Hypomagnesemia / Grass Tetany (acute)

Hypomagnesemia, commonly known as grass tetany or grass staggers, is an acute and frequently fatal metabolic emergency caused by critically low blood magnesium levels in ruminant livestock. This condition typically occurs in lactating cattle grazing lush, rapidly growing pastures in spring or fall, though it can occur under various circumstances where magnesium intake is insufficient to meet the demands of lactation. The name grass tetany reflects the characteristic muscular hyperexcitability and tetanic seizures that occur when blood magnesium drops below levels necessary for normal neuromuscular function. Without immediate treatment, affected animals often die within hours of symptom onset.

Grass tetany primarily affects mature lactating cows, with beef cattle on spring pastures being the classic at-risk population. However, the condition also occurs in dairy cattle, particularly those grazing pasture or fed high-moisture forages. Sheep and goats develop hypomagnesemia under similar circumstances, though the condition is less commonly recognized in these species. The highest risk period is typically early spring when animals are turned onto rapidly growing cool-season pastures, though fall regrowth can pose similar risks. The condition affects an estimated 1 to 3 percent of at-risk cattle populations annually, but mortality rates among affected animals approach 20 to 30 percent even with treatment.

The economic impact of hypomagnesemia includes both direct losses from mortality and the significant costs of prevention programs. Sudden death of valuable breeding cows during the critical calving and breeding season represents a substantial loss. Orphaned calves require expensive care and often have reduced growth performance. Subclinical hypomagnesemia, though less dramatic than acute grass tetany, may affect animal performance, reproduction, and susceptibility to other diseases. Prevention through magnesium supplementation is relatively inexpensive but requires consistent implementation throughout risk periods.

Recognition of early signs and immediate treatment are critical for survival, but the rapid progression of grass tetany means that many affected animals are found dead or dying with little opportunity for intervention. Prevention through appropriate magnesium supplementation is therefore the most effective management strategy. Understanding the risk factors, implementing appropriate supplementation programs, and recognizing early warning signs allow producers to protect their herds from this devastating condition.

The primary cause of grass tetany is inadequate magnesium absorption from the diet, leading to depletion of blood magnesium stores and subsequent neuromuscular dysfunction. Unlike calcium, magnesium is not stored in significant mobilizable reserves in the body, so animals depend on daily dietary intake to maintain blood levels. When dietary magnesium is insufficient or absorption is impaired, blood magnesium levels can fall rapidly to dangerous levels. Lactating animals are at highest risk because milk production represents a significant magnesium drain that cannot be reduced even when blood levels are falling.

The forage factors that predispose to grass tetany involve both low magnesium content and factors that impair absorption. Rapidly growing cool-season grasses, particularly during wet, cool spring weather, often contain low magnesium concentrations despite appearing lush and nutritious. High potassium levels in these same forages directly interfere with magnesium absorption in the rumen. High nitrogen fertilization increases forage potassium while potentially reducing magnesium uptake by plants. Low soil magnesium in some geographic regions results in chronically low forage magnesium. The combination of high potassium and low magnesium creates the classic grass tetany scenario.

Animal factors influence individual susceptibility to hypomagnesemia. Lactation dramatically increases magnesium requirements, making lactating cows far more susceptible than dry animals. Older cows appear more susceptible than younger animals, possibly due to decreased absorptive efficiency. High milk production increases magnesium output and risk. Poor body condition reduces any marginal reserves of magnesium and other nutrients. Stress from weather, handling, or transportation increases magnesium requirements and may precipitate acute tetany in marginally deficient animals. Concurrent hypocalcemia, which frequently accompanies hypomagnesemia, worsens clinical signs.

Environmental and seasonal factors create predictable risk patterns. Spring turnout onto lush pastures is the classic high-risk period, particularly when preceded by several days of cool, wet weather that promotes rapid grass growth. Fall regrowth creates similar forage conditions and risk. Wheat pasture and other cereal grain pastures pose extremely high risk, hence the term wheat pasture poisoning sometimes used for grass tetany. Sudden weather changes, particularly cold snaps during grazing season, may precipitate acute tetany in marginally deficient animals. These predictable seasonal patterns allow targeted prevention efforts during highest-risk periods.

The pathophysiology of hypomagnesemia involves the essential role of magnesium in neuromuscular function and cellular metabolism. Magnesium stabilizes nerve and muscle cell membranes, and when blood magnesium falls below approximately 1.0 mg/dL (normal is 1.8-2.4 mg/dL), neuromuscular excitability increases dramatically. This manifests as the characteristic hyperexcitability, muscle tremors, and tetanic seizures of grass tetany. Magnesium also influences calcium and potassium handling at the cellular level, and concurrent abnormalities in these electrolytes compound the clinical effects. The combination of neuromuscular hyperexcitability and cardiovascular effects can cause rapid death through respiratory failure, cardiac arrhythmias, or injury during violent seizures.

Early warning signs of hypomagnesemia may be present for hours to days before acute crisis, but are subtle and easily missed in extensive grazing operations where animals are not closely observed. Affected cows may appear nervous, excitable, or hyperalert, startling easily at sounds or movements that would not normally disturb them. Decreased milk production may be noted in dairy cattle or as poor calf condition in beef herds. Reduced feed intake and grazing time may occur. The cow may walk with a stiff, stilted gait. Ear flicking and frequent blinking or twitching of facial muscles represent early signs of neuromuscular hyperexcitability. These early signs are often recognized only in retrospect after acute tetany develops.

As hypomagnesemia progresses, symptoms become more dramatic and recognizable. The cow becomes increasingly agitated and excitable, often aggressive if approached or handled. Muscle tremors become visible, particularly affecting the head, neck, and flank regions. The gait becomes progressively more abnormal with stiffness, incoordination, and staggering, hence the term grass staggers. Excessive salivation and teeth grinding may occur. The cow may bellow or vocalize abnormally. Heart rate increases and becomes irregular. Any stimulation or stress may precipitate progression to full tetanic seizure.

Acute grass tetany presents as a dramatic neurological and muscular crisis. The cow collapses suddenly, often triggered by the stress of being approached, moved, or handled. Violent convulsive seizures occur with rigid extension of the legs, arching of the back (opisthotonus), and paddling movements. Chewing movements produce froth at the mouth. The eyes roll back and the pupils dilate. Heart rate is very rapid and irregular, and the heart sounds may be abnormal. Body temperature rises rapidly to 104-106°F due to intense muscular activity. Between seizures, the cow may lie quietly but remains hyperresponsive to stimulation.

Physical examination during or between seizures reveals characteristic abnormalities. Body temperature is markedly elevated during active seizures but may normalize between episodes. Heart rate is typically 100-120+ beats per minute with potentially serious arrhythmias. Respiratory rate increases during and after seizures. Mucous membranes may be congested or cyanotic during seizures. Muscle tone is dramatically increased during tetanic episodes with rigid extension of limbs. Reflexes are exaggerated and easily triggered. The animal may respond violently to minor stimulation including sounds, touch, or light.

Symptom progression in untreated grass tetany is rapid and often fatal. Seizures become more frequent and severe. Between seizures, the cow becomes increasingly depressed and may enter a coma-like state. Cardiac arrhythmias may cause sudden death during or between seizures. Respiratory failure from prolonged seizure activity or aspiration can occur. Hyperthermia from prolonged muscular activity can cause heat stroke. Injury during violent seizures, including fractures and internal hemorrhage, may prove fatal. Many affected animals are found dead without observed clinical signs, having died within hours of developing acute tetany.

Certain presentations indicate critical emergency requiring immediate intervention. Active seizure activity represents a life-threatening emergency requiring immediate magnesium administration and management of concurrent problems. A cow found down and paddling after apparent seizure requires immediate treatment. Any cow in a high-risk group showing excessive agitation, tremors, or staggering should be treated presumptively for hypomagnesemia before progression to seizures. Concurrent hypocalcemia is common and should be addressed in treatment.

Clinical diagnosis of grass tetany is based on the characteristic presentation, timing, and circumstances combined with response to treatment. A lactating cow grazing lush spring pasture who presents with agitation, muscle tremors, staggering, and seizures is presumptively diagnosed with hypomagnesemia and treated accordingly. The rapid response to magnesium therapy serves as confirmation. However, the speed of progression often means that diagnosis is made post-mortem when animals are found dead, and the circumstances must guide retrospective diagnosis and preventive measures for herdmates.

Diagnostic testing confirms hypomagnesemia and guides treatment but is rarely available quickly enough for initial clinical management. Blood magnesium measurement is definitive, with clinical tetany typically occurring when levels drop below 1.0 mg/dL. However, blood magnesium levels may be artificially elevated by stress and muscle activity at the time of sampling, potentially masking the severity of deficiency. Concurrent measurement of calcium is valuable because hypocalcemia frequently accompanies hypomagnesemia and affects treatment requirements. Magnesium measurement in vitreous humor (eye fluid) post-mortem provides a more stable indicator of ante-mortem magnesium status than blood samples. Urine magnesium measurement can assess magnesium status but is not useful for emergency diagnosis.

Differential diagnosis considers other conditions causing sudden neurological signs, seizures, or death in grazing cattle. Acute lead poisoning causes neurological signs and seizures but usually affects multiple animals and has a different history. Nervous ketosis in dairy cattle causes neurological abnormalities but typically occurs earlier in lactation and responds to glucose therapy. Polioencephalomalacia (thiamine deficiency) causes neurological signs but usually has more gradual onset. Rabies should be considered in any bovine with unusual neurological signs. Acute hypocalcemia (milk fever) causes weakness and recumbency but typically lacks the hyperexcitability and seizures of grass tetany. Plant poisonings affecting the nervous system should be considered. History, response to treatment, and laboratory testing differentiate these conditions.

Herd-level assessment is essential when grass tetany occurs because other animals are likely at risk. Evaluation of pasture conditions, including grass species, growth stage, and recent fertilization, identifies risk factors. Forage testing for magnesium, potassium, and nitrogen content confirms tetany-prone conditions. Assessment of current supplementation programs identifies gaps in prevention. Blood magnesium testing of clinically normal herdmates identifies subclinically deficient animals. This investigation guides implementation or adjustment of prevention programs to protect remaining at-risk animals.

Emergency treatment of grass tetany requires immediate intravenous or subcutaneous administration of magnesium to restore blood levels and terminate neuromuscular hyperexcitability. Calcium-magnesium solutions (such as those containing calcium borogluconate with magnesium) are preferred because concurrent hypocalcemia is extremely common. Treatment should be initiated with subcutaneous injection if intravenous access is difficult in a seizuring animal, as some absorption will occur and this route is safer than attempting venipuncture during violent movements. Once seizures are controlled, intravenous administration provides more rapid correction of severe deficiency.

Medical management protocols address both the acute crisis and ongoing magnesium deficiency. Intravenous calcium-magnesium solutions are administered slowly over 15-20 minutes while monitoring heart rate and rhythm, as both calcium and magnesium can cause cardiac effects with overly rapid administration. Subcutaneous injection of magnesium sulfate solution provides slower, sustained release and should follow intravenous treatment. Sedation with xylazine or other appropriate agents may be necessary to control violent seizure activity and reduce injury risk. Subsequent oral magnesium supplementation helps prevent relapse during the period of ongoing high risk.

Supportive care during and after the acute crisis improves outcomes and prevents complications. Animals having seizures should be protected from self-injury by removal from fences, rocks, and other hazards when safely possible. Restraint of seizuring animals is extremely dangerous and should be avoided. After seizures are controlled, the animal should be positioned in sternal recumbency with the head elevated to prevent aspiration. Shade and cooling may be needed for animals that developed hyperthermia during prolonged seizures. Quiet handling minimizes stimulation that could retrigger seizures. Food and water should be offered once the animal is stable and able to swallow safely.

Monitoring during recovery guides ongoing management. Heart rate should normalize within hours of treatment, and persistent tachycardia or arrhythmias warrant veterinary evaluation. Return of normal reflexes and responsiveness indicates resolution of neuromuscular hyperexcitability. Ability to stand and walk without ataxia confirms recovery of motor function. Appetite typically returns within 12-24 hours. Animals should be monitored for 24-48 hours for signs of relapse, which is common without ongoing magnesium supplementation.

When grass tetany occurs, herd-level treatment and prevention measures must be implemented immediately. All animals in the affected group should be assumed to be at risk. Removal from high-risk pasture to a grass tetany prevention program should occur immediately if possible. If animals must remain on pasture, immediate implementation of magnesium supplementation is essential. Oral magnesium oxide in a molasses-based supplement or free-choice mineral mix provides ongoing prevention. Treatment of any animals showing even subtle early signs prevents progression to acute tetany.

Treatment decisions must account for the practical challenges of treating seizuring animals and the potentially rapid fatal course. Many animals with grass tetany are found dead or in terminal stages where treatment is unlikely to succeed. Prompt treatment of animals showing early signs dramatically improves outcomes compared to treatment of animals in full tetanic crisis. Economic considerations rarely drive treatment decisions given the high value of breeding cows and the relatively low cost of treatment. However, euthanasia is appropriate for animals with severe complications from seizure activity including fractures, severe hyperthermia, or neurological damage.

Recovery from grass tetany is generally rapid and complete in animals that survive the acute crisis and receive appropriate treatment. Most treated animals show significant improvement within 30 minutes to 2 hours of magnesium administration, with resolution of tremors, hyperexcitability, and seizure tendency. Return of normal gait and coordination occurs over several hours. Appetite typically returns within 12-24 hours. Animals treated early, before severe seizure activity, often recover without complications and can return to normal productivity.

Post-treatment care focuses on preventing relapse and ensuring complete recovery. Relapse is common within 24-48 hours if ongoing magnesium supplementation is not provided, because the underlying dietary deficiency persists. Oral magnesium supplementation should continue throughout the risk period, typically until pastures mature or animals are moved to safer feeding programs. Animals should be kept in low-stress environments with minimal handling for 24-48 hours post-treatment. Monitoring for complications from seizure activity, including lameness, evidence of muscle damage, or respiratory problems, guides ongoing care.

Prognosis depends heavily on the severity of the episode and promptness of treatment. Animals treated while showing early signs (agitation, tremors, staggering) before seizure activity have excellent prognosis with rapid complete recovery. Animals in active seizure at treatment have guarded prognosis with survival rates of 70-80% with appropriate treatment. Animals found down and unresponsive have poor prognosis, with many failing to respond to treatment. Complications from seizure activity including fractures, hyperthermia, or aspiration pneumonia significantly worsen prognosis. Animals that survive severe episodes may have lasting neurological deficits or secondary complications.

Return to production following grass tetany recovery requires careful management of ongoing hypomagnesemia risk. Affected animals should not return to high-risk pastures without confirmed adequate magnesium supplementation. Milk production may be reduced for several days to a week after severe episodes. Breeding should be delayed until complete recovery is confirmed, as stress could precipitate relapse in marginally supplemented animals. Long-term productivity is generally not affected in animals that recover without major complications. However, affected animals may be more susceptible to future episodes and warrant heightened attention to magnesium supplementation.

Prevention of grass tetany through strategic magnesium supplementation is far more effective than treatment of clinical cases and should be the primary management focus. Supplementation programs should begin before the high-risk season (typically 2-4 weeks before spring turnout to pasture) and continue throughout the risk period. Magnesium oxide is the most common and cost-effective supplement, though its bitter taste can reduce palatability. Delivery methods include free-choice minerals, molasses-based lick blocks or tubs, addition to grain or silage, and water supplementation in some systems.

Biosecurity considerations for grass tetany prevention involve pasture and grazing management to reduce exposure to high-risk forages. Avoiding grazing of very lush, rapidly growing cool-season pastures during high-risk periods reduces exposure. Delaying spring turnout until pastures have matured somewhat reduces risk. Rotating animals between high-risk pastures and safer feeds dilutes exposure. Providing access to hay or other supplemental feeds reduces reliance on tetany-prone pastures. Avoiding grazing of wheat pasture or other cereal grain pastures without aggressive supplementation is essential in areas where this is practiced.

Nutritional management beyond magnesium supplementation supports overall mineral balance and reduces risk. Ensuring adequate dietary calcium supports both calcium and magnesium metabolism. Avoiding excessive potassium in other dietary components reduces interference with magnesium absorption. Adequate energy nutrition supports overall metabolic function. Avoiding sudden dietary changes reduces metabolic stress. Salt supplementation ensures adequate sodium intake, as sodium deficiency can worsen magnesium absorption.

Management practices complement supplementation for comprehensive prevention. Identifying high-risk animals (older lactating cows, high producers) allows targeted supplementation. Avoiding stress during high-risk periods reduces magnesium requirements and precipitation of acute tetany. Providing wind breaks and shelter reduces cold stress that can trigger tetany. Gradually introducing animals to lush pastures rather than abrupt turnout allows some metabolic adaptation. Feeding hay before morning turnout onto wet pastures may reduce the impact of high-risk forage consumption.

Monitoring and testing protocols support prevention program effectiveness. Blood magnesium testing of representative animals during the risk period confirms adequate supplementation. Forage testing for magnesium, potassium, and tetany ratio (K/(Ca+Mg)) identifies high-risk pastures. Monitoring supplement consumption ensures animals are receiving intended amounts. Observation for early warning signs allows intervention before acute crisis. Documentation of cases and near-misses guides prevention program adjustments.

Daily management during grass tetany risk periods requires heightened observation and consistent supplementation. Animals should be observed at least twice daily, with attention to behavior changes, gait abnormalities, or reduced grazing activity. Supplement consumption should be monitored to ensure animals are receiving adequate magnesium. Rapid weather changes, particularly cold snaps, should trigger increased vigilance. Any animals showing early signs should be treated immediately rather than waiting to see if signs progress. Having treatment supplies readily available enables rapid response.

Housing and environmental management for pasture-based cattle during high-risk periods focuses on providing safe alternatives to exclusively grazing high-risk pastures. Access to hay or other supplemental feeds provides magnesium and reduces exclusive reliance on lush grass. Loafing areas with mineral feeders encourage supplement consumption. Adequate shelter reduces weather stress. Water availability should be confirmed as animals may reduce intake when grazing lush pastures, concentrating their exposure to problematic forages.

Herd health programs should include grass tetany prevention as a specific seasonal focus. Written protocols specify when supplementation begins, supplement type and delivery method, target consumption levels, and monitoring procedures. All personnel should understand the importance of consistent supplementation and recognize early warning signs. Emergency treatment supplies including calcium-magnesium solutions should be stocked before the risk season. Veterinary consultation on prevention programs optimizes effectiveness for local conditions.

Record keeping supports grass tetany management through documentation of prevention efforts and any cases that occur. Supplement purchase and delivery records document implementation of prevention programs. Supplement consumption monitoring identifies potential gaps. Case records including affected animals, severity, treatment, and outcomes guide future prevention efforts. Pasture records document fertilization and management that affects tetany risk. These records support continuous improvement of prevention programs and provide documentation if losses occur.

Economic considerations for grass tetany management strongly favor prevention over treatment. The cost of magnesium supplementation is minimal compared to the value of breeding cattle at risk. Treatment costs, while not excessive, are compounded by the high mortality rate even with treatment. Orphaned calf care adds substantial cost when cows die. Lost production and reproduction from subclinical hypomagnesemia, though difficult to quantify, likely exceeds the obvious costs of clinical cases. Investment in effective prevention programs provides excellent return through reduced losses and improved overall herd health during the spring production season.

Grass tetany can affect any breed of cattle, sheep, or goat, but risk is primarily determined by production factors rather than breed genetics. Lactating beef cattle grazing spring pastures are the classic high-risk population, with Angus, Hereford, and other common beef breeds frequently affected. High milk production within any breed increases risk due to greater magnesium output. Dairy cattle face similar risk when grazing pastures or receiving high-moisture forages, with Holstein and other high-producing breeds at particular risk due to their high milk output. Dual-purpose and dairy-beef crossbred cattle may have elevated risk due to higher milk production than traditional beef breeds.

Production type and management intensity determine risk more than breed characteristics. Beef cow-calf operations with spring calving and spring pasture turnout face the classic grass tetany scenario. Fall calving herds face risk during fall pasture regrowth. Stocker operations grazing wheat pasture or cool-season annual pastures face severe risk, with wheat pasture poisoning being a form of grass tetany. Dairy operations using intensive rotational grazing of lush pastures face significant risk during peak grass growth. Sheep and goat operations face similar risk patterns based on lactation status and pasture conditions.

Age and parity significantly influence individual susceptibility within breeds. Older cows (generally over 6 years) are more susceptible than younger animals, possibly due to decreased magnesium absorption efficiency. First-calf heifers have relatively lower risk due to lower milk production and potentially better absorption. High-producing individuals within any age group face elevated risk. Body condition influences susceptibility, with thin cows having less reserve capacity. Genetic selection specifically for grass tetany resistance is not practiced, but awareness of individual and family history allows targeted management of high-risk animals within herds.

Several conditions commonly co-occur with or complicate hypomagnesemia in ruminants. Hypocalcemia is extremely common as a concurrent finding in grass tetany cases, as magnesium deficiency impairs calcium metabolism and both conditions have similar risk factors. Treatment protocols should always address potential concurrent hypocalcemia. Ketosis may occur concurrently in early lactation dairy cattle with multiple metabolic stresses. Fatty liver may be present in high-producing cattle with concurrent energy and mineral deficiency. Immune suppression from mineral deficiency may increase susceptibility to infectious diseases.

Conditions presenting similarly to grass tetany must be differentiated for appropriate treatment. Acute lead poisoning causes neurological signs including apparent blindness, seizures, and death but typically affects multiple animals with a common exposure source. Polioencephalomalacia (thiamine deficiency) causes neurological signs and may respond to thiamine therapy. Nervous ketosis in dairy cattle causes neurological abnormalities but responds to glucose administration. Rabies should be considered in any bovine with abnormal behavior or neurological signs. Eclampsia in ewes presents similarly to grass tetany and is treated with calcium. Tetanus causes characteristic rigid muscular spasms but has a wound history and specific postural changes. Careful history and response to specific treatments differentiate these conditions.

Complications and sequelae of grass tetany result from the violent seizure activity and metabolic crisis. Fractures may occur during seizures, particularly affecting long bones and the spine. Muscle damage from prolonged tetanic spasms can cause myoglobinuria and potentially kidney damage. Hyperthermia from seizure activity can cause heat stroke if animals are not cooled. Aspiration pneumonia may develop from inhalation of rumen contents during seizures or recumbency. Neurological damage from severe or prolonged seizures may cause lasting deficits. These complications significantly affect prognosis and may warrant euthanasia even in animals that survive the initial crisis.