Magnesium deficiency can lead to grass tetany

Lush pastures can be a perfect setting for fatal conditions

cattle on a lush green pasture

Every time I drive past lush green May pastures I think of grass tetany in beef cows.

That’s because it is essentially a magnesium deficiency in lush pasture that can have fatal effects upon a cow herd. Who wants to come across a dead pregnant or fresh cow in a field due to grass tetany? I recommend producers know some of the symptoms of grass tetany as well as the pasture conditions that expose their cattle. They should also implement a good mineral program to ensure magnesium requirements of beef cows are met in order to prevent dangerous grass tetany.

Not always clear cut

Last year about this time, I received a phone call from a producer who operates a 300-head cow-calf ranch across the border into the United States. He was concerned many of his pre- and just-fresh cows had grass tetany on sprouting pasture. Over the phone, he related many of his older cows collapsed on their front legs and had a hard time getting back up. Some animals collapsed on pastures, while others loss muscle control in front of the water trough. He said one or two cows died.

A day after the call, I drove out to his farm. I did not see any cows collapsing as he experienced, but one or two of his cows “appeared” walking with a stiff and staggering gait, “appeared” nervous and had muscular tremors; all possible signs of a magnesium (Mg) deficiency or grass tetany. Furthermore, this herd had a higher percentage of older cows and heavy milkers that are very susceptible to this metabolic springtime disease.

This producer also informed me he was working with a local veterinarian, who drew some blood samples from several of the “afflicted” cows. The blood results showed deficient magnesium levels as well as marginal deficient levels of copper and zinc. Because this producer was feeding only trace mineral salt blocks on pasture, I recommended he feed a magnesium-enriched commercial loose cattle mineral (with organic selenium, and chelated copper and zinc).

A few weeks later, I learned that his veterinarian took more blood samples, which confirmed the collapsing cows suffered from anaplasmosis (re: a protozoa induced anemia that tends to appear in older cows and make them very weak). Whether these cattle also had clinical grass tetany remained inconclusive due to the overshadowing of an apparent larger problem.

Because this particular herd was on a questionable mineral program (re: trace-mineral salt blocks), it is my belief this cow could have an underlying grass tetany issue as well as poor copper/zinc status, which has been proven in university and extension research to negatively affect cow herd health, milk production and reproduction. In a challenging feeding situation such as this one, it is often difficult to separate several issues at hand. For example, grass tetany, and milk fever are two metabolic diseases that produce tetany-like symptoms. It takes an experienced person to determine which one of the two caused specific cases of pasture-death in cattle.

Preventable measures

Regardless of another nutritional or other problem affecting the cow herd at the same time, preventing grass tetany on sprouting spring pastures should be straightforward: gestating beef cows require about 15 grams of Mg per day, while lactating beef cows need closer to 20 to 25 grams of Mg daily.

A low magnesium status in beef cows that often leads to actual grass tetany can be traced back to low forage Mg levels found in rapidly growing pasture grasses during a typical spring of cool, cloudy, and rainy weather. Such conditions favourable to developing Mg deficiencies in cattle are often seen on orchardgrass, perennial ryegrass, timothy, tall fescue, crested wheatgrass and bromegrass pastures. A lack of dietary Mg can also be seen on grazed cereal pastures; oat, barley and rye.

Forages containing adequate levels of magnesium (0.2 per cent Mg) have also been shown to cause grass tetany in cattle because this dietary magnesium was rendered biologically unavailable. Lush pastures frequently contain high levels of rumen-degraded protein that is digested into ammonia. If too much ammonia is present in the rumen, it can convert dietary magnesium into an insoluble hydroxide form, which is metabolically unavailable and fails to contribute to the animals’ Mg requirement.

Tips for preventing grass tetany

Here are five things that can be done to help assure that magnesium requirements of beef cows grazing potentially dangerous grass tetany pastures are met:

  1. Feed a commercial beef mineral formulated with eight to 12 per cent magnesium at three to four ounces per head per day from three to six weeks before the beef herd is allowed out on pasture.
  2. Formulate the entire diet to insure the ratio of potassium/ (calcium + magnesium) is less than 2.2 in their total diet (estimate pasture intakes on a dm basis). The Ca/P ratio of this diet should also be around 2:1.
  3. Feed salt blocks or in loose form on pasture; salt (NaCl) has been shown to reduce the incidence of grass tetany in beef cattle.
  4. Continue to feed dry hay to the beef cowherd at the beginning of the pasture season. Wait to put the cattle on pasture until the grass is four to six inches tall.
  5. Adjust calving season to avoid high-risk grass tetany periods (first two to three weeks of the grazing season).

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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