Grass tetany — known formally as hypomagnesaemia or hypomagnesaemic tetany — is a metabolic disease of cattle caused by critically low blood magnesium levels. It is responsible for significant cattle deaths across the temperate and subtropical regions of Australia every year, concentrated in specific seasonal windows and cattle classes that are highly predictable once you understand the underlying biology. Unlike many livestock diseases where early diagnosis and treatment give good outcomes, grass tetany can kill within hours of the first clinical signs, and clinical signs can be subtle before the animal enters the acute phase. Prevention is the only reliable management strategy.

What causes grass tetany?

Magnesium is an essential mineral for cattle, required for normal nerve and muscle function. Unlike calcium, which the body can draw from bone reserves during periods of dietary insufficiency, cattle have no practical storage reservoir of magnesium — they depend on a continuous daily dietary supply. When dietary magnesium intake falls below the body's requirements, blood magnesium levels drop and neuromuscular function becomes impaired. Below a critical threshold, spontaneous and uncontrolled nerve discharge causes the muscle tremors, spasm, and convulsions that characterise clinical tetany.

The specific conditions that drive grass tetany are well-understood. The disease occurs when cattle are grazing pastures that are simultaneously high in potassium and/or nitrogen and low in available magnesium — a nutritional combination that dramatically reduces the proportion of dietary magnesium that the rumen can absorb into the bloodstream. The trigger is not simply low-magnesium pasture; it is the interaction between specific pasture chemistry and the cow's physiological state that creates the problem.

The high-risk scenario: lush, rapidly growing grass pasture following rain or fertilisation with nitrogen or potassium fertiliser, grazed by lactating cows in early-to-mid lactation. Rapidly growing grass has high crude protein, high potassium, low structural carbohydrate, and critically — low magnesium availability. Lactation dramatically increases the cow's daily magnesium requirement (a cow producing 10 litres of milk per day loses 0.25 to 0.30 g of magnesium in that milk alone, on top of daily maintenance requirements). The combination of increased demand and reduced dietary availability creates a negative magnesium balance that can develop into clinical tetany within days.

When and where grass tetany occurs in Australia

In temperate southeastern Australia, grass tetany risk is highest in autumn (March to May) and spring (August to October), coinciding with the periods of rapid pasture growth following rain or irrigation after dry periods. Autumn is the most dangerous period because lactating cows are most common (spring calving systems have cows at peak lactation in March to April), and rapidly growing autumn pasture on recently fertilised paddocks creates exactly the high-potassium, low-magnesium conditions described above.

Winter-calving operations in subtropical Queensland face their highest tetany risk in spring (August to September) when warm-season grass growth accelerates. Properties in the high-rainfall zone of Victoria, southern NSW, Tasmania, and the southeast of South Australia — particularly those running cows on ryegrass and clover pastures — are in the highest-risk regions in Australia. Kikuyu-dominant pastures in coastal subtropical NSW and Queensland also carry significant tetany risk despite their warm-season dominance, because kikuyu is notoriously low in available magnesium.

Risk is highest on: improved pastures recently fertilised with potassium or nitrogen, pastures growing after cold, cloudy weather followed by rapid warming, paddocks with heavy clover content (high nitrogen fixation drives potassium uptake), and any situation where cattle are moved from poorer dry pasture onto a lush improved paddock suddenly.

Which cattle are highest risk?

Not all cattle are equally susceptible. The highest-risk class by a significant margin is mature lactating cows in early to mid lactation — typically 1 to 2 months after calving. Their daily magnesium requirement is highest (combined maintenance plus milk production), they have the least body fat buffering (magnesium is mobilised with fat reserves to a small degree), and they are most likely to be on lush early-growth pasture. Older cows (6+ years) appear to have reduced gut magnesium absorption efficiency and are more susceptible than younger cows at equivalent lactation stage.

Dry cows, steers, and young cattle are at substantially lower risk — their magnesium demands are lower and they are rarely on lush high-risk pasture in circumstances that also deny adequate dietary magnesium. However, any class of cattle can develop tetany in extreme conditions — this is not exclusively a lactating cow disease, just one where lactating cows are disproportionately affected.

Clinical signs: what to look for

Grass tetany progresses rapidly once clinical signs begin, often from first observation to death within 2 to 4 hours without treatment. Recognising the earliest signs is critical because animals caught in the early stage have an excellent prognosis with prompt treatment; animals found in convulsions or coma have a much poorer outlook.

Early signs (hours before convulsions): a cow that is hanging back from the mob, appears nervous or hypersensitive to noise and disturbance, is walking with a stiff or unusual gait, or is twitching her ears and facial muscles more than usual. She may have a slightly wild-eyed appearance and be reluctant to let you approach normally. These signs are easy to miss because they are non-specific and the cow is still standing and alert.

Intermediate signs: incoordination that becomes obvious — stumbling, weaving, and apparent difficulty walking in a straight line. Muscle tremors visible in the flanks and hindquarters. Increased sensitivity — startles strongly to sound or touch, which can trigger a brief spasm. Still standing but clearly unwell.

Acute signs: collapse followed by paddling movements with all four legs, severe muscle spasms across the body, opisthotonos (head thrown back), and frantic eye movement. Convulsive episodes may be triggered by noise or touch. Between convulsive episodes the animal may be in a quiet coma — do not be misled into thinking recovery is occurring.

When you see a cow in this condition, treat for grass tetany immediately — do not wait for a definitive diagnosis. Delay costs lives.

Emergency treatment

The first-aid treatment for grass tetany is subcutaneous (under the skin) injection of calcium-magnesium solution, which provides both magnesium and calcium (since hypocalcaemia often accompanies hypomagnesaemia). The standard first-aid product for Australian producers is 400 mL of calcium-magnesium borogluconate (Calcigol CMB or equivalent) given subcutaneously across four injection sites (100 mL per site) on the neck or shoulder. This delivers magnesium through slow subcutaneous absorption rather than intravenous delivery — IV administration of magnesium by an untrained operator is dangerous because the rate of magnesium entry into the bloodstream can cause cardiac arrest.

Call your vet immediately when you find an affected cow. Describe the signs clearly — a vet will typically guide you through subcutaneous administration over the phone if they cannot attend immediately, and will follow up with IV administration if the cow does not respond to subcutaneous treatment. Keep at least 4 to 6 bottles of a registered calcium-magnesium product on hand during high-risk periods — you cannot drive to a produce store when a cow is convulsing in the paddock.

After treatment, move recovered animals into a paddock with lower tetany risk (ideally one with poor or native pasture rather than high-potassium improved pasture) and monitor closely for 24 to 48 hours. Relapse is common if the underlying dietary magnesium deficit is not addressed.

Prevention: magnesium supplementation programs

Prevention is the only reliable strategy for grass tetany management — by the time clinical signs appear, the disease has already progressed to a life-threatening stage. Effective prevention requires providing supplementary magnesium to at-risk cattle during the risk period, in a form and dose that cattle actually consume consistently.

Magnesium oxide (causmag) in loose lick: the most widely used supplementation method in Australian beef operations. Magnesium oxide (typically 50 to 55% elemental magnesium) mixed into a loose mineral lick or dusted onto pasture. The target intake is 30 to 60 grams of magnesium oxide per cow per day — equivalent to 17 to 33 g of elemental magnesium. The key limitation of loose lick supplementation is variability in voluntary intake: dominant cows may consume more than the target, subordinates less, and cattle generally consume mineral licks inconsistently from day to day. Set out enough lick points that subordinate animals can access the supplement without competition.

Magnesium oxide dusted on pasture: broadcasting causmag at 15 to 30 kg per hectare across the pasture using a spreader or modified fertiliser spreader. Cattle consume the magnesium as they graze — distribution is more even than lick supplementation because dominant animals cannot monopolise a single source. More expensive and labour-intensive than lick, but more reliable for consistent intake in mob situations.

Magnesium bullets/rumen boluses: slow-release magnesium bullets that sit in the rumen and dissolve over 30 to 90 days, providing consistent daily magnesium. More expensive upfront than lick or dusting but provide reliable continuous supplementation without the intake variability of loose lick. Well-suited to small mobs of high-value breeding cows during defined high-risk windows.

Begin supplementation 2 to 3 weeks before the predicted onset of tetany risk, and continue until risk conditions have passed — typically until the rapid-growth flush ends and pasture matures. Do not wait until the first death in the mob to begin supplementing.

For more on cattle health management, see our Cattle Health Guide. For seasonal management planning, use our Livestock Seasonal Calendar.

For more, see our Bloat in Cattle: Causes, Prevention and Emergency Treatment for Australian Producers and Sick Cattle Symptoms: 10 Signs That Need Attention on Your Australian Property.