Grass tetany and mineral supplementation

Producer Question from 2012

Q.  I have a pasture that cows seem to get grass tetany when they are grazing it. When and how long before I turn out on this pasture should I start supplementing with mineral? (May 30, 2012)

A.  When conditions for occurrence of tetany are suspected, cows should be provided mineral mixes containing 12 to 15 percent magnesium oxide and be consumed at 3 to 4 ounces per day. It is best for the supplements to be started at least 30 to 45 days ahead of the period of tetany danger so that proper intake can be established. Continue to feed the grass tetany mineral until the growth of the cool-season grass becomes consistent, which is usually by the middle of May in Eastern Nebraska. Because tetany can also occur when calcium is low, calcium supplementation should also be included. 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

Grass tetany, caused by magnesium deficiency, typically occurs in beef cows during early lactation and is more prevalent in older cows. The reason is thought to be that older cows are less able to mobilize magnesium reserves from the bones than are younger cows. Grass tetany most frequently occurs when cattle are grazing lush immature grasses (bromegrass) or small grains pastures and tends to be more prevalent during periods of cloudy weather. Symptoms include incoordination, salivation, excitability (aggressive behavior towards humans) and, in final stages, tetany, convulsions and death.

It is known that factors other than simply the magnesium content of the forage can increase the probability of grass tetany. High levels of potassium in forages can decrease absorption of magnesium and most lush, immature forages are high in potassium. High levels of nitrogen fertilization have also been shown to increase the incidence of tetany although feeding protein supplements has not. Other factors such as the presence of certain organic acids in tetany-causing forages have been linked with tetany. It is likely that combinations of factors, all related to characteristics of lush forage are involved.

Cows grazing lush small grain pastures should be fed mineral mixes containing calcium and magnesium.

Dr. Rick Rasby Dr. Rick Rasby, Professor of Animal Science
Animal Science, University of Nebraska–Lincoln, Lincoln, NE