, concentrate or another palatable carrier. Without ongoing magnesium, recurrence can occur within approximately 36 hours. (Merck Veterinary Manual)

This outbreak-level recommendation should not be confused with the amount of a commercial mineral product. Magnesium oxide is an ingredient, while commercial mineral intake includes salt, calcium, trace minerals and carriers.

Recheck the Cow

Monitor for:

Recurrent twitching
Nervousness
Reduced feed intake
Stiffness
Weakness
Difficulty rising
Further seizures

Any relapse requires immediate veterinary reassessment.

What if the Cow Does Not Respond?

Failure to improve may mean:

The magnesium dose was inadequate.
Treatment occurred too late.
Severe brain or muscle injury developed.
Hypocalcaemia remains significant.
Another electrolyte abnormality is present.
The diagnosis is incorrect.
A toxic or infectious disease is occurring.
The cow sustained trauma during a seizure.

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 begins immediately.
The cow remains standing.
She responds without repeated convulsions.
Oral magnesium follows treatment.
The herd is moved onto safer feed or supported with hay.

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 convulsions.
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 remains comatose.
Severe aspiration or bloat develops.
There is major head, spinal or limb trauma.
Normal breathing cannot be maintained.

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

One affected cow should be treated as a warning for the whole group.

Inspect the Highest-Risk Cows

Prioritise:

Older cows
Cows in the first weeks of lactation
High-milking cows
Previous tetany cases
Cows with reduced feed intake
Cows behaving differently from the group

Observe them calmly from a distance before walking through the herd.

Provide Magnesium Immediately

Do not wait for forage-analysis results.

Use a veterinarian- or nutritionist-approved delivery method capable of providing dependable daily intake.

Offer Dry Hay

Hay can:

Increase dry-matter intake
Dilute the mineral imbalance of lush pasture
Reduce reliance on rapidly growing forage
Act as a carrier for magnesium oxide

Review Pasture and Fertiliser History

Document:

Nitrogen applications
Potassium applications
Manure applications
Recent rainfall
Pasture maturity
Forage availability
Date cattle entered the field

Monitor at Least Twice Daily

Early signs may be brief. Increase observation during cold, wet weather or after a sudden flush of pasture growth.

How Can Grass Tetany Be Prevented?

Start Magnesium Before Turnout

Introduce a high-magnesium mineral approximately three to four weeks before cattle enter high-risk pasture.

Current extension recommendations range from two to four weeks, with one month providing useful time to identify poor intake and adjust feeder placement or palatability. Continue supplementation throughout the risk period and early lactation. (extension.okstate.edu)

Choose a Proper High-Magnesium Mineral

Commercial grass-tetany minerals commonly contain approximately 10% to 15% magnesium, with magnesium oxide as the usual source.

A common target intake is approximately 3 to 4 oz, or 85 to 115 g, of mineral per cow each day, depending on the formulation.

For lactating cows grazing small cereal crops, one Nebraska example recommends a four-ounce mineral containing at least:

10% magnesium
8% calcium
20% to 25% salt

The exact formulation must be matched to the forage, cattle and total diet. (UNL Beef)

Magnesium Versus Magnesium Oxide

These terms are not interchangeable.

A mineral labelled as 10% magnesium generally contains considerably more than 10% magnesium oxide because magnesium oxide is only partly elemental magnesium.

Producers should compare the guaranteed magnesium percentage and expected daily product intake rather than looking only for the ingredient name.

Calculate Actual Mineral 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 ÷ 50 ÷ 7 = 1.6 oz per cow daily.

That is well below a three- to four-ounce target.

Do not assume mineral is being consumed adequately because the feeder occasionally looks empty. Calves, wildlife, rain and spillage can account for part of the disappearance.

Remember Individual Variation

A satisfactory herd average can conceal:

Cows consuming almost none
Dominant cows consuming more than intended
Newly calved cows avoiding the feeder
Cattle grazing far from the mineral
Cows satisfying their salt appetite elsewhere

Provide adequate feeder space and use more than one location when the group or paddock is large.

Improve Palatability

Magnesium oxide is bitter.

Intake may improve by:

Introducing the mineral before turnout
Keeping the product dry and fresh
Including salt within the formulation
Adding an approved palatable carrier
Mixing magnesium through concentrate or pellets
Placing feeders near water or frequently used grazing areas
Adding a modest amount of corn, distillers grain or another suitable ingredient to the mineral when advised

Nebraska Extension suggests that approximately one pound of corn or dried distillers grains per 50 pounds of mineral may improve intake when consumption is below target. (UNL Beef)

Do Not Add Unnecessary Potassium

Check the mineral label carefully.

A grass-tetany mineral should not add substantial potassium to a diet that is already excessively high in potassium. (UNL Beef)

Use Controlled Feeding When Free-Choice Intake Is Unreliable

Where the risk is high and free-choice intake is poor, consider:

Magnesium mixed into 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

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

Maintain Adequate Sodium and Calcium

High-risk forage may be low in sodium and calcium as well as marginal in magnesium.

A complete prevention programme should therefore evaluate:

Magnesium
Calcium
Sodium
Potassium
Total intake
The balance of the complete ration

Supplementing magnesium while ignoring severe calcium or sodium deficiencies may provide incomplete protection. (UNL Beef)

Provide Palatable Hay

Hay is especially useful when cows are:

Grazing green cereal crops
On potassium- or nitrogen-fertilised pasture
Experiencing cold, wet weather
At risk of inadequate dry-matter intake
Recovering after a clinical case

Hay cannot replace magnesium supplementation in a high-risk herd, but it can increase intake and dilute the lush forage. (Merck Veterinary Manual)

Delay Turnout When Appropriate

Allowing pasture to reach approximately four to six inches before turnout can support root development, improve forage availability and reduce some of the nutritional stress associated with very short, rapidly growing grass. (UNL Beef)

Do Not Confuse Tetany Prevention With Bloat Prevention

Grass tetany and pasture bloat are separate problems.

Magnesium and calcium supplementation can help prevent tetany, but a high-magnesium mineral should not be relied upon as the primary bloat-control measure. (extension.okstate.edu)

Match Cow Groups to Pasture Risk

Where possible, reserve lower-risk forage for:

Older lactating cows
Previous tetany cases
High-milking cows

Higher-risk paddocks may be better suited to:

Dry cows
Younger cattle
Steers
Cows well past peak lactation

Maintain Adequate Forage Availability

Short pasture reduces total dry-matter and magnesium intake.

Even forage with a reasonable mineral concentration cannot meet requirements when there is too little of it for cows to consume sufficient dry matter. (Merck Veterinary Manual)

Protect Intake During Bad Weather

Cold rain, snow and wind can reduce grazing and increase maintenance requirements.

Provide:

Shelter
Dry hay
Supplemental feed
Accessible water
Reliable magnesium

Common Grass Tetany Mistakes

Waiting Until the Cow Is Down

Twitching, nervousness or stiffness is already an emergency.

Chasing the Cow to the Yards

Movement and stimulation can trigger fatal convulsions.

Treating With Calcium Alone

Concurrent calcium deficiency may exist, but magnesium must also be replaced.

Pouring Oral Magnesium Into a Recumbent Cow

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

Describing a Mineral as 10% Magnesium Oxide

The important guaranteed analysis is the percentage of elemental magnesium in the complete mineral. Magnesium oxide is the ingredient supplying it.

Assuming Offered Mineral Equals Consumed Mineral

Calculate actual intake and investigate spillage or poor access.

Starting Mineral on Turnout Day

Introduce high-magnesium mineral several weeks earlier.

Using One Feeder for a Large or Dispersed Group

Some cows may never receive the intended amount.

Offering Plain Salt Separately Without Reviewing Intake

Cows may consume the salt while avoiding the high-magnesium mineral.

Relying Only on the Tetany Ratio

Forage testing is valuable, but cow age, lactation, weather, intake and supplementation determine the real risk.

Moving the Herd Abruptly After One Case

A rushed movement may trigger another affected cow. Make management changes quietly and provide hay and mineral where the cattle already are when immediate movement is unsafe.

Sampling Only Postmortem Blood

Serum magnesium may appear normal 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, hay, silage and sudden intake disruption can also produce disease.

A Practical Grass Tetany Prevention Plan

Three to Four Weeks Before High-Risk Grazing

Test the pasture or conserved forage.
Review magnesium, calcium, potassium and sodium.
Select an appropriate high-magnesium mineral.
Confirm that it does not add unnecessary potassium.
Introduce the mineral.
Measure baseline consumption.
Identify older and previously affected cows.
Confirm the emergency veterinary protocol.

One Week Before Turnout

Recalculate mineral intake.
Add feeders where access is poor.
Correct palatability problems.
Arrange a reliable hay supply.
Check water and fencing.
Review the clinical warning signs with staff.

During the High-Risk Period

Maintain uninterrupted magnesium access.
Calculate intake at least weekly.
Observe cows at least twice daily.
Increase surveillance after cold rain or rapid pasture growth.
Keep hay available.
Avoid unnecessary transport or fasting.
Record abnormal behaviour immediately.

After a Clinical Case

Treat the affected cow immediately.
Intensify magnesium delivery for the entire group.
Supply hay.
Sample forage, mineral and water.
Collect appropriate diagnostic samples.
Recheck every high-risk cow.
Review feeder placement and actual intake.
Maintain prevention throughout the remaining danger period.

Frequently Asked Questions

Can grass tetany occur when forage magnesium looks normal?

Yes. High potassium and nitrogen may prevent adequate magnesium absorption even when forage magnesium appears acceptable. Intake, sodium, calcium, stage of lactation and the complete mineral balance also matter. (Merck Veterinary Manual)

How quickly can grass tetany kill a cow?

A cow may progress from apparently normal grazing to convulsions and death within a few hours. In some cases, sudden death is the first recognised sign. (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 reliable daily magnesium because readily available body stores are limited. (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 primarily caused by low calcium and more commonly produces quiet weakness and flaccid recumbency. Both conditions may 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 begin immediately, and stimulation should be kept to a minimum.

Final Thoughts

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

The most important points are:

Grass tetany is caused by inadequate available magnesium.
Mature lactating cows are at greatest risk.
High potassium and nitrogen may prevent adequate magnesium absorption.
Lush cool-season grasses and cereal crops are common high-risk forages.
Early signs include twitching, nervousness, stiffness and incoordination.
Chasing or stressing an affected cow can trigger a fatal seizure.
Some cows are found dead without recognised warning signs.
Grass tetany and milk fever may occur together.
Clinical cases require emergency parenteral calcium and magnesium.
Intravenous treatment must be administered slowly with cardiac monitoring.
Oral mineral must not be forced into a down or seizuring cow.
Recovery may be slower than after treatment of uncomplicated milk fever.
Recurrence is possible without continued magnesium support.
One affected cow means the rest of the group is at risk.
A high-magnesium mineral works only when cattle consume the intended amount.
Start supplementation several weeks before high-risk turnout.
Forage testing, hay, feeder placement and fertiliser planning all contribute to prevention.
Grass tetany risk can be reduced substantially, but no purchased mineral can compensate for poor intake or an unmonitored herd.

The most dangerous mistake is trying to move the cow before treating her. With grass tetany, keeping the situation quiet is not merely good stockmanship. It may be the difference between recovery and a fatal convulsion.

ASK A VET™ can help organise forage analyses, mineral labels, paddock 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.

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About the author

Dr Duncan Houston

Veterinarian · Founder of ASK A VET

Every ASK A VET article is written and reviewed by qualified veterinarians.

About Dr. Duncan →drduncanthevet