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Nip risk of grass tetany in the bud

Provide a tablespoon or two daily of magnesium mineral supplement before turnout

Grass tetany isn’t a huge problem, but where or when conditions develop, it can be corrected.

Every spring, I think about the possibilities of grass tetany in the cow herds going out to graze in on lush prairie grass. While nobody wants to find a fresh cow that died under mysterious circumstances, there is no need to panic. Fortunately, most threats of fatal grass tetany can be reduced by providing beef cows with a well-balanced mineral fortified with magnesium and covered by some sound management tips.

That’s because grass tetany or hypomagnesaemia is essentially a metabolic disorder which affects most mature beef cows grazing lush, and yet magnesium-deficient pastures.

Symptoms of grass tetany in beef cows may start with extreme nervousness and progress to a lack of muscle co-ordination, spasms, staggering and finally failure to stand. If not immediately treated, most afflicted animals often die.

While grass tetany can be a real problem on some farms, be aware that misdiagnosis of grass tetany is always a possibility in similar-looking cases. Some clinical magnesium-deficiency symptoms of grass tetany are mimicked in other nutrition imbalances such as milk fever, selenium toxicity (blind staggers), nitrate-tetany, polio-encephalomalacia (PEM) as well as relatively rare lead poisonings.

Despite these reservations, I can think of a recent story told to me by a beef producer in southeastern Manitoba that took place at his farm last year. I strongly believe it was a case of classic spring grass tetany. It happened to his herd of about 60 Angus brood cows during a particularly cool and wet spring, which happened about two months before my own visit to his farm.

The producer told me he let his cow herd out to graze about 20 acres of lush green swamp grass. At the time, these cows had just calved in the last month and still had access to a commercial mineral (with vitamins) in an adjacent dry lot. The operator told me that within a couple of days of grazing, several of the older cows were very excitable to the point he could not go near them. That night, an older cow died in a convulsive fit. The producer said he chalked it up to one of those “things” and took no further action.

I can understand why this producer did not call his veterinarian, because grass tetany can leave an animal dead within hours of the onset of its symptoms, making detection difficult and treatment improbable. However, I wish he had called his vet in order to confirm this incident as a true case of grass tetany. I could have helped him by setting up a proper mineral-feeding program and likely prevent grass tetany in his cows’ future.

My mineral feeding program would include elevated levels of magnesium (Mg) to assure that his late-gestation cows receive their requirement of 15 grams (about a tablespoon) of Mg per day, while the lactating beef cows would receive closer to 20-25 grams Mg daily.

In reformulating his new feeding program I would have also taken into account that some pasture grasses may contain adequate levels of magnesium (0.20 per cent) but still cause grass tetany, because its dietary magnesium is rendered biologically unavailable in some way.

Some management tips

Whether one’s grass tetany situation is a simple magnesium deficiency caused by lush pasture grasses or a more complex one; here are a few points that I recommend to assure magnesium requirements of beef cows grazing potentially dangerous grass tetany pastures are met:

  • Feed a “high magnesium” mineral (with vitamins) that contains at least 15-20 grams of magnesium oxide in every 100 grams of mineral mix. Provide this mineral mix to cattle about two or three weeks before cattle are released to pasture. Continue to feed this high-Mg mineral for the first few weeks of the pasture season, when the grasses are in their lush growing stages.
  • Assure that all cattle are consuming about 100 grams of this mineral mix in order that they receive the recommended amount of dietary magnesium. In addition to the prescribed cattle mineral, feed salt (sodium chloride) at the rate of 30-45 grams per head per day in loose form (although block salt is fine too). Note: feeding salt (NaCl) has been shown to reduce the incidence of grass tetany in beef cattle on its own in some cases.
  • Consider turning cattle out to pastures at a later date. Once grass plants are over six inches high, much of the inherit risk of grass tetany is past. Some producers continue to feed bales of grass hay during the early days of the grazing season. Other people avoid high-risk pastures all together, which are known to cause a high incidence of grass tetany (lush orchardgrass pastures) among cattle.
  • Lastly, it is important to work with your cattle nutritionist and veterinarian when implementing these grass tetany preventative measures. As a result, you should be able to significantly reduce potential cattle cases of grass tetany grazing spring pastures.

About the author


Peter Vitti is an independent livestock nutritionist and consultant based in Winnipeg. To reach him call 204-254-7497 or by email at [email protected]



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