Grass tetany is caused by magnesium deficiency and can occur in cattle grazing lush, growing cool-season pastures. It can occur 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 or small grains pastures and tends to be more prevalent during periods of cloudy weather. Symptoms include incoordination, salivation, excitability (aggressive behavior toward 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 a combination of factors, all related to characteristics of lush forage are involved.

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