Hypomagnesemic tetany (commonly known as grass tetany, grass staggers, lactation tetany, or wheat pasture poisoning) is an acute, often fatal metabolic disorder of ruminants characterized by hypomagnesemia (low serum magnesium) and associated...
Overview and Clinical Importance
Hypomagnesemic tetany (commonly known as grass tetany, grass staggers, lactation tetany, or wheat pasture poisoning) is an acute, often fatal metabolic disorder of ruminants characterized by hypomagnesemia (low serum magnesium) and associated neurological signs including hyperexcitability, muscular spasms, convulsions, and death. This condition is one of the most important metabolic emergencies in bovine practice and represents a high-yield topic for the NAVLE examination.
The disease primarily affects lactating beef and dairy cattle grazing lush, rapidly growing pastures in spring and fall. Understanding the pathophysiology of magnesium metabolism, recognizing clinical signs, and implementing appropriate treatment and prevention strategies are essential competencies for veterinarians working with cattle.
| Transport Mechanism |
Characteristics |
| Potential-Dependent (Active) |
Primary mechanism at normal ruminal Mg concentrations
Driven by electrical gradient (apical membrane potential)
INHIBITED by high potassium - K+ depolarizes the apical membrane
Energized by basolateral Na+/Mg2+ exchanger |
| Potential-Independent (Passive) |
Secondary backup mechanism
Requires 4X higher ruminal Mg concentration to function
K+-insensitive but less efficient
Relies on chemical concentration gradient |
Etiology and Pathophysiology
Magnesium Homeostasis in Cattle
Magnesium (Mg) is the fourth most abundant cation in the body and the second most abundant intracellular ion. It is essential for numerous enzymatic reactions including ATPases, kinases, and phosphatases, as well as normal nerve conduction, muscle contraction, and bone mineral formation. Approximately 60-70% of total body magnesium is bound in bones, but this pool is not readily mobilizable in adult cattle.
High-YieldUnlike calcium, there is NO specific hormonal regulatory mechanism for magnesium homeostasis. Cattle depend entirely on daily dietary intake and absorption to maintain serum magnesium levels. This is why consistent daily supplementation is critical during high-risk periods.
Sites and Mechanisms of Magnesium Absorption
The rumen is the primary site of magnesium absorption in cattle (unlike monogastrics where intestinal absorption predominates). Two transport mechanisms exist across the rumen epithelium:
NAVLE TipThe NAVLE frequently tests the concept that high dietary potassium inhibits magnesium absorption by depolarizing the rumen epithelial cell membrane. This is why lush, rapidly growing spring grasses (which are high in K+ and low in Mg2+) are so dangerous.
Risk Factors for Hypomagnesemic Tetany
Types of Hypomagnesemic Tetany
| Animal Factors |
Dietary/Forage Factors |
Environmental Factors |
| Lactating cows (peak demand)
Older cows (greater than 6 years)
Early lactation (6-8 weeks postpartum)
Beef breeds more susceptible than dairy
Poor body condition |
Forage Mg less than 0.2% DM
Forage K greater than 3% DM
High N fertilization of pastures
Low Na and low P in forage
K/(Ca+Mg) ratio greater than 2.2
Cereal grain pastures (wheat, rye, oats) |
Cool, wet spring weather
Rapid pasture growth
Sudden diet changes
Inclement weather reducing grazing
Transport stress
Heavy K or N fertilizer application |
Clinical Signs
Clinical signs result from low magnesium concentration in the cerebrospinal fluid (CSF), which causes spontaneous neuronal activation and uncontrolled muscle stimulation. Signs typically occur when plasma Mg falls below 1.2 mg/dL (0.5 mmol/L) in cattle.
Clinical Presentations by Form
High-YieldThe classic NAVLE presentation is an older, lactating beef cow on spring pasture found down with tetanic seizures, paddling, opisthotonus, and tachycardia (heart rate 150 bpm audible without stethoscope). The hyperexcitable, aggressive behavior before collapse may be mistaken for rabies.
| Type |
Description |
Key Features |
| Grass Tetany (Spring Tetany) |
Most common form; occurs when lactating cows graze lush spring pastures high in K and N, low in Mg and Na |
Peak incidence: 2-8 weeks postpartum; Cool-season grasses; Rapid onset |
| Wheat Pasture Poisoning |
Occurs in cattle grazing early-growth cereal crops (wheat, oats, rye, barley) |
Often concurrent hypocalcemia; May resemble milk fever; Any age affected |
| Winter Tetany |
Occurs in cattle on poor quality winter feeds (hay, corn stalks) with inadequate Mg supplementation |
Chronic energy deficiency; Triggered by stress (cold weather, handling); Subclinical hypomagnesemia for weeks |
| Milk Tetany (Calves) |
Affects 2-4 month old calves fed only milk or milk replacer; decreased Mg absorption with age |
Mg absorption efficiency drops from 87% (2-3 weeks) to 32% (7-8 weeks); Often post-diarrhea |
| Transport Tetany |
Occurs after stressful events: transport, handling, adverse weather, sudden dietary changes |
Stress-induced; Adrenaline shifts Mg intracellularly; Pre-existing borderline hypomagnesemia |
Diagnosis
Diagnostic Approach
Diagnosis is typically based on history, clinical signs, and response to treatment. Laboratory confirmation is often retrospective due to the emergency nature of the condition.
NAVLE TipFor POSTMORTEM diagnosis, vitreous humor Mg is the MOST RELIABLE sample - stable for up to 48 hours after death if temperature remains below 23 degrees C. Serum Mg from dead animals is UNRELIABLE due to muscle damage causing Mg leakage from intracellular stores, which falsely elevates levels.
Forage Risk Assessment
The tetany ratio (K/(Ca+Mg) calculated in milliequivalents) can assess forage risk. Values greater than 2.2 indicate high risk for grass tetany. Forages with less than 0.2% Mg, greater than 3% K, and greater than 4% N (25% crude protein) on dry matter basis are especially dangerous.
Differential Diagnosis
| Form |
Clinical Signs |
| Peracute/Acute (Tetanic Form) |
Sudden onset - animal may be found dead without premonitory signs
Throws head up, bellows loudly, gallops blindly in frenzied manner
Falls with severe paddling seizures (legs extended rigidly)
Opisthotonus (head thrown back)
Chomping of jaws, frothy salivation
Nystagmus, rapid eyelid fluttering, snapping retraction of third eyelid
Tachycardia (up to 150 bpm - audible without stethoscope)
Hyperthermia (up to 105 degrees F/40.5 degrees C from muscle activity)
Death within 30 minutes to several hours if untreated |
| Subacute (Paretic Form) |
Facial and shoulder muscle twitching
Hyperesthesia (hypersensitive to touch and sound)
Stiff gait, reluctance to move
Frequent urination
Unusual alertness, staring expression, erect ears
Nervous, excitable, may become aggressive (mistaken for rabies)
May progress to acute form if stressed or handled |
| Subclinical |
Depression of appetite
Decreased milk yield
Slight nervousness
Udder edema, anemia (rare)
Represents the "tip of the iceberg" - many subclinical cases for every clinical case |
Treatment
Hypomagnesemic tetany is a TRUE VETERINARY EMERGENCY. Death can occur within 30 minutes to several hours. Treatment must be initiated immediately, and affected animals should be handled with minimal stimulation to avoid triggering fatal seizures.
Emergency Treatment Protocol
High-YieldResponse to treatment is SLOWER than with hypocalcemia alone because it takes time to restore Mg levels in the CSF. Expect at least 1 hour for recovery. Relapses are common within 12-36 hours if oral Mg supplementation is not provided post-treatment.
NAVLE TipIf a veterinarian cannot reach the farm in time, an EMERGENCY field treatment is 200 mL of saturated (50%) MgSO4 (Epsom salts) solution given SUBCUTANEOUSLY. This raises blood Mg within 15 minutes. MgSO4 can also be given as an ENEMA but is toxic if given orally to a recumbent animal.
| Sample |
Normal Values |
Diagnostic Threshold for Tetany |
| Serum/Plasma Mg (cattle) |
1.8-2.4 mg/dL (0.75-1.0 mmol/L) |
Less than 1.2 mg/dL (less than 0.5 mmol/L) |
| CSF Mg |
0.62-0.81 mmol/L |
Less than 1.0 mg/dL (less than 0.4 mmol/L) |
| Urine Mg |
Detectable (reflects dietary excess) |
Undetectable (kidneys conserve Mg) |
| Vitreous Humor Mg (PM) |
0.84-0.90 mmol/L |
Less than 0.55 mmol/L (less than 1.34 mg/dL) |
| Serum Calcium |
8-10.5 mg/dL |
Often concurrently low (less than 8 mg/dL); Up to 80% have hypocalcemia |
Prevention
Prevention is the cornerstone of grass tetany management since treatment success depends heavily on early intervention. Begin supplementation 30 days before calving and continue through the high-risk period.
Magnesium Supplementation Strategies
Management Strategies
- Delay turnout to spring pasture until grass is 4-6 inches tall
- Provide hay when grazing lush pastures (slows passage, provides additional Ca/Mg)
- Limit grazing time to 2-3 hours/day on high-risk pastures initially
- Add legumes to pastures (higher Mg and Ca than grasses)
- Avoid excess K and N fertilization especially in spring; Soil test and apply dolomitic limestone
- Provide adequate salt (NaCl) - sodium enhances Mg absorption
- Graze less susceptible animals on high-risk pastures (dry cows, heifers, steers)
- Consider ionophores (monensin, lasalocid) - increase Mg absorption by 15-20%
| Condition |
Key Differentiating Features |
Diagnostic Test |
| Hypocalcemia (Milk Fever) |
Flaccid paralysis, NOT tetany; Cow is weak/lethargic not hyperexcitable; S-shaped neck; Usually periparturient dairy cows |
Serum Ca less than 5 mg/dL; Responds rapidly to Ca alone |
| Acute Lead Poisoning |
More seizure activity; Blindness common; GI signs (salivation, bruxism); History of lead source access |
Blood lead levels; Kidney/liver lead at necropsy |
| Polioencephalomalacia |
Usually young animals; Blindness with intact PLR (cortical); Dorsomedial strabismus; Responds to thiamine |
Response to thiamine; Cortical autofluorescence at necropsy (UV light) |
| Rabies |
Progressive course over days; Paralysis more common than excitement; Exposure history; Always fatal |
FA or IHC on brain (postmortem) |
| Tetanus |
Progressive stiffness; Sawhorse stance; Prolapsed third eyelid; Wound history; No frenzied episodes |
Clinical diagnosis; History of wound |
| Nervous Ketosis |
High-producing dairy cow; Bizarre behavior, licking, chewing; Ketotic breath; Usually 2-6 weeks postpartum |
Blood/urine ketones elevated |
Prognosis
Prognosis depends heavily on the time from onset to treatment. Animals down for more than 12 hours often have extensive muscle damage ("downer cow syndrome") and poor prognosis. Early intervention with combined Ca-Mg therapy generally has a favorable outcome, though relapses within 12-36 hours are common without continued Mg supplementation. Case fatality rates range from 5-30% depending on how quickly treatment is initiated.
| Step |
Treatment |
Details |
| 1. Sedation (if seizing) |
Chloral hydrate IV or sedative to control seizures before treatment |
Prevents fatal seizures during handling and IV administration |
| 2. IV Calcium-Magnesium |
400 mL of 40% calcium borogluconate
+ 50 mL of 25% magnesium sulfate
Given SLOWLY IV while monitoring heart |
Adult cow needs 1.5-2.25 g elemental Mg
MgSO4 is only 9.7% elemental Mg
Monitor for arrhythmias during Ca administration |
| 3. SC Magnesium |
120-400 mL of 25% MgSO4 SC (or 200 mL of 50% MgSO4 SC) |
Provides prolonged Mg absorption; IV effect lasts only 6-12 hours |
| 4. Rectal/Oral Mg (adjunct) |
MgCl2 enema: 60 g in warm water (large intestine absorbs Mg rapidly)
Once swallowing reflex returns: oral MgO or Mg gel |
Reduces relapse rate; Provides longer-term Mg supplementation |
| 5. Post-Treatment Care |
Prop in sternal recumbency
Leave undisturbed - NO stimulation
Remove calves temporarily (suckling may trigger relapse) |
Recovery takes at least 1 hour (time for CSF Mg to normalize)
WARNING: Treated cows may become extremely aggressive! |
| Method |
Dosage/Details |
Notes |
| High-Mag Mineral Mix (Preferred) |
12-14% Mg mineral mix
Target: 4 oz/head/day
Provides approximately 13-20 g Mg/day |
Use MgO source (not dolomitic limestone); Keep feeders full; Multiple locations in pasture |
| MgO on Hay (Causmag) |
60 g MgO/head/day sprinkled on hay |
MgO is unpalatable; Mix with molasses or feed to improve intake |
| Grain Mix |
5 lb MgO per 50 lb DDGs; Feed 1 lb/head/day (provides 22 g Mg) |
Ensures consistent intake; Good for problem herds |
| Water Treatment |
MgCl2 or MgSO4 at 3 g/L; Target 20-40 g Mg/cow/day |
Ensure no alternative water sources; Greater than 40 g/day MgSO4 may cause scouring |
| Intraruminal Boluses |
Slow-release Mg boluses; Effective for up to 90 days |
Ensures consistent delivery; Useful for extensive grazing systems |