Lush, green, rapidly growing grasses present a considerable risk for grass tetany, regardless of season. The combination of high potassium and low magnesium and calcium in these forages is thought to be the primary cause of grass tetany.
Grass tetany tends to occur most often in the early- to mid-spring following a cool period (< 60° F), when grasses are lush and growing rapidly. It can also occur in the fall in new growth of both small cereal grains (e.g., wheat, oats, rye) and cool-season grasses, especially crested wheatgrass.
One method to minimize grass tetany risk is to delay turnout until the forage is more mature and is past the rapid growth stage (at least 6 inches tall). Grazing non-lactating animals on high-risk pastures can reduce or eliminate problems with grass tetany. Grass tetany occurs most frequently in cows that are nursing young calves. Growing cattle, dry cows, or cows with calves older than four months of age are generally at lower risk for grass tetany.
One of the most effective prevention strategies involves supplementing magnesium while the cattle are grazing high-risk forages. Cattle should receive at least .5 to 2 oz of supplemental magnesium per head per day to prevent tetany. Magnesium is not effectively stored in the body; beginning supplementation more than 2 weeks prior to the grazing period is not advantageous.
Cattle should be observed at least twice a day when they are first turned out on spring pasture. There are some clinical signs that can be used as indicators of grass tetany; however, the first sign is usually a dead animal. An animal with grass tetany frequently stops grazing and appears nervous or high-headed. It will likely respond quickly and erratically to the slightest stimulus. As the condition advances, it may begin to stagger or experience twitching of the skin, followed by stiffening of the muscles and violent jerking convulsions with the head pulled back. It may also lie down and “pedal” with its legs and chew to the point of frothing at the mouth. If the convulsions subside, the animal may appear more relaxed; however, noises or touching of the animal, even to administer a treatment, may result in violent reactions.
In live animals, decreased serum magnesium concentrations may be indicative of inadequate magnesium intake, therefore increasing the risk of grass tetany. In animals currently undergoing convulsions, magnesium may be falsely elevated due to magnesium leakage out of damaged muscle cells. In dead animals, magnesium levels in urine, cerebrospinal fluid (up to 12 hours after death), and vitreous fluid (from the rear chamber of the eyeball; up to 48 hours after death) may be measured to diagnose grass tetany.