Grass tetany is caused by magnesium deficiency. It typically occurs in beef cows during early lactation and is more prevalent in older cows. Calves and yearling cattle rarely are a concern. The reason is that older cows are less able to mobilize magnesium reserves from the bones than younger cows are. Grass tetany most frequently occurs when cattle are grazing lush, immature grasses such as brome grass or small grains pastures. It tends to be more prevalent during periods of cloudy weather, as in early spring. Symptoms include incoordination, salivation, excitability such as aggressive behavior towards humans, and, in final stages, tetany, convulsions and death. Feeding a mineral supplement containing more than eight to 10 percent magnesium for two to three weeks prior to grazing lush forages is recommended. See http://www.oznet.ksu.edu/library/crpsl2/s115.pdf> (PDF)a> for more information.
Factors other than the magnesium content of the forage alone 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 12 to 15 percent magnesium oxide, consumed at three to four 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. Because tetany can also occur when calcium is low, calcium supplementation should 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.
Cows grazing lush, small-grain pastures should be fed mineral mixes containing both calcium and magnesium.
