Grass tetany is a nutrition-related health condition that generally occurs when cows are grazing cool-season grasses in early spring or wheat pasture in the fall.

In reality, tetany can occur in some form at any time of the year. What's needed is feed with low magnesium levels and high calcium and potassium levels, or levels that are out of proportion during stress situations.

“Winter tetany” occurs in wintertime when cows are fed harvested feeds. Grass hays, including cereal grain hays, tend to be low in magnesium and need to be properly supplemented. Feeding legume hay may alleviate the problem, but it will not fix an immediate problem.

Non-legume hays may average 0.18% magnesium, but some hays may be as low as 0.03 to 0.05% magnesium on a dry matter basis. Forage levels below 0.18% magnesium are marginal, while levels less than 0.12% will predispose an animal to winter tetany. Low calcium levels and high potassium levels in the feed may also be contributing factors.

Cereal-grain hays and grass hays may be high in potassium, but calcium and magnesium levels may be low. A plant magnesium level of 2-2.5% is considered a safe level. Normal blood levels are 1.7-3.2 mg./dl in mature cows.

Mature animals are far more susceptible than younger animals because of their inability to mobilize magnesium from their bones to meet their requirements. This is especially critical during lactation.

Cows with young calves are more at risk than steers, heifers, dry cows and cows with calves more than 4 months of age, but heavy milking cows are the most susceptible to tetany. Also, cows that develop tetany are more prone to do so again.

Tetany-afflicted cows may show signs of nervousness, reduced feed intake, reduced milk production and muscular twitching along the face, shoulder and flank. It progresses to staggering, when cattle fall on their sides with the head thrown back, excessive salivation and grinding of the teeth. The time between the first signs and death may be as short as four to eight hours.

Therefore, treatment must begin as soon as possible. It's important to quickly get some form of magnesium into the animal and relocate the cattle until preventive measures can be taken.

The treatment of choice is an intravenous (IV) injection with calcium-magnesium gluconate because it gives the most rapid response. Drenching the animal with a source of magnesium, such as magnesium sulfate, or using a rectally-infused enema of magnesium sulfate are other options.

With an IV treatment, the blood levels rise rapidly but fall back to the previous level within three to six hours, so additional measures must be taken.

To prevent winter tetany on tetany-prone grass or harvested feeds (grasses, cereal grain hays), feeding alfalfa or other legume hay may reduce the risk. Cows at this time of year (precalving) should always have a mineral source available to them that includes a source of magnesium and calcium.

If a problem is suspected, additional magnesium may be added. In tetany-prone situations, chelated magnesium, which is more readily absorbed, may be called for. Remember, magnesium must be provided on a daily basis to prevent tetany problems.

Producers are the best defense against tetany. As part of management, a complete feed analysis should have been run on the feed or pastures at some time. If a producer has a known problem with tetany or suspects a problem, the feeds in question should be analyzed specifically for magnesium, calcium, nitrate and potassium.

When the feed analysis information is known, a “tetany ratio” can be calculated by the producer, vet or nutritionist to see if the forage is tetany-prone. The formula is:

tetany ratio = % potassium

% calcium + % magnesium

If the ratio is greater than 2.2, then the forage is tetany prone and preventive measures should be taken.

If grazing wheat pasture, crested wheat or tall fescue — or feeding straw, cornstalks or other low-quality, tetany-prone roughage — the producer should be prepared ahead of time to plan preventive measures and reduce losses.

David Wieland is a nutrition consultant specializing in cow/calf, feedlot and horses. Based in Shepherd, MT, he also publishes a subscription newsletter. Contact him at 406/373-5512 or