Ask the Vet

Grass Tetany Risks

The risk factor for grass tetany in spring-calvers is higher, but can be offset with minerals. (DTN/Progressive Farmer photo by Becky Mills)

QUESTION: Are spring-calving operations more prone to having their cows suffer from grass tetany? If so, what can a producer do to offset the chances of losing an animal to this condition?

ANSWER: Yes, I think the risk factors for grass tetany are greater with spring-calving cows. Grass tetany results from low magnesium levels in the bloodstream. Cool-season grasses, or winter annuals, tend to have lower levels of magnesium in winter and early spring under lush growing conditions.

Grass tetany is most common in mature cows in early stages of lactation. Heavy milkers are at greater risk since magnesium and calcium requirements of lactating cattle are far greater than those of dry cattle.

Most cases of grass tetany I have seen have been on pastures with unbalanced soil nutrients. The soils tended to be low in pH, deficient in magnesium and phosphorus, and high in potassium. In many cases, the cattle had access to high-magnesium, or hi-mag, minerals, yet still developed tetany. Soil testing is often overlooked as a key to preventing grass tetany. Legumes are another element producers can add to help, since they are high in magnesium.

In most cases, a well-balanced mineral fed all year long will provide adequate magnesium. In some high-risk pastures, or in cases where a herd has a history of grass tetany, hi-mag minerals often are needed in late winter and spring.

Remember that magnesium is not palatable. So even if the herd seems to be consuming adequate amounts of a hi-mag mineral, individuals still may be at risk. Carefully observe cows to be sure all of them are eating the mineral.