Grass tetany is a potentially fatal metabolic disorder in cattle and sheep caused by dangerously low magnesium levels in the blood. It strikes most often in spring and fall when livestock graze on lush, fast-growing pastures that are high in potassium but low in magnesium. Without rapid treatment, an affected animal can die within hours.
Also called hypomagnesemia or “grass staggers,” the condition costs the Australian beef industry alone more than $20 million per year in livestock losses, and it’s a significant concern for cattle and sheep producers worldwide.
How Low Magnesium Causes Tetany
Magnesium plays a critical role in keeping nerve and muscle cells stable. When blood magnesium drops, so does the magnesium concentration in the fluid surrounding the brain and spinal cord. At low enough levels, neurons begin firing spontaneously, triggering uncontrolled muscle contractions, the hallmark “tetany” that gives the condition its name. This is why affected animals show staggering, muscle twitching, and eventually violent convulsions.
Unlike calcium, which the body stores in large quantities in bone and can mobilize when needed, cattle have very limited magnesium reserves. They depend on a steady daily intake from their diet. If absorption drops or dietary supply falls short even briefly, blood levels can plummet fast.
Why Certain Pastures Are Dangerous
The real problem usually isn’t that pasture grass contains zero magnesium. It’s that high levels of potassium in the forage block magnesium from being absorbed in the rumen. Cool, wet weather drives rapid grass growth, and those young, lush plants tend to accumulate potassium while remaining relatively low in magnesium and calcium.
Nitrogen fertilization makes the situation worse. Adding nitrogen to pastures increases the soil’s ratio of potassium relative to calcium and magnesium. When that ratio climbs above a critical threshold, the forage becomes a grass tetany risk. Researchers use a formula called the tetany ratio, expressed as potassium divided by the sum of calcium and magnesium (K/(Ca + Mg)). A forage ratio above 2.2 is considered dangerous, meaning the potassium content is high enough to interfere with magnesium absorption in the gut.
Soil testing offers an early warning. Soil K/(Ca + Mg) ratios above 0.09 are classified as dangerous, while ratios between 0.07 and 0.09 fall into the hazardous-to-critical range. Ratios below 0.06 are generally safe. These numbers let producers assess risk before turning animals onto a pasture, rather than waiting for clinical signs.
Which Animals Are Most at Risk
Lactating beef cows in their first few weeks on spring pasture are the classic victims. Milk production drains magnesium from the body at high rates, and older cows absorb magnesium less efficiently than younger ones. A mature cow nursing a calf on lush spring grass is, in effect, losing magnesium faster than she can replace it.
Sheep are also susceptible, particularly ewes in late pregnancy or early lactation. Cold, wet, windy weather increases the risk in both species because stress hormones and reduced grazing time compound the magnesium deficit. Animals that have been transported, yarded, or otherwise stressed are more vulnerable as well.
Signs to Watch For
Grass tetany can progress from subtle to fatal remarkably quickly. Early signs are easy to miss: nervousness, reduced grazing, twitching ears or facial muscles, and a stiff, uncoordinated gait. An animal may separate from the herd or appear unusually alert and jumpy.
As magnesium levels drop further, the animal staggers, falls, and develops full-body muscle spasms. In the final stage, the animal lies on its side with legs paddling, head thrown back, and may experience violent seizures. Death can follow within minutes to hours once convulsions begin. Some animals are simply found dead in the paddock with no observed warning signs at all, particularly if the drop in magnesium was rapid overnight.
Emergency Treatment
An animal showing signs of grass tetany needs immediate veterinary attention. Treatment involves intravenous solutions containing both magnesium and calcium, administered slowly. Recovery from grass tetany is slower than from a pure calcium deficiency because it takes time for magnesium levels to rebuild in the fluid around the brain and spinal cord.
One critical detail: affected animals must be kept as calm and unstimulated as possible during and after treatment. Noise, rough handling, or sudden movements can trigger fatal seizures even while treatment is underway. If you find a down animal you suspect has grass tetany, approach quietly, minimize stress, and get veterinary help on the way before attempting to move or handle it.
Prevention Through Supplementation
Because treatment is difficult and often comes too late, prevention is the real strategy. Cattle need magnesium at 0.10% to 0.20% of their total diet on a dry matter basis. During high-risk periods, supplemental magnesium oxide is the most common and cost-effective way to fill the gap.
The challenge is palatability. Magnesium oxide is bitter, and cattle will avoid it if given the choice. Mixing it into a molasses lick, grain supplement, or loose mineral mix improves intake. Some producers add it to water troughs, though this requires careful management to ensure consistent dosing. The key is getting every animal in the herd to consume enough magnesium daily, since even a few days without adequate intake can push a high-risk cow into danger.
For feedlot cattle, research has tested magnesium oxide inclusion rates ranging from 0.25% to 0.75% of the diet without toxicity concerns. Toxicity is generally not a risk in beef cattle at dietary levels below 1.0%, so there’s a wide margin of safety for supplementation.
Pasture and Soil Management
Supplementation addresses the animal side of the equation, but managing pastures reduces the underlying risk. Avoiding heavy potassium and nitrogen fertilization on pastures grazed by lactating cows in spring is one of the most effective steps. Applying magnesium-containing fertilizers like dolomite can help rebalance soil mineral ratios over time.
Maintaining a diverse pasture with legumes mixed into the grass also helps, since legumes generally have higher calcium and magnesium content and lower potassium than grasses. Delaying turnout onto lush spring pastures until grass has matured slightly reduces risk, as older growth tends to have less extreme potassium levels. Providing hay alongside fresh pasture during the transition period gives animals an alternative forage source with a safer mineral profile.
Producers in high-risk areas benefit from routine soil and forage testing in early spring and again in fall, the two peak danger periods. Knowing the tetany ratio of your pastures before the season starts gives you time to adjust supplementation, fertilization, or grazing plans rather than reacting to a dead cow in the paddock.