Winter Grass Tetany Can Be a Problem for Livestock

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Grass tetany is considered a problem that usually occurs when cattle or sheep are eating lush, spring grass or annual cereal forages such as rye, wheat or triticale; but it can also occur when cattle are being fed harvested forages.

Grass tetany, sometimes called grass staggers or hypomagnesaemia, is a metabolic disorder of cattle related to a deficiency of magnesium (Mg). Magnesium is a critical mineral to the nervous system and muscle function. Low levels of magnesium can result in cattle that exhibit hyper-excitability, reduced feed intake, and muscle twitching, especially around the face and ears. Cattle may also appear uncoordinated and walk with a stiff gait.

Grass tetany is most often associated with cattle grazing immature cool season grasses or lush annual forages. However, tetany can also occur during the winter when cattle are being fed grass hay, alfalfa hay or annual forages harvested for hay. This is especially true if these forages are being fed in a dry lot situation where they are the only source of feed.

Grass, alfalfa and cereal grains harvested for hay can be low in magnesium. A mineral analysis showing less than 0.15% magnesium in hay is considered low. When hay is low in magnesium and calcium while being simultaneously high in potassium, tetany can occur.

Forages likely to cause grass tetany are often borderline to low in magnesium and sodium while having excess levels of potassium. Because high potassium levels interfere with magnesium absorption, it’s the excess potassium that induces tetany. An imbalance of potassium, calcium and phosphorus in feed can hinder magnesium absorption from the digestive system into the bloodstream, magnifying the problem of a low intake of magnesium. Sodium is important in transporting magnesium into cells, so it is crucial to provide adequate sodium (salt) to insure proper magnesium utilization.

To prevent winter tetany from harvested forages, consider the following:

1. Test hay for mineral concentrations to identify if an imbalance of magnesium, potassium and calcium is present.

2. If hay tests low in magnesium and calcium and high in potassium, consider feeding a high magnesium mineral supplement.

3. Examine the concentration of potassium in mineral supplements. If feeds are already high in potassium, feeding additional potassium in a mineral only aggravates the problem.

4. Consider feeding hay that is higher in calcium and magnesium with the hay that is low in magnesium. Alfalfa can be high in potassium as well, so be sure to test prior to feeding.

Winter tetany can be an unexpected problem, as most producers are not looking for it at this time of year. Through forage testing for levels of calcium, potassium, phosphorus and magnesium present, producers can determine if action may be needed to prevent winter tetany from occurring.

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