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Proper mineral supplement can prevent grass tetany

Follow these management tips to help protect against deficiencies

cattle on a pasture

Many parts of western Canada suffered through one of its longest and coldest winters in decades. It’s anybody’s guess that arctic temperatures and belly-high snows predispose beef cows to grass tetany when green lush grass sprout across pastures. However, proper nutrient supplementation to the cows’ early spring diet eliminates risk of this highly preventable magnesium-deficiency disease.

Reported cases of grass tetany or hypomagnesaemia often affect mature cattle grazing rapidly growing, magnesium deficient pastures. Symptoms of grass tetany may start with extreme nervousness, and then progress to a lack of muscle coordination and spasms, staggering, and finally failure to stand. If not immediately untreated, most animals suffering from tetany symptoms die.

The risk of grass tetany in beef cattle seems to be higher in a number of grazed cool-season pastures, namely: fescues, timothy, orchard grasses, brome-grass, and vested wheatgrass. It rarely appears in legume pastures containing clovers or alfalfa.

Normal magnesium levels for these common grass and legume species usually measure 0.20 – 0.25 per cent, which is enough to support the magnesium requirements of beef cows. NRC Mg requirement is about 13 – 15 g/hd/d for gestating cows, and about 20 – 22 g/hd/d for early stages of lactation. When grass tetany develops in the high-risk pasture grasses, their magnesium levels are measured below 0.10 – 0.12 per cent — not enough to support magnesium requirements.

Fortunately, it takes a combination of specific climatic and field conditions to drop magnesium to deficient levels in pastures to cause grass tetany.

Grass suddenly appears

The highest risk occurs in cattle grazing succulent grasses that appear from nowhere in warm periods of 10 – 15 C after weeks of cool, wet and cloudy weather. If there are high potassium and nitrogen levels in the soil (such as from previous fall-applied fertilizers), there is an added risk that these two elements interfere with magnesium’s uptake by the plants’ roots. The grass tetany risk seems to decrease substantially when day and nighttime temperatures rise above 20 C, as grass is able to draw more magnesium from the soil.

Keep in mind, some of the conditions that predispose cattle to grass tetany occur after the cow takes its first bite of lush pasture! When beef cows graze magnesium-deficient pastures, these grasses often contain very high levels of potassium that tend to follow and interfere with the ruminal absorption of what little magnesium is ingested.

As far back as 1957, research illustrated when the ratio of dietary potassium to the sum of calcium and magnesium or “tetany ratio” was less than 2.2, the incidence of grass tetany affected less than one per cent of cattle. The incidence of grass tetany increased to nearly seven per cent in other cattle when the tetany ratio was greater than 2.2. (Note: The exact equation to determine the tetany ratio is based upon milli-equivalents of potassium, magnesium and calcium, which takes into account the molecular weight and electrical charge of each of these elements.)

Regardless of the “tetany ratio,” the predictability of grass tetany has remained unclear. Some newer field trials have shown that cattle can be affected in some magnesium-deficient pastures, while not affected in others. It appears cattle fed loose salt (sodium chloride) on a free-choice basis are rarely affected by grass tetany. However, cattle provided with magnesium supplements on pasture with low sodium or salt intake may come down with grass tetany.

Speculation is that lush pastures that cause grass tetany are very high in potassium. This high concentration of potassium may interfer with magnesium absorption in the rumen. But when salt is fed on the same pasture it counteracts excessive dietary potassium, while restoring any metabolic electrolyte imbalance in the cattle caused by the potassium in the first place. As a result, there is more magnesium absorption and the threat of grass tetany is decreased.

A beef cow that receives inadequate magnesium or utilizes it poorly in its body from grass tetany pastures usually comes down with deficiency symptoms, quickly. Although, 70 per cent of magnesium is stored in the skeleton, with the remainder in the soft tissues and fluids, this macro-mineral is poorly retained compared to other minerals stored in the body. Luckily, there are several methods to prevent grass tetany on pastures each spring.

Management tips

Sound grass tetany preventative measures include:

  • 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 to three weeks before cattle are released to pasture. Continue to feed a high Mg mineral for the first part of the pasture season, when grasses are lush and growing.
  • Ensure all cattle are consuming about 100 grams of this mineral mix to receive the recommended amount of dietary magnesium. In addition to the prescribed cattle mineral, feed salt (sodium chloride) at the rate of 15 – 30 grams per head per day, preferably in loose form. This salt might also be mixed with the high magnesium mineral.
  • Consider turning cattle out to pastures at a later date. Once grass plants are more than six inches tall, much of the inherit risks of grass tetany are past. Some producers continue to feed some grass hay during the early parts of the grazing season.
  • Make a point to observe beef cows at least two times per day when grass tetany is high risk. Symptoms of grass tetany are similar to milk fever and other ‘downer cow’ syndromes. Consult your veterinarian. Beef cows diagnosed early with hypomagnesaemia are often treated with intravenous or subcutaneous administration of magnesium-containing solutions.

Older beef cows grazing lush cool season pastures can be susceptible to grass tetany during early spring. Even without seeing one affected cow, grass tetany can be effectively prevented by making sure that all essential nutrients (including magnesium) are balanced in the beef cows’ overall diet.

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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