Mineral nutritioin (Part 1): Macrominerals (Proceedings)

Double trouble

We often go to great extremes to do the best for our animals. We build enormous barns, we raise superb pastures, we give medicines and vaccines, we stay up long nights teaching their orphans to drink from bottles, we feed tasty grain supplements, we provide minerals and vitamins. And sometimes our well-meaning efforts get us into nutritional hot water, especially with minerals and vitamins. Actually more times than you can imagine. I call this problem Double Trouble.

The basic principle of minerals and vitamins is quite simple: livestock should get enough of them each day. Because only some minerals and vitamins are stored effectively in the body, our best strategy is to give them daily access to minerals — either free-choice or in prepackaged amounts — and assume that the mixture takes care of their needs. I won't go into details about individual minerals here, but I'll say this: with one important exception, livestock don't have "nutritional wisdom" for minerals and vitamins. If offered a selection of trays, each containing an individual mineral, our animals won't select what they need in the correct amounts. In fact, since many mineral compounds are quite unpalatable, animals will stubbornly avoid those trays.

The one clear exception is white salt (which means old-fashioned sodium chloride. In fact, "salt" is the official and legal feedtag name for sodium chloride). Livestock obviously relish salt. They will seek it out when they need it, and they won't over-consume it to toxicity as long as they drink enough water to excrete the excess. The feed industry universally recognizes this feature. Companies mix salt with other less-palatable minerals (and vitamins and drugs) and sell the product as a Trace Mineral Mixture. The percentage of salt in this mixture is not as critical as you might think. I've seen successful TM mixtures with salt levels ranging from 4% to 96%. Each company formulates its own recipes, and each mineral recipe is carefully designed for an expected level of intake. In any case, the underlying concept for these TM mixes is that salt is the driving force of mineral intake.

And this defines the Double Trouble problem. If you offer animals two or more sources of salt, what will happen to the intake of your main TM mix? Either (a) mineral intake will go down or (b) mineral intake will become more variable — over time (some days very little, some days very high) or within the flock or herd (individual animals responding differently to these choices). Or all of the above. You will have lost control of your mineral intake. And if you depend on that TM mix to supply doses of critical items like selenium or drugs like Bovatec® or antibiotics, what will happen with those ingredients? Their dosages will also decrease or become more variable — thus increasing the risks of mineral deficiencies, reduced drug effectiveness, and increased microbial resistance to drugs.

Now let's talk about practical Double Trouble scenarios that occur on farms and ranches in the real world.

The most obvious is to feed extra white salt. Yes, some people do this, either because they think their TM mixture doesn't contain enough (or any) salt, or to save money ("hey, my animals eat less of that expensive mineral when I offer white salt"), or they simply "heard" that white salt is a good thing. A variation of this scenario is to offer three or four or even more mineral mixtures — just to "make sure". One cure for this problem is to read the feedtag of the original TM mix. If the feedtag specifically gives directions to feed white salt, then of course follow the directions. But if there are no such directions, then study the list of ingredients. If you see the term "salt", then you know that sodium chloride is already in the original mixture, and you don't have to feed any additional salt. But if you are feeding extra white salt to reduce the intake of the original mineral, then you are actually diluting the original mineral intake, defeating the goals of the company nutritionists, and exposing your animals to all those health risks. And if you are feeding three or four different mineral mixtures at the same time, then mineral nutrition really becomes a tangled mess.

Another scenario: some folks routinely feed a grain or protein supplement. Sometimes this is necessary for production, sometimes not. But in either case, look at the feedtag. Straight corn or oats or other grain is not a problem, but a commercial grain mixture may contain salt. Grain mixtures are very palatable. If animals eat one pound of a yummy supplement that contains salt, they're also consuming extra salt. Again, how will this effect the consumption of a free-choice TM mix? Many times I have visited a ranch where the owner proudly shows me how he feeds a little of this, a little of that, a scoop of this other stuff, and also a cupful of a special mix from that bag in the corner. Oh my — enough said.

Do you use a lick tank as an energy or protein supplement? A lick tank usually contains molasses and urea and perhaps some other ingredients or drugs. But you should read the label — does it also contain salt?

Here's something that may be a specialty of the Pacific Northwest — salted hay — although I suspect it is used elsewhere. On the west side of the Cascade Mountains we sometimes get a bit of rain during the haymaking season. (That's a joke. Laugh. We always get rain during the haymaking season). Sometimes the square bales are too wet to stack safely in the barn, so we do this: as we lay down a layer of hay in the barn, we sprinkle white salt on top of that layer. We do this for each layer of hay. Our hope, of course, is that the salt will draw enough moisture out of the bales to prevent the barn from exploding in flame. The existence of long-standing barns in the PNW is kind of a backhanded proof that this technique works. But a side result is that the hay contains salt. When that hay is fed months or years later, folks may have forgotten about the salt, and soon the animals begin suffering from weird mineral deficiencies. Double Trouble, again.

A nice variation of this scenario occurs if that salted hay is sold. The unsuspecting buyer gets a truckload of hay, feeds it out, and unexpectedly runs into mineral problems like selenium deficiency. How can you protect yourself? Well, since hay does not usually come with a user guide, the most practical way is to monitor the intake of the minerals. If mineral intake suddenly goes awry when a new source of hay is fed, you should become concerned. (The official recommendation, of course, would be to test that hay in a lab. That's fine and dandy — as long as your sampling technique is good enough that you can fully depend on the results).

Here's an interesting Double Trouble scenario: the ocean. There's a big world out there, with more than 70% of it covered with water, salt water. All along the coastline of North America are fields exposed to ocean fog, spray, and wind. When I work near the ocean, I like to take a grab sample of the growing forage and analyze it for minerals, especially sodium. Background sodium levels are generally lower than 0.20%, dry matter basis. A sodium level higher than 0.40% is a red flag. Salt in growing grass is still salt, and perhaps something to watch.

This salt interference theme has nearly endless variations, like bloat blocks, salt licks, high-salt streams, etc. But aside from identifying it, what can we do about it? Some scenarios are quite easy to fix — for example, it's easy to stop feeding the extra bag of white salt — but what about those situations where we can't easily reduce the second source of salt?

Let's return to the original concept of trace mineral mixtures. If mineral intake is driven by salt, and something interferes with the importance of salt as an intake stimulus, then we should try changing the driving force of intake. Find an alternative TM mix that contains other tasty ingredients, like flavor additives or molasses. The mixture will probably also include salt, but this salt is just along for the ride, just another required nutrient. The real intake stimulant is something else. Something that can get you out of Double Trouble.

Too little, too much, early spring

Early spring — the first full warmth of sunshine, trees with their bright green leaflets, the iridescent green of early grass pastures. But one morning you walk outside and notice your cows or ewes trembling, walking unsteadily. Then suddenly one falls down with an awful tetany, frothing at the mouth, convulsing, and pawing the air. You frantically call your veterinarian. She comes right away, inserts an IV tube, and an hour later, the animal is back on its feet, looking rather pert and kind of surprised by the ordeal. "Oh yes," you think, "early spring ... magnesium tetany. Where is that nutrition book?"

This is a problem with lots of names — magnesium tetany,grass staggers, grass tetany, hypomagnesemia, winter tetany, even wheat pasture poisoning. Whatever we call it, it's the result of low blood levels of magnesium, and it involves forages. Since magnesium is tightly associated with nerve impulses, affected animals show dramatic neural symptoms like agitation, incoordination, and convulsions.

But magnesium tetany is not just a simple lack of magnesium; it's a complex syndrome that is still not completely understood. Treating the problem is, however, relatively straightforward — we supplement magnesium to our livestock. For animals showing overt tetany symptoms, veterinarians intravenously dump a huge load of magnesium into the blood (usually as calcium-magnesium gluconate), which usually gives spectacular results — blood magnesium rises, and the animal quickly gets back on its feet and regains coordination. For the rest of the herd or flock, we add extra magnesium to the diet for the duration of the high-risk period, either as magnesium oxide in the trace mineralized mixture or as a topdress on the hay or grain.

Magnesium tetany occurs sporadically. Some years seem worse than others; some animals seem more susceptible than others. We usually view this problem in terms of risk, with many factors influencing that risk. The most important risk factor is the level of magnesium in the forage. A simple rule-of-thumb for assessing forages: Magnesium levels above 0.18% are generally safe. Levels between 0.12–0.18% are medium risk. Levels below 0.12% are high risk.

But magnesium tetany is a tangled web with other risk factors. For example, we know that tetany usually occurs in the early spring on lush, young pasture. More cases coincide with cool, rainy weather. But magnesium tetany can also occur at other times of the year, which is confusing. We also know tetany primarily affects mature animals rather than young animals, especially during late gestation or early lactation, probably because older animals are slower than juveniles to mobilize extra magnesium from their bones to supplement falling blood levels. And we also know that other nutrients can influence magnesium uptake and metabolism, such as potassium, calcium, and even nitrogen.

Let's look at potassium — in the forage and in the soil. High forage levels of potassium can cause a double whammy: (1) they cause lower magnesium levels in the forage by reducing magnesium absorption from the soil, and (2) they depress blood magnesium in animals by interfering with magnesium absorption across the intestinal tract.

How high is high? Well, I get concerned about forage potassium levels above 3.0% (dry matter basis). Grasses can be particularly guilty of this problem. Grasses, being grasses, love potassium in a way that some people seem to love chocolate — they'll eagerly consume more than they need. Although grasses require potassium for growth, if the soil contains excess potassium, grasses will accumulate potassium above their requirements without showing additional yield. In grasses, this phenomenon is called luxury consumption. (In humans, it can still be called luxury consumption, but the effects of excess chocolate are definitely not agronomic). Legumes are not guilty of this sin, and thus potassium levels in clovers and alfalfa are never high enough to interfere with magnesium.

One common recommendation for reducing the risk of magnesium tetany is to add legumes to the pasture. Well, that's a nice thought, but generally it's not very practical. Remember that tetany usually appears in animals grazing the earliest spring growth — when soil temperatures are still relatively low. Grasses grow at lower temperatures than legumes, so this early spring growth is — what? — grass. Any legumes in that pasture haven't yet begun to grow. Interesting.

But why is high forage potassium an issue, when so many farms and ranches must struggle to improve their low soil levels by including potassium in fertilizer? Because in reality, some fields may actually contain high levels of potassium, either because those fields overlay high-potassium subsoils, or because they have not been harvested for hay, or because they've received lots of waste sludge from confinement dairy or hog operations. In general, soil potassium levels of 150–200 ppm will support good forage yields. But I've seen soil tests with potassium levels higher than 600 ppm. That's very high. I would want to check the potassium and magnesium of any forages growing on these fields, especially in the early spring.

Forage calcium also influences magnesium tetany. Experience has shown that tetany can depend on a three-way interaction between forage calcium, magnesium, and potassium. Relatively low levels of calcium, combined with high levels of potassium, seem to cause tetany problems, at least in forages with marginal levels of magnesium. But in assessing risk, most of us aren't comfortable with the phrase "seems to", so agronomists have derived a handy formula called the Tetany Ratio, which combines the levels of these three minerals into a single number. A ratio above 2.2 indicates a high-risk situation.

The Tetany Ratio gives us a numerical tool to assess risk. It's particularly useful for marginal forages — forages with magnesium levels between 0.12–0.18% (although I would apply it to any forage with less than 0.22%). We don't need this ratio for the extreme magnesium values. Forages with less than 0.12% are clearly high-risk situations, and forages with more than 0.22% magnesium I would consider safe.

So what exactly is the formula for the Tetany Ratio? Oops, there's a problem here. Quite a few websites and printed documents do list a formula, but unfortunately their formula is wrong because it is based on simple, uncorrected percentages. Using raw percentages gives misleadingly high values. Try this yourself. Create a simple spreadsheet to make the calculations easier and plug in a few values. You'll quickly see that even safe forages will give ratios over well 3.0, which suggests a high-risk situation, even though the reality is a no-risk situation. So what is wrong?

Because the real formula for the Tetany Ratio is based on something called an "equivalence basis" which most of those published formulas ignore. In chemistry, "electrical equivalents" is a technical way of saying that the raw, simple percentages must be corrected for atomic weight and electrical charge. Each of the three elements has a different atomic weight and a different electrical charge. In other words, the formula must contain some correction factors. Here is the correct formula for the Tetany Ratio:

Tetany Ratio =

(potassium % x 256) divided by the sum of (calcium % x 499) plus (magnesium % x 823)

When this formula is applied to marginal forages, results greater than 2.2 indeed represent a good estimation of increased risk. In other words, the results coincide with reality, which is rather comforting.

Why has an incorrect formula been so widely distributed? Perhaps this is an example of people repeating each other without checking the results. Publishing something doesn't necessarily make it true or accurate. Be careful.