Aerial view of cows in a grassy field

Spring brings a higher risk of grass tetany

March 20, 2023 3:00 p.m.

In spring, as grasses and cereal grains are immature and rapidly growing, the risk increases that mature cows can become sick and, in extreme cases, even die from a syndrome called grass tetany.

What is grass tetany?

Grass tetany is a magnesium deficiency syndrome which primarily affects mature cattle grazing on lush, succulent forages. Grass tetany and milk fever (caused by sudden calcium deficiency) have similar symptoms, but cows with milk fever are lethargic, while cows with tetany exhibit more aggressive and violent symptoms. Cows can have calcium and magnesium deficiency at the same time, which is why supplements containing both minerals are often used.

Magnesium is a mineral that is necessary in nerve function and muscle contraction. Mature animals are far more susceptible to grass tetany than younger ones because of their inability to mobilize magnesium from their bones to effectively balance water in their bodies, called body fluid homeostasis.

The most consistent clinical sign of grass tetany is hypomagnesemia, and typical grass tetany is frequently described as hypomagnesemic tetany, an acute neurological condition due to low dietary intake of magnesium.

Forages or complete rations with magnesium (Mg) levels of 0.2 to 0.25 percent constitute a safe level to prevent tetany.

The following criteria have been used to evaluate tetany danger in forage for lactating or pregnant beef cows.

  • Tetany prone less than 0.12% Mg
  • Marginal 0.12 to 0.18% Mg
  • Adequate more than 0.18% Mg

Cattle with grass tetany become excitable, develop muscle tremors and exhibit difficulty walking and breathing which can lead to convulsions, recumbent paddling and thrashing, coma, and death.

When to watch for grass tetany

Tetany usually occurs near the time when a cow is giving birth until approximately two months postpartum, around peak milk production. The frequency usually increases with older, high-milking cows. Tetany occurs most frequently in beef cows grazing pastures, and cool, rainy weather generally accelerates the occurrence of grass tetany.

Preventing grass tetany

Rancher holding out hay to cattle in feedlot


Although tetany is caused by a metabolic deficiency of magnesium and calcium, it is difficult to prevent it with short term supplemental magnesium and calcium in mineral mixes. These minerals, especially magnesium, need to be supplied daily. Cattle should be fed a high magnesium mineral mix a month or more prior to spring turnout and while grazing tetany-prone pastures.

Payback Hi-Mag minerals are an ideal option for supplementing magnesium to beef cattle to assist in preventing magnesium deficiency and grass tetany.

To learn more about how to effectively reduce your risk of grass tetany in the spring, reach out to your local Payback dealer or CHS animal nutrition sales consultant. Find your local Payback Dealer.