The pastures that look the best in June are the ones that can put a cow flat on her side. Lush, fast-growing cool-season grass after a warm rain carries a lot of water and not much magnesium, and a cow can’t store magnesium the way she stores calcium. She has to get it every day from what she eats. When the grass quits delivering, blood magnesium drops, and the first sign a lot of ranchers get is a good cow staggering or already down.
Grass tetany is easiest to think of as a spring problem, and most years the worst of it comes with the first flush of green. But it hangs on wherever the grass stays lush and immature—irrigated meadows, subirrigated bottoms, hay aftermath, and any pasture that greened up late after June rains. If you turned cows onto that kind of feed in the last few weeks, you’re still in the window.
Who goes down, and why
It isn’t random which cow falls. Grass tetany hits the older cows nursing the biggest calves, because milk pulls magnesium out of her hard and she’s already got less reserve than a young cow. A first-calf heifer can get it, but bet on the six- and eight-year-olds first. The cows in the best flesh giving the most milk are the ones you’ll find.
The grass matters as much as the cow. Cool-season grasses—brome, wheatgrass, fresh regrowth—running fast on nitrogen and potassium tie up magnesium so the cow can’t use what’s there. Cold, wet, cloudy stretches make it worse because the grass grows watery and the cow eats less dry matter. A three-day cold rain in June that greens everything up is a classic setup. Legumes and mature, stemmy grass are safer; it’s the young, leafy, rank stuff that gets you.
What it looks like before she’s down
Early tetany is a behavior problem before it’s a health problem. A cow will hold her head up too high, walk stiff and jerky, and act nervous or spooky when she’s normally quiet. Some bawl and wander off from the bunch. If you push her, she may go into muscle tremors—a fine twitch through the flank and shoulders—and that’s the warning that she’s close to going down.
Once she’s down, she’ll often paddle, throw her head, and thrash, sometimes hard enough to look like a seizure. That’s a bad sign, because a cow flat and convulsing on green grass in June is grass tetany until proven otherwise, and she can die within an hour or two. Cattle found dead with the ground torn up around them—torn-up grass, leg marks in the dirt—died fighting it.
The trouble with the down cow is that handling her can kill her. Excitement and stress spike the problem. If you rope her, run her, or crowd her into a chute, you can push her over the edge. Move slow, keep it quiet, and if she’s still up, don’t chase her into a corner.
Getting magnesium into her, and keeping it there
A cow that’s actually down with tetany is a call-your-vet situation, and it’s one where minutes count. Treatment is getting magnesium and calcium back into her, and it needs to go in carefully—magnesium and calcium given too fast can stop a heart. This is not a case to guess at with the bottle you’ve got in the truck. If you run cattle on tetany-prone ground, it’s worth asking your vet ahead of time what to keep on hand and how to use it, so you’re not reading the label while a cow convulses.
The real answer is not treating them, it’s not having them go down in the first place, and that comes down to magnesium in the mineral. A high-magnesium mineral—the ones sold as “hi-mag” for exactly this reason—fed through the risky weeks keeps blood levels up. The catch is that magnesium oxide tastes bad and cows won’t line up for it, so you have to make them eat it. Get it out before you turn onto lush grass, not after you find the first cow, and check the tubs and feeders often enough to know they’re actually cleaning it up. Mixing it with something palatable, or putting it where they loaf and water, helps get it in them.
Watch the weather as much as the calendar. A warm, dry stretch that lets grass mature drops the risk; a cold, wet spell that pushes another flush of green brings it back. Keep the mineral in front of them until the grass toughens up and the milk load on the cows starts to ease off. By the time the pastures go summer-brown, the danger’s mostly behind you—but the cow you lose in June was carrying your best calf, and she’s the one you can least afford to find dead under a good stand of grass.
