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 hays 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 also low in calcium (less than 0.4%), while being simultaneously high in potassium (greater than 2.5%), tetany is more likely to occur.  Recent forage test results, observed in the Nebraska panhandle, have indicated that this could potentially be an issue with some hays harvested this year.

Forages likely to cause grass tetany are often borderline to low in magnesium while having excess levels of potassium.  Usually, forages also tend to be low in sodium content. 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 blood stream, 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:

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

If hay tests low in magnesium (<0.15%) and calcium (<0.40%) and high in potassium (>2.5%), consider feeding a high calcium, high magnesium mineral supplement (10-13% Mg for a 4 oz. target intake mineral) that also contains salt.  Because magnesium oxide is bitter, adding dried distillers grains or soybean meal at the rate of 1 pound to 50 pounds of the mineral and salt mix can help to increase intake if consumption is not at targeted levels.

In instances where cattle are being hand fed a protein or energy supplement, supplemental magnesium can also be delivered with the feed.  If this occurs, make sure access to loose salt is provided as well.

Be sure to 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.

Another option to consider when managing hay that is high in potassium and low in magnesium, is feeding hay that is higher in calcium and magnesium together with it. Alfalfa can be high in potassium as well, so be sure to test it 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.

For more information on management and prevention of grass tetany, please see these articles at beef.unl.edu.

Grass Tetany Considerations with a Late Spring
Preventing Grass Tetany in the Lactating Beef Cow this Spring

Interviews with the authors of BeefWatch newsletter articles become available throughout the month of publication and are accessible at https://go.unl.edu/podcast.

University of Nebraska-Lincoln