This could be the year for grass tetany
Grass tetany is a serious, and often fatal, metabolic disease caused by low blood levels of magnesium. It is also known as grass staggers or wheat pasture poisoning. It most commonly affects older cows early in lactation, but it may also occur in any cattle. It occurs most commonly when cattle are grazing lush, immature grass and it often affects the best cows in the herd. It is typically a late winter or early spring problem coinciding with the rapid growth of cool season grasses. Across much of Oklahoma this winter, anyone would have a hard time classifying pastures as “lush,” but don’t let the first look fool you. In many pastures, the old growth from last summer is gone and cattle are eating almost exclusively new green growth. Recent temperature and moisture conditions are conducive to rapid growth of the cool season grasses and I have already heard of several herds experiencing dead or down cows with low blood magnesium levels. High levels of nitrogen fertilization reduce magnesium availability, especially in soils that are high in potassium or aluminum. While in veterinary practice in northwest Arkansas, we always expected to see multiple grass tetany cases when the fescue started rapid growth after application of poultry litter. In western Oklahoma, it is common in cows grazing wheat pasture when moderate temperatures are combined with high levels of nitrogen application.
Signs of grass tetany begin with an uncoordinated gait and end up with recumbence, convulsions and death. The course can progress rapidly and animals are often found dead without any prior signs being observed. Evidence of thrashing will usually surround the dead animal. Intravenous calcium may cause a response in a few cases but more often has no effect.
Luckily, prevention is more effective than treatment. Graze less susceptible cattle, such as calves, yearlings, or dry cows on pastures where problems are likely to occur. Dolomite or high Mg limestone can increase the availability of magnesium while raising the pH of the soil. On high risk pastures, the use of a mineral with high magnesium content increases the blood magnesium levels and alleviates the potential for problems. Since there is very little capacity for storage and rapid mobilization of magnesium, supplemental magnesium must be consumed on a daily basis. If the supplement is allowed to run out, the cattle are suddenly again at risk.
In these times of high cattle prices coupled with high input costs, every calf counts. By the time you realize you have a grass tetany problem, it is probably too late to do much except dispose of the carcasses and figure out what you are going to tell the banker. In this case, an ounce of prevention truly does equal a pound of cure. — Dave Sparks, DVM, Oklahoma State University Area Extension Food Animal Quality and Health Specialist