Grass Tetany: When To prepare

By Adele Harty

The temperatures in South Dakota have been warmer than average this spring, which has brought on rapid growth of the cool season grasses. With that green-up and cows lactating heavily comes the concern of grass tetany.

Grass tetany is a metabolic disorder associated with lush pastures due to low concentrations of blood magnesium, which results in nerve impulse failure in cattle. With adequate moisture and warm temperatures, grasses will begin to grow rapidly. The concern of grass tetany isn’t normally seen until May, but taking steps to prevent it now will be more effective in the long run. What steps are being taken or should be taken to prevent grass tetany? Even though it may seem premature, it is never too early to plan and ensure proper management practices are in place.

Causes

For a better understanding of grass tetany and be able to manage its risk, one needs to understand what factors play a role in it. These include:

• Low magnesium (Mg) content of rapidly growing grasses and pastures
• High potassium (K) content of rapidly growing grasses and pastures
• High crude protein content of grasses and pastures
• Bad weather, storms, stress, etc., that cause cattle to be “off feed” for 24-48 hours
• Lactation: losses of Mg and calcium (Ca) in milk
• Various combinations of the above factors resulting in low blood Mg or Ca

Protection & Prevention

The key to prevention is to be proactive. Measures should be taken to minimize risks associated with cows grazing lush pastures. One long-term approach is to incorporate more legumes into pasture mixes. Legumes have higher levels of Mg and Ca than do immature grasses resulting in a better balance across the pasture. If possible, delay turnout into lush pastures until plants are 4 to 6 inches tall. This will reduce the occurrence of tetany, in addition to giving drought-stressed pastures a little more time to rest. The reality is that many producers need to utilize pastures when grasses begin to green-up and the risk of tetany is most prevalent.

If delayed grazing is not an option, other management tools should be utilized. First, always provide a high magnesium (Mg) mineral supplement or mineral mix containing at least 8-12% Mg. This needs to be provided two to three weeks before turnout or before tetany is likely to occur. Palatability and adequate intake can be challenging, resulting in some of the animals consuming inadequate amounts of the mineral on a daily basis. Make sure all animals have access to the mineral supplement prior to and while grazing tetany-prone pastures, as this will help decrease the occurrence. Another potential tool is to provide hay while cattle are on lush pastures; however, cattle are not likely to eat hay unless forced to. Dry forages can act as carriers to provide the animals additional Mg and Ca at critical times. If the drinking water source can be controlled (i.e., water tanks), soluble Mg salts may be added to the water. Some examples of soluble Mg salts are magnesium acetate, magnesium chloride, and magnesium sulfate (Epsom salts). The most common form of Mg, magnesium oxide, is not soluble in water and therefore cannot be used for this purpose.

Symptoms & Treatment

Older lactating cows with calves younger than 2 months of age have the greatest susceptibility to tetany; while steers, heifers, dry cows, or cows with calves older than 4 months of age are less susceptible. Mature cows are more susceptible because they are less able to mobilize Mg from bones to maintain the necessary level in their system. Also, cows within two months after calving have increased milk production and require additional Ca and Mg.  

Cattle will exhibit symptoms of grass tetany, but they may not be observed as death may occur relatively rapidly (within 4 to 8 hours). An affected animal will exhibit a series of progressive signs. These include grazing away from the herd, irritability, muscle twitching in the flank, wide-eyed and staring, muscular incoordination, staggering, collapse, thrashing, head thrown back, coma, and finally death. Affected animals should be handled calmly, since sudden death can occur if animals are stressed.