This syndrome has lots of names – magnesium tetany, grass staggers, hypomagnesemia – but each year, we call the veterinarian to save those animals with magical intravenous treatments, we put out high-magnesium trace minerals for the rest of the herd, and perhaps we try to increase forage magnesium levels by adding magnesium to the soil as dolomitic limestone or Epsom salts.
When we eventually get through the tetany season, we breathe a sigh of relief and move on.
So, this year, here we go again? Yes … and no. This year there is a new wrinkle to the magnesium tetany story – one which we should discuss, something to add to our tetany toolkit.
First, however, a very quick overview. The symptoms of magnesium tetany result from a reduced level of magnesium in the blood, usually in older animals. But this is not just a simple nutritional deficiency of magnesium; lots of other factors come into play.
This syndrome usually occurs in very early spring, when the young forage is primarily lush grass containing high levels of nitrogen and certain minerals, especially potassium.
It tends to strike older animals, probably because they are less able to mobilize extra magnesium from their bones as easily and quickly as younger animals. The primary factor, of course, is the actual magnesium level in the forage.
Traditionally, we consider forage magnesium levels above 0.18 percent (of the dry matter) as generally safe, although higher is better – 0.12 to 0.18 percent as marginally risky and less than 0.12 percent as high-risk.
In practice, we typically cope with magnesium tetany by flooding the system with as many magnesium atoms as possible and hoping for the best. Nothing subtle here; it’s the good ol’ sledgehammer approach.
Even though other factors influence this syndrome, we try to solve it by overwhelming the system with lots and lots of magnesium atoms. Sometimes it works, but sometimes it doesn’t. As an added wrinkle, magnesium salts are quite unpalatable, so getting animals to eat enough of them in a mineral mixture is sometimes a real challenge.
Potassium and salt
But let’s stop for a moment and consider one other factor that influences grass staggers: a high level of forage potassium. Grasses can accumulate lots of potassium, and I consider levels greater than 3 percent a critical threshold.
We know that high potassium levels can reduce magnesium uptake by plants, and they can also reduce magnesium absorption by the animal so fewer magnesium atoms cross into the blood from the digestive tract. Let’s hold this thought for a moment because the effect of high potassium levels may be less important than a low level of something else.
A few years ago, Thomas Swerczek, a veterinary pathologist at the University of Kentucky, came up with an intriguing theory: The syndrome of magnesium tetany was somehow related to the amount of salt in the diet.
Wait ... salt? Old-fashioned sodium chloride (NaCl) related to magnesium? Swerczek postulated that the lack of salt was a major factor in the occurrence of this syndrome. This, in all honesty, sounded strange to me, and to others as well because there is a lot of skepticism in the field. But read on.
Swerczek’s conclusions were based on his extensive field observations with beef cattle. He noted that herds with good access to salt showed fewer cases of magnesium tetany than herds where the cows did not eat much salt.
Sometimes these latter herds were consuming standard trace mineral mixtures, but those mixtures contained only low percentages of salt. (In the feed world, the word “salt” officially means “sodium chloride.”) His observations made for an interesting argument, but without disciplined experimental evidence, his theory was just another unusual twist on a perplexing problem.
I remained skeptical until I carefully reviewed the scientific literature. But what I found was clear: Independent scientists in Europe have conducted good laboratory experiments on the effects of NaCl and have developed sound physiological models to support this theory. In brief, here is the rest of the story.
Sheep and cattle are ruminants. Their fabulous organ of the rumen is great for digesting fiber, converting nonprotein nitrogen to protein, etc. But as part of their rumination process, ruminants also secrete lots of saliva, gallons and gallons every day. This saliva lubricates the feed, especially useful for chewing cud, but it also contains enzymes and buffers and minerals.
In fact, we can think of ruminant saliva as a type of recycling system; minerals from the blood diffuse into the salivary glands and then into the fluid saliva. The saliva carries these minerals back into the rumen.
They can then be reabsorbed across the rumen wall into the blood, or they can be lost when the unabsorbed atoms are swept down the gastrointestinal tract and excreted in the manure. This recycling system occurs with magnesium and also, in a much bigger way, with sodium. In fact, under normal conditions, the rumen fluid contains quite a lot of sodium. This is important.
But let’s focus for a moment on magnesium. In the case of magnesium, the absorption machinery in the rumen wall includes a molecular pump that actively ferries magnesium atoms from the rumen across the rumen wall into the blood.
This pump is especially important when the rumen level of magnesium is low – like when animals consume early spring grass that is deficient in magnesium. This molecular pump helps actively pull magnesium atoms into the blood in spite of its low concentration in the rumen.
One key facet of this picture is that sodium – remember sodium? – is critically important here. To operate properly, the magnesium pump requires relatively high levels of sodium in the rumen.
This high sodium level sets up an electrical difference across the rumen wall that drives the pump. And the sodium recycled in the saliva helps keep those rumen levels high.
But here’s the rub: We know that sodium is so important to animals they will go to great physiological extremes to avoid a deficiency. If mammals – sheep, cattle, mice, humans – do not consume enough salt in their diets, their bodies switch into a high-conservation mode: They conserve their sodium atoms by seriously reducing the amount of sodium leaving the body in the urine and sweat and … saliva.
Therefore, if a cow doesn’t consume enough sodium in her diet, the sodium level in the saliva drops, and then those sodium atoms are replaced by potassium atoms.
Shutting down without sodium
Rumen levels of sodium decline while the saliva supplies more and more potassium into the rumen. In addition, if the forage also contains a high level of potassium – recall the threshold of 3 percent in spring grasses? – the rumen then contains high levels of potassium and relatively low levels of sodium.
This situation causes the electrical difference across the rumen wall to collapse, and the magnesium pump effectively shuts down, which seriously reduces the amount of magnesium crossing into the blood. The result: magnesium tetany.
So … is magnesium tetany caused by insufficient salt? No, not exactly. The situation is more complex than that. If the forage levels of magnesium are sufficiently high, then enough magnesium atoms cross the rumen wall anyway by diffusion, and no tetany occurs.
But if the forage magnesium levels are low or marginal so the animal must rely on the magnesium pump to pull atoms from the rumen, and there is not enough dietary salt to maintain good levels of sodium in the rumen, and the forage contains relatively high levels of potassium – then, as they say, we have a situation.
All these factors come together. The magnesium pump shuts down, the level of blood magnesium drops, and the animal suffers from the symptoms of magnesium tetany.
Salt is cheap. I would recommend that during the tetany season, we should carefully check that our trace mineral mixture contains enough salt, and that the cattle are actually consuming it.
I could end this article by saying you should take all this with a grain of salt, but that pun is too easy, so I won’t.
Woody Lane is a livestock nutritionist and forage specialist in Roseburg, Oregon. He operates an independent consulting business and teaches workshops across the U.S. and Canada. His book, From The Feed Trough: Essays and Insights on Livestock Nutrition in a Complex World, is available through Woody Lane, PhD.
PHOTO: The primary factor of grass staggers is the actual magnesium level in the forage. Staff photo.
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