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Get magnesium levels up before cattle turnout

Mineral deficiency can lead to sudden and fatal grass tetany

A magnesium deficiency can lead to health problems when cattle move into lush new grass growth.

It wasn’t unusual that many producers fed a lot of straw and screenings to their beef cows this winter. That’s because their last year’s hay or grain crop was so poor. Some told me that spring couldn’t come soon enough when green grass starts to sprout on pasture. As a beef nutritionist, I warned them: Don’t let your cattle graze any lush pasture without first implementing a good magnesium-mineral feeding program to prevent grass tetany.

Grass tetany is essentially a severe magnesium deficiency which can have sometimes fatal consequences on an unprepared cow herd. Nursing mature cows are particularly susceptible because they require high levels of magnesium to support milk for their newborn calves. For example, a beef cow in peak lactation (six to eight weeks, postpartum) requires about 25 grams of magnesium compared to 15 grams needed as an overwintering pregnant cow.

Inadequate magnesium status in beef cows that lead to actual grass tetany can be traced back to low forage magnesium levels found in rapidly growing pastures, particularly those under frequent cloudy and rainy skies. These weather conditions are favourable to induce grass tetany on orchardgrass, perennial ryegrass, timothy, tall fescue, crested wheatgrass, and bromegrass pastures. A lack of dietary magnesium can also be found on cereal grain pastures.

That’s not all. Even some pastures that have been tested to contain adequate levels of magnesium of 0.20 per cent have still been responsible for causing grass tetany. Research shows that some lush pastures contain components that render their magnesium biologically unavailable to cattle.

Case in point: some lush pastures contain such high levels of soluble protein that when digested by cattle is converted into high pools of ammonia. Such excessive ammonia converts magnesium from grazed forages into a metabolically unusable form which contributes to the cow’s failure to meet its dietary magnesium requirement. Similar high forage potassium levels (greater than three per cent) also have proven to inhibit absorption across the cow’s rumen wall.

Regardless of how cattle become deficient, most afflicted cows exhibit similar grass-tetany symptoms. They may start with nervousness and progress to a lack of muscle co-ordination, staggering and finally a failure to stand. If not immediately treated, most animals die. Some of these clinical symptoms of grass tetany are mimicked in other nutritional imbalances such as milk fever, selenium toxicity, nitrate tetany as well as lead poisoning.

Grass-tetany not common, but it happens

Throughout my career as a beef nutritionist, I have only come across a half-dozen cases with a strong grass-tetany connection. For example, a few years ago, I met a producer that grazed about 60 Angus brood cows and their calves on 20 acres of lush swamp pasture during a particularly cool and wet spring. Within a week of moving his cows onto this piece of land, he told me that many of his older cows became so skittish that he could not come within several yards of them. He didn’t think much of it until he found a couple of dead cows, both of which looked like they pawed the ground while lying on their side before they died.

To help producers found in a similar situation as well as prevent other possible cases, here are a few suggestions to implement into a practice magnesium-feeding program for nursing beef cows:

  • Feed a cattle mineral formulated with eight to 12 per cent magnesium at three to four ounces per head per day from three weeks before cows and calves are allowed on pasture. In addition, their mineral program should have a dietary ratio of potassium (calcium + magnesium) of less than 2.2 in their total diet (estimate pasture intakes on a DMI basis). The Ca/P ratio of this diet should also be around 2:1.
  • Feed salt blocks or in loose form on pasture. Salt (NaCl) has been shown to reduce the grass tetany in beef cattle because in heavily nitrogen-fertilized pastures, sodium binds nitrates rather than the nitrate binding up magnesium and making it biologically unavailable.
  • Consider turning cattle out to pastures later. Once grass plants are over six inches tall, much of the initial risk of grass tetany is finished. Some producers also continue to feed grass-hay bales during the early days of the lush grazing. Other people avoid high-risk pastures altogether.

By working with your nutritionist and veterinarian to implement these sound magnesium suggestions, you should be able to significantly reduce the risk of any grass tetany cases on 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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