Because of drought and short forage supplies in 2011-2012, more than ever Oklahoma producers are looking to wheat pasture for winter feed for mature beef cows. Grass tetany, caused by magnesium and/or calcium deficiency, does not seem to be a major problem in Oklahoma although occasional cases are reported. It 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 and calcium reserves from the bones than are younger cows. 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 lack of coordination, salivation, excitability (aggressive behavior towards humans) and, in final stages, tetany, convulsions and death. If you see these symptoms contact your local veterinarian immediately. Afflicted cows can recover if treated soon enough. 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.
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 a combination of factors, all related to characteristics of lush forage are involved.
When conditions for occurrence of tetany are suspected, cows should be provided mineral mixes containing 8 to 15 percent magnesium and be consumed at 3 to 4 ounces per day. It is best for the mineral supplements to be started at least a month ahead of the period of tetany danger so that proper intake can be established. Because tetany can also occur when calcium is low, calcium supplementation should also be included. Much more information about mineral supplementation and suggested mineral mixes to prevent tetany can be found in the Oklahoma State University Extension Circular E-861: Vitamin and Mineral Nutrition of Grazing Cattle.
Source: Glenn Selk, Oklahoma State University Emeritus Extension Animal Scientist