COLUMBIA, Mo. – If you’ve ever seen a cow suddenly collapse in early spring, seemingly out of nowhere, you might have witnessed a case of grass tetany, says University of Missouri Extension state beef nutritionist Eric Bailey.

This metabolic disorder, historically linked to magnesium deficiency, can be a problem for cattle grazing lush cool-season grasses. Understanding the causes, symptoms and prevention strategies for grass tetany can help cattle producers avoid costly losses, says MU Extension state forage specialist Harley Naumann.

What is grass tetany?

Grass tetany, also known as grass staggers, is caused by low levels of magnesium in the blood. It is most commonly observed in lactating cows grazing early spring growth of cool-season grasses like tall fescue. These pastures, while high in energy and protein, often have low magnesium levels and excessive amounts of potassium, which can interfere with magnesium absorption.

Environmental stressors such as frost and drought further alter the mineral composition of forages. Spring-calving cows are especially vulnerable due to the high magnesium demands of lactation, which can quickly deplete their reserves.

Recognizing the symptoms

Grass tetany doesn’t always announce itself with clear warning signs, but knowing what to look for can make the difference between saving and losing an animal, says Bailey. Early symptoms include:

• Increased excitability.
• Teeth grinding.
• Excessive salivation.
• Muscle spasms.
• Loss of body control.
• Collapse and death in severe cases.

Without immediate intervention, death can occur within hours.

Traditional solutions: Do they work?

For years, the standard recommendation was to supplement cattle diets with additional magnesium. Magnesium oxide is commonly added to mineral mixes or mixed into feed to increase intake. However, magnesium supplements are often unpalatable, leading to inconsistent intake among cattle, Bailey says. Additionally, despite supplementation, some cases of grass tetany still occur, indicating other factors are at play.

In acute cases, intravenous magnesium and calcium solutions administered by a veterinarian can treat the condition. However, prevention remains the best approach since once symptoms become severe, survival rates decline significantly.

The role of potassium and sodium in grass tetany

Research suggests potassium and sodium play a crucial role in magnesium absorption, and their balance in forages may be as important as magnesium itself.

Potassium: The hidden culprit

While potassium is essential for plant growth, high levels in pastures can exacerbate magnesium deficiency. Potassium competes with magnesium for absorption sites in the rumen, so even when magnesium levels in forage are adequate, excessive potassium can block magnesium from leaving the rumen, increasing the risk of grass tetany.

Fertilization practices can further contribute to the problem. Heavy potassium fertilization, especially in early spring, may increase the risk of grass tetany by skewing the mineral balance in pastures, says Naumann.

Sodium’s overlooked role

Sodium plays a less obvious but equally important role in magnesium absorption. Sodium helps transport magnesium across the rumen wall, improving its availability to the animal. However, freeze-damaged grasses often lose significant amounts of sodium, potentially reducing magnesium uptake even further.

Low sodium intake in cattle diets can compound the issue. Providing free-choice salt or increasing the amount of salt in your free-choice mineral can support magnesium absorption and be a cost-effective strategy for reducing the risk of grass tetany, says Bailey.

What about phosphorus?

Soil phosphorus may play a critical role in preventing grass tetany in ruminants. MU research indicates that fertilizing tall fescue pastures to reach adequate soil phosphorus levels enhances the availability of magnesium in the forage.

This improvement in magnesium availability helps ruminants maintain proper magnesium levels in their diets, which is crucial for preventing grass tetany.

Prevention strategies

Managing grass tetany requires a multifaceted approach. Bailey and Naumann offer some key strategies:

Provide high-magnesium supplements: Make sure cows receive sufficient magnesium, especially during high-risk periods in early spring. Most “hi-mag” minerals have 6%-10% magnesium.

Increase salt consumption: Most mineral supplements contain 15%-30% salt. Choose a mineral that is closer to 30% salt, if possible. If not, consider mixing a bag of salt and a bag of mineral for cows in March and April.

Limit excessive potassium fertilization: Be mindful of pasture fertilization practices. Avoid heavy applications of potassium in early spring, which can suppress magnesium absorption in cattle.

Graze mixed pastures: Introducing legumes such as clover into pastures can help balance mineral content, as they generally contain higher levels of magnesium than grasses alone.

“Grass tetany remains a significant challenge for cattle producers, but with careful management and strategic supplementation, its impact can be minimized,” says Bailey. “Understanding the interplay between magnesium, potassium and sodium in forage systems is key to developing effective prevention strategies.”

For more information, contact your local MU Extension agronomy or livestock specialist.