Grass Tetany

Lance Kennington, Ph.D.
CHS Nutrition

A common question asked beef nutritionists this time of year is this: "How can I prevent grass tetany in my beef cow herd? Grass tetany is a condition seen in cows consuming lush, early spring pastures or in cows that are fed low-magnesium diets in confinement. Signs of grass tetany include frothing at the mouth, salivation, convulsions, anorexia, and excitability. If left untreated, grass tetany can cause substantial death loss in the beef cowherd. This is totally unnecessary because grass tetany can usually be easily prevented. However, to properly prevent and control grass tetany incidence, one must first understand the underlying causes of this condition.

Grass tetany is the condition resulting from magnesium deficiency. The Beef NRC (1996) recommends early lactating cows consume diets containing 0.20% magnesium on a dry matter basis. Magnesium functions within the body as a major mineral both intra- and extracellularly. Intracellularly, magnesium is a major mineral necessary as a component in many enzymatic reactions within the cell. Without magnesium, biochemical reactions within the cell would cease and death would occur. Wacker (1980) found over 300 enzymes that require magnesium for activation. Extracellularly, magnesium is essential for muscle function, nerve cell conduction, and bone formation. Although around 70% of the magnesium in the body is contained in the bone, this magnesium is not very available to the animal in times of magnesium deficiency. Therefore, it is essential cows receive supplemental magnesium in the diet when faced with a deficiency.

Cattle absorb magnesium provided in the diet from the rumen. As a result, many dietary conditions can make the problem worse. For instance, diets high in nitrogen, calcium, potassium, and phosphorus may reduce absorption and utilization of magnesium. Grass tetany is particularly a problem in pastures that have been fertilized with nitrogen and potassium. Also, lush, green pasture grass increases the rate of passage in the rumen resulting in less time for digestion and absorption of nutrients. As a result, cows may be consuming enough magnesium but may not be absorbing enough magnesium.

Despite these concerns, preventing grass tetany caused by magnesium deficiency is not particularly difficult. If your operation has only experienced mild problems with grass tetany in the past, feeding free-choice mineral that supplies 13-15 g (1/2 oz) of magnesium per head per day for 30 days prior to and 30 days after turnout will control most grass tetany problems. If you feel the problem is more severe in your area, providing the cows with high magnesium cake (15-30 g of magnesium per head per day) can be an option. This strategy may be useful to be sure all cows are receiving supplementation because some cows may not be consuming adequate free-choice mineral. In conclusion, it is important to remember that prevention of grass tetany is vital because many times the grass tetany symptom most producers observe is death. The Nutrition Consultant in your area can help you identify the product that fits your operation.