Reference Library

GRASS TETANY
THE VET'S BAG, April, 1996
by Stuart J. Burns, D.V.M.

There are few words to describe the feeling a stockman has when observing a down cow from midwinter through early spring. It is awful. If she is still alive when you find her, it is an emergency of the first order.

One of the most probable causes will be grass tetany, and it is worth our while to become familiar with it. Too often the only thing a herdsman will see is a dead cow. Before death, other signs that may be observed are excitability, loss of coordination, and muscular tremors. The problem is most frequent in older or higher producing cows that are nursing young calves. Grass tetany can be traced to low blood levels of magnesium. In mid-winter, it means the hay is low in magnesium or, in spring, that forage plants in the lush pasture being grazed are low in magnesium. This is a condition most frequently observed when the temperature average is below 40 degrees F. and/or when the cow is grazing rapidly growing early spring grass.

Treatment, if provided quickly enough, is quite rewarding. The cow is restrained and given 500 cc to 1000 cc of a calcium-magnesium solution, I.V., but at a fairly slow rate of delivery. Too rapid an injection can result in death, also. This approach to management is not very useful as a first-line defense unless one is willing to live in the pasture with the herd during the colder months.

The immediate goal of prevention can be had by increasing the amount of magnesium in the diet. This must be provided as a daily intake because the cow does not have the ability to store magnesium in her body for use at a later time. The most common source of more magnesium is through the addition of magnesium oxide in the ration. First, we must recognize that magnesium oxide is rather bitter, and cow's don't like it very much if the concentration is too high. The compound is frequently incorporated into mineral blocks, but if supplied in the block form alone, a cow may not be able to lick enough of it to obtain a satisfactory amount. This is the reason that it is wise to also provide a loose mineral mix that is blended with magnesium oxide. A cow can get a lot more from the mineral mix than a solid block. A third delivery system incorporates magnesium added to a molasses mix that the cows like for its flavor. Note that the best source of supplementation is not always that product with the greatest amount of magnesium oxide in it. If the final product is too bitter because there is too much magnesium in it, the cows won't eat it.

An awareness of plant physiology is helpful in understanding the problem on early spring pastures. Grass tetany is more common on grass pasture than legumes, as the legumes tend to have higher magnesium levels in their leaves. The condition is associated with cool weather because the metabolism of the plant is slower, and its mineral uptake from the soil is lower, leading to lower magnesium in the forage that the cow is eating. There is a relationship between soil phosphorus content and magnesium uptake in the forage. If phosphorus is low, even if soil magnesium is adequate, the plant won't take up the magnesium in adequate amounts to meet the cow's needs. Early spring top dressing of the fields with phosphorus will help reduce grass tetany problems, while fertilization with nitrogen and potassium tends to increase the frequency of grass tetany due to rapid for-age growth with a poor ability to move soil magnesium into the plant's leaves.

Some breeders who have pastures dominated by cool season grasses will leave a high magnesium mineral mix out all through the year. It is probably helpful, and it can't hurt anything. Knowing what we do about grass tetany, it's very frustrating to continue to lose stock to it. I lost one myself this winter, even with high magnesium tubs out...the dam horses like to lick the tubs, too, and kept at least one cow away from it enough that she got grass tetany.

Is there a genetic tendency for grass tetany? Possibly so, as I lost this cow's mother the same way last year. Made me think of that old saying, "White man fool Indian once, white man's fault. White man fool Indian twice, Indian's fault!"

Keep the magnesium available until the nighttime temperatures are consistently above 40 degrees F. Good luck!