With 2021 shaping up to be a drought year, cattle ranchers might not be worrying about grass tetany. It’s a condition normally associated with wet years, says cow-calf specialist Adele Harty, “when we’re going to have a lot more of that early-spring growth with our cool-season grasses.”

Harty, with South Dakota State University Extension at the Rapid City Regional Center, says the metabolic disorder associated with grazing lush, rapidly growing pastures can occur any year — even in drylot situations.

This metabolic disorder is caused by low blood magnesium. “If we look even further into the definition of tetany, it’s really a continuous spasm of skeletal muscles,” she says. Tetany can be devastating to a cattle herd if widespread among the animals.

Observe cattle for clues

Effects of grass tetany can progress rapidly, moving from an initial symptom of an animal grazing away from the herd to showing irritability, muscle twitching and staggering. “Eventually, the animal will collapse,” Harty says.

“Once those animals are down, you’re going to see them paddling and thrashing with their head,” she says, adding that the animal’s head tends to lean toward its back. “If these animals aren’t treated immediately, they will become comatose and die.”

Because this series of symptoms can progress in only four to eight hours, Harty stresses the need to closely monitor a herd. A blood test determines if an animal is suffering from tetany; however, ranchers would need to begin treatment before the test results are returned because of the disorder’s rapid progression.

“If your serum magnesium levels are below 1.1 milligrams per deciliter, that confirms that you have an aggressive tetany issue,” she says. “If your levels are at 1.5 or below, supplemental magnesium is actually going to be beneficial for milk production, so you may not be at a true deficiency level that would cause grass tetany. But you’re still in deficiency level that you’re going to get improved milk production if you do provide supplemental magnesium.”

For extremely deficient animals, they will require an intravenous injection of a magnesium solution with veterinary supervision, Harty says, warning that the animals can become aggressive.

If caught in time, cattle can see a quick turnaround upon receiving the IV; however, she says the animals can relapse if the dietary deficiencies are not corrected.

Some veterinarians may also inject additional magnesium subcutaneously, orally or rectally “to try to overcome any short-term deficiencies, but then you still have to focus on getting the dietary balance correct,” she says.

Harty reminds ranchers that grass tetany normally doesn’t show up until May, and that older cows in early lactation are more prone to the disorder “because they have a constant drain on their system for magnesium due to that milk production,” she says. “And typically it’s going to be some of our older cows that are higher producing, and so they need higher levels of magnesium on a daily basis to maintain those blood magnesium levels.”

Furthermore, older cows, she says, “are not going to be able to readily mobilize magnesium from their bone stores, like some of the younger animals can, which makes them more dependent on that daily intake of magnesium.”

Where the conundrum lies, is that while the older cows need more intake of magnesium, the rapidly growing grasses in the spring are typically low in magnesium, increasing the grass tetany risk.

“In the typical spring when the grass is growing rapidly and the soil temperatures are still cool, those plants are actually going to take up higher levels of potassium, because it’s more readily available than the magnesium in the soil, which then results in low-magnesium forages,” she says. “Additionally, if there’s anyone who fertilizes your pastures, specifically some of the introduced pastures, with NPK, that can actually increase the risk of grass tetany as well,” she says.

Supplement before turnout

Prevention is the best way to minimize the risk of tetany. Harty suggests delaying cattle turnout onto pastures until plants are 4 to 6 inches tall. “But in a drought year, it may not get to be 6 inches tall,” she says. In a press release that she issued in March, Harty says if turnout can’t be delayed, the cows can be provided a high-magnesium supplement with 8% to 12% at 3 to 4 ounces daily intake.

She suggests that these supplements be offered two to three weeks before turnout, making sure that all animals have access to the supplement prior to and while grazing pastures that are prone to grass tetany.