Cattle producers looking to use forage as cheap winter feed for their beef cows need to practice sound mineral feeding protocols to reduce the risk of grass tetany next spring.

Grass tetany, caused by magnesium deficiency typically is not a major problem in Oklahoma although occasional cases are reported, typically occurs in beef cows during early lactation and is more prevalent in older cows.

“That means at and after calving time next January, February and March, grass tetany problems could occur for some producers,” said Glenn Selk, Oklahoma State University Cooperative Extension emeritus animal scientist and editor of the weekly OSU Cow-Calf Newsletter.

When conditions for occurrence of tetany are suspected, cows should be provided mineral mixes containing 12 to 15 percent magnesium and be consumed at three to four ounces per day.

“It is best for the supplements to be started a couple of months ahead of the period of tetany danger so that proper intake can be established,” Selk said. “Because tetany also can occur when calcium is low, calcium supplementation should be included as well.”

Symptoms of tetany from deficiencies of both minerals are indistinguishable without blood tests and the treatment consists of intravenous injections of calcium and magnesium gluconate, which supplies both minerals.

“The reason older cows are thought to be vulnerable is due to the fact that they are less able to mobilize magnesium reserves from the bones than are younger cows,” Selk said.

Grass tetany most frequently occurs when cattle are grazing lush immature grasses or small grains pastures and tends to be more prevalent during periods of cloudy weather. Symptoms include incoordination, salivation, excitability – including aggressive behavior towards humans – and in the final stages, convulsions and death.

In addition, factors other than the magnesium content of the forage can increase the probability of grass tetany. High levels of potassium in forages can decrease absorption of magnesium. Most lush, immature forages are high in potassium.

High levels of nitrogen fertilization also have been shown to increase the incidence of tetany. However, feeding protein supplements has not. Other factors such as the presence of certain organic acids in lush forages have been linked with the occurrence.

“It is likely that a combination of factors, all related to characteristics of lush forage, are involved,” Selk said.

Additional information about mineral nutrition of grazing cattle is available online at by downloading OSU Extension Circular E-861, “Vitamin and Mineral Nutrition of Grazing Cattle.”