The Digestive Tract
Prevent grass tetany in beef cows grazing spring forages.
March 20, 2026
Grass tetany is a costly and often sudden challenge when beef cows transition onto lush, cool‑season pastures in the spring. Although the condition can be fatal, it is also highly preventable with thoughtful planning and consistent magnesium (Mg) supplementation. Preparing for spring turnout by ensuring adequate intake of high‑magnesium minerals remains one of the most effective strategies producers have to maintain magnesium status and prevent deficiency during this high‑risk window.
Understanding grass tetany
Grass tetany, which is more accurately described as secondary magnesium deficiency, is most associated with cows grazing lush, rapidly growing cool‑season forages. While low magnesium is the immediate cause, the disorder is best understood as a problem of impaired magnesium absorption rather than simply inadequate dietary supply.
Magnesium is essential for normal nerve transmission and muscle function. This explains the classic clinical signs of acute grass tetany, which may include excitable or aggressive behavior, nervousness, reduced feed intake, muscle twitching (especially around the face, ears and flanks), blind staggering, convulsions and — in advanced cases — death.
Unfortunately, many affected cows are found dead with few or no preceding symptoms.
Why spring forages increase risk
Despite its name, grass tetany is driven largely by the mineral composition of lush spring forage, particularly high nitrogen (N) and potassium (K) levels, which interfere with magnesium absorption across the rumen wall, even when forage magnesium appears adequate. This risk is heightened in improved pastures that have received nitrogen and/or potassium fertilization.
Lush spring forage also tends to be high in moisture and rapidly fermented, reducing effective magnesium intake. Cool, cloudy and wet weather can further limit dry-matter intake and increase metabolic stress, compounding the risk.
At greatest risk
Lactating cows, especially older mature cows, are the most vulnerable. Magnesium requirements rise sharply with lactation, increasing from roughly 0.12% of diet dry matter (DM) in late gestation to about 0.20% during lactation. Demand peaks around calving, as colostrum contains nearly three times the magnesium concentration of mature milk.
Key forage risk factors
Grass tetany risk is driven by the balance of magnesium, calcium (Ca), nitrogen and potassium — not magnesium concentration alone. The disorder is most associated with forages containing low magnesium (< 0.15%), low calcium (< 0.40%) and high potassium (> 2.5%).
Even nutritionally “adequate” forages can induce magnesium deficiency when these imbalances reduce ruminal absorption.
Supplementation
Supplementing additional magnesium is the cornerstone of grass tetany prevention. During high-risk periods, minerals should typically contain 10%-15% magnesium, with a target intake of approximately 4 ounces per cow per day, alongside adequate calcium and salt levels.
When evaluating commercial mineral products, pay close attention to potassium content. Supplementing additional potassium in the mineral or supplement is counterproductive and increases tetany risk.
Magnesium oxide is the most used magnesium source and is effective. However, it is bitter and unpalatable (i.e., cows don’t necessarily like to eat it). When feeding a high-magnesium mineral, reduced mineral intake is a common challenge, making intake management just as important as mineral formulation.
To improve and support intake, introduce high-magnesium mineral several weeks before turnout to allow cows to adapt. If intake is low, improve intake by adding 1 to 2 pounds (lb.) of dried distillers’ grains per 50 lb. of mineral.
Other considerations
Providing dry hay alongside lush spring pasture can help slow rumen passage rate, improve rumen function and enhance mineral absorption (particularly during cold, wet weather). Incorporating legumes into pasture systems can also help dilute potassium levels and improve overall mineral balance.
Grass tetany remains one of the most preventable causes of death in grazing beef cows. The condition is driven not only by magnesium deficiency, but by forage mineral imbalances that impair magnesium absorption during periods of high physiological demand.
By planning ahead, introducing high-magnesium minerals before turnout and closely monitoring mineral consumption, you can reduce the risk of grass tetany during the critical spring grazing period.
Editor’s note: “The Digestive Tract” is a regular column focused on nutrition for the beef cattle life cycle. Travis Mulliniks is the Glenn & Mildred Harvey Professor of Beef Cattle Management, as well as the associate head of the Department of Animal and Rangeland Sciences, at Oregon State University.
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