This winter and looking towards spring, forecasts predict warmer than average temperatures across the state. With warmer temperatures and recent precipitation, ranchers need to be proactive in mitigating grass tetany risk. Cool-season grasses are beginning to green up, posing a potential risk for cows with young calves. As the temperatures continue to rise, cool-season grasses, such as crested wheatgrass, Kentucky bluegrass and bromes, will grow rapidly, increasing that risk. Having a solid understanding of the risk factors and how to mitigate risk are key.

What is Grass Tetany?

Grass tetany is a metabolic disorder associated with grazing lush, rapidly growing pastures, resulting in low concentrations of blood magnesium (Mg), which results in nerve impulse failure in animals. With adequate moisture and warm temperatures, grasses grow rapidly. Grass tetany isn’t normally seen until May, however, taking steps early to prevent it will be more effective in the long run. It is never too early to plan and ensure proper management practices are in place.

Understanding factors that influence the progression of grass tetany will help manage risk. These factors include:

  • Low Mg coupled with high potassium (K) content of rapidly growing forages
  • High crude protein content of forages
  • Bad weather, storms, stress, etc., that cause cattle to be “off feed” for 24–48 hours
  • Lactation decreases Mg and calcium (Ca) levels as they are transferred to the calf through milk
  • Various combinations of the above factors resulting in low blood Mg or Ca

Older, lactating cows with calves younger than two months-of-age have the greatest susceptibility to tetany. Mature cows are less able to mobilize Mg from their bones to maintain blood Mg levels, making them more susceptible, along with cows within two months post-calving due to increased milk production, which requires additional Ca and Mg. Steers, heifers, dry cows, cows with calves older than four months-of-age and bulls are less susceptible.

Management Considerations

Prevention is key to minimizing risks associated with lactating cows grazing lush pastures. If possible, delay turn-out until plants are four to six inches tall. This will reduce the occurrence of tetany and give pastures more rest and recovery. Unfortunately, the reality is that many pastures are needed when green-up begins and the risk for tetany is highest.

If delayed turn-out is not an option, other management tools should be utilized. First, provide a high-magnesium (Mg) supplement containing 8–12% Mg at three to four ounces daily intake. For a cooked molasses product with a recommended intake of approximately one pound, the guaranteed analysis for Mg should be approximately 4%. Read the label to ensure adequate Mg levels and know the recommended intake. Magnesium supplements need to be offered two to three weeks prior to turn-out, or before tetany is likely to occur to allow time for reserves to build in the cow’s system. Palatability and adequate intake can be challenging, resulting in some animals consuming an inadequate amount of mineral on a daily basis. Ensure all animals have access to the supplement prior to and while grazing tetany-prone pastures to help decrease occurrences.

Another option is to provide hay while cattle are grazing lush pastures; however, cattle are not likely to eat hay unless forced to. Dry forages can provide additional Mg and Ca at critical times. If the water source is contained (i.e., water tanks), soluble Mg salts can be added. Some examples of solubles are magnesium acetate, magnesium chloride and magnesium sulfate (Epsom salts). The most common form of Mg found in feed supplements, magnesium oxide, is not soluble in water and therefore cannot be used for this purpose.

A long-term approach is to incorporate more legumes into pasture mixes. Legumes have higher Mg and Ca than do immature grasses, resulting in a better balance across the pasture. As the pastures green-up, cattle will have access to both types of forage, helping alleviate risk.

Symptoms and Treatment

Death can occur very rapidly, therefore symptoms may not be observed. Symptoms progress over four to eight hours as follows: 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 because stress can cause sudden death.

Treatment options are available, but effectiveness depends on the clinical stage when it is administered. If treatment is started one or two hours after clinical signs develop, the results are usually a quick recovery. Treatment is not effective if delayed until the coma stage. Grass tetany can be treated with an intravenous dextrose-based commercial preparation of magnesium and calcium purchased from a local veterinarian.

Remember, cattle are more susceptible to grass tetany in the spring, with weather events increasing risk. Determine and implement prevention practices, monitor cattle for symptoms, and treat as soon as possible according to a protocol developed with your veterinarian.

Source:  Adele Harty, SDSU Extension Cow/Calf Field Specialist