As spring approaches with warmer temperatures and grass growth, the risk for grass tetany in cattle accelerates. Grass tetany is a metabolic disorder that occurs when blood magnesium concentrations are low after cattle graze on lush, immature grasses. The disorder results in a nerve impulse failure and possible death if not caught early enough.

Adele Harty, South Dakota State University (SDSU) extension cow-calf field specialist, notes in a recent SDSU Extension iGrow livestock newsletter article ways to mitigate the risk of the disorder. She explains that understanding the risk factors associated with grass tetany helps to reduce its occurrence.

Harty shares that there are multiple influential factors that exasperate the onset of grass tetany. These include:

• Low magnesium (Mg) coupled with high potassium (K) content of rapidly growing forages

• High crude protein content of forages

• Bad weather, storms, and stress that cause cattle to be “off feed” for 24 to 48 hours

• Lactation, which creates losses of Mg and calcium (Ca) in milk

• Various combinations of the above factors resulting in low blood Mg or Ca

Mature cattle are less able to mobilize Mg from their bones to maintain necessary blood levels in their system, which makes them more susceptible to grass tetany. The most susceptible to the disorder are older, lactating cows with calves younger than 2 months old. These cows have higher milk production, requiring more Ca and Mg. The least susceptible are steers, heifers, dry cows, bulls, and cows with calves older than 4 months.

Minimize risk

“Prevention is key to minimizing risks associated with lactating cows grazing lush pastures,” Harty notes. “If possible, delay turnout until plants are 4 to 6 inches tall. This will reduce the occurrence of tetany in addition to giving pastures more rest and recovery. Unfortunately, the reality is that many pastures are needed when grasses begin to green up and the risk for tetany is highest,” she adds.

If delaying turn-out is not an option, Harty recommends changing management to reduce the risk of grass tetany. Provide supplements with high Mg levels. She suggests supplements containing 8% to 12% Mg with a 3- to 4-ounce intake. Offer supplements two to three weeks before turn-out or when tetany is likely to occur.

Another option Harty suggests is providing hay while cattle are grazing lush pastures. Dry forages provide additional Mg and Ca. However, the specialist notes that cattle are unlikely to eat hay unless forced to. If the water source is contained, dissolving soluble Mg salts is another good option for additional Mg.

“A long-term approach is to incorporate more legumes into pasture mixes,” Harty explains. “Legumes have higher levels of Mg and Ca than 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 to alleviate the tetany risk.”

Treat early

Harty notes that treatment options are available, but the effectiveness depends on the stage it is administered. Death can occur rapidly, and symptoms progress over four to eight hours. Symptoms begin with cattle grazing away from the herd and advance to irritability, muscle twitching in the flank, wide-eyed staring, muscular incoordination, staggering, collapse, thrashing, head thrown back, coma, and finally death.

Treatment given within the first two hours of symptom development results in a quick recovery; however, treatment is not effective when delayed until the coma stage.

Harty concludes by reminding producers to implement prevention practices now to reduce future grass tetany problems. During the spring, keep an eye on cattle for symptoms and treat as soon as possible.

Michaela King

Michaela King served as the 2019 Hay & Forage Grower summer editorial intern. She currently attends the University of Minnesota-Twin Cities and is majoring in professional journalism and photography. King grew up on a beef farm in Big Bend, Wis., where her 4-H experiences included showing both beef and dairy cattle.