Mineral nutrition (Part 2): Microminerals (Proceedings)
If you own sheep, you know about copper.
If you own sheep, you know about copper. Every shepherd, it seems, can tell horror stories about unfortunate producers who've lost sheep to chronic copper toxicity (CCT). So the word is out, "Toxic! Toxic! Avoid copper at all costs!" Easy to repeat, like a mantra, but unfortunately, not quite true, nor easily followed. Yes, copper is readily toxic to sheep, but it is also a required nutrient for them, and the difference between the two levels is rather small. But new research may make it harder to keep sheep away from high levels of it. Oh, the tangled web it weaves!
This topic is fairly involved, so let's all get onto the same playing field. Most livestock species need 6-11 ppm copper in their total diet. Copper deficiencies show up as neurological problems (swayback in newborn lambs), loss of pigment in hair and wool, reduced immune function, reduced fertility, spontaneous bone fractures, etc. High levels of certain other minerals like molybdenum, sulfur, or iron will interfere with copper absorption across the gut wall. Conversely, very low levels of these minerals will potentiate copper absorption — i.e. make copper more available to an animal. And some geographical regions are actually copper deficient, either because the forage levels of copper are too low, or very often, because the levels of molybdenum, sulfur, and/or iron are too high. Therefore, any feed analysis that doesn't list all four minerals is generally worthless for determining the true copper status of a ration.
Some more points: The liver stores excess copper and then releases it for excretion or metabolic use. Blood is just the transport system. Insufficient dietary copper may indeed reduce blood copper levels, but ironically, high levels of dietary copper will not increase blood copper until the very end, because the liver sequesters extra copper away from the blood until the last stages of toxicity. Although blood copper levels may be useful for detecting a copper deficiency, they are useless for predicting a copper toxicity.
Finally — and this is the well-known sheep fact — sheep are particularly sensitive to CCT because, over time, sheep tend to accumulate copper in their livers faster than other livestock species. CCT ends in metabolic disaster. When liver copper reaches a toxic threshold (usually above 800 ppm dry weight), the liver suffers catastrophic damage and dumps huge amounts of copper directly into the blood, causing a sudden rise of blood copper levels and the classic hemolytic crisis, which includes jaundice and death.
Therefore, sheep producers hear one recommendation over and over — feed "sheep salt" to sheep rather than a standard trace mineral mixture designed for beef cattle. Actually, sheep salt is really just a standard TM mixture without any added copper. This recommendation is okay in areas where forages contain sufficient copper (or low levels of molybdenum), because TM mixtures for beef cattle all contain copper, which may add too much copper to a sheep's ration. But many producers, especially those who run both sheep and cattle, have historically never stocked two different minerals. For years, they've fed the same beef cattle TM mixture to all their livestock without incident. But things are changing in the cattle world, and this feeding strategy may now cause problems for sheep.
Among its many metabolic roles, copper is used by a number of enzymes in the immune system. Over the past ten years, researchers have observed better immune responses in dairy cattle fed diets containing more than 10 ppm copper. In response to this data, the new 2001 NRC Nutrient Requirements of Dairy Cattle (National Research Council) recommends 12-16 ppm copper in the total diet for dairy animals. In an analogous situation with beef cattle, some university bulletins now recommend that free-choice TM mixtures for beef cattle should contain 1,000-2,000 ppm copper, and I've even seen recommendations as high as 5,000 ppm. Some feed companies have already increased copper levels in their cattle mineral mixtures.
What do these new recommendations mean for people who raise sheep? Simple — it's now much riskier to offer sheep any mineral mixtures designed for cattle. Some cattle mineral mixtures may now contain much higher levels of copper than previously. If sheep consume mineral mixtures containing copper at 1,000 ppm or more, well, quite a few sheep producers may become distressingly acquainted with the details of copper toxicity.
I say may, because not all cattle minerals contain these higher levels of copper. It depends on the company and the region. Usually, these higher copper levels are listed on the feedtag, maybe even with a warning "Do Not Feed to Sheep." But not always.
What about the neighbor who says, "Heck, I've fed the same mineral to my sheep and cattle for years, and I've never seen a problem"? Well, consider those years as history. He probably fed trace mineral mixtures with lower levels of copper.
Today, any shepherd who wants to feed a beef cattle mineral to sheep should read the feedtag very carefully, and also know the background levels of copper, molybdenum, sulfur, and iron in the rest of the diet.
What about operations which run both sheep and cattle? Unlike sheep, cattle can handle the extra copper and are fairly resistant to CCT. But we do have some options. First, know the mineral levels in the total ration. Then you can decide which minerals are needed in the supplementary TM mixture and which ones should be avoided. Secondly, plan to feed two different mineral mixtures — one to the cattle, one to the sheep. This plan, of course, may be easier said than done, given the complexities of running a ranch or farm. Thirdly, look at other options for supplementing copper to cattle. Veterinary supply houses now carry slow-release boluses that contain tiny needles of copper oxide. These boluses lodge in the rumen and release their copper over six months or more. The boluses aren't cheap, but they may be less expensive than extra fence or labor, or losing sheep.
The alternative — of simply feeding a cattle TM mixture to sheep and taking a wait-and-see approach — is not a good idea. Remember that CCT develops slowly. Excess copper insidiously accumulates in the liver over months or years, with no overt symptoms. By the time the first sheep shows a hemolytic crisis, every other sheep in the flock may also have livers loaded with copper, just waiting for the other shoe to drop. Copper toxicity in sheep is very bad news. Avoid it at all costs.
But there is more to this story. Copper nutrition is complex, and the risks of CCT depend on more than just feeding sheep salt. So let's tidy up some loose ends — let's examine some other potential sources of extra copper.
If you've been reading agricultural discussions on the Internet, you could conclude that copper was the most horrible toxin known to sheep and of course no one should ever feed it. Well, that's not quite true. Copper is required by all mammals, including sheep who need it at 7–11 ppm of their diet (from the 1985 NRC). But excess copper accumulates in the liver, and some genetic lines of sheep have trouble getting rid of it. Given enough time, liver copper levels can rise to a toxic threshold. Then those liver cells break open and release their copper into the blood, causing a catastrophic hemolytic crisis which is usually fatal. Shepherds in toxicity-prone areas generally know about this problem and routinely feed their animals sheep salt, which is a trace mineral mixture that contains no added copper. So far, so good.
But this feeding strategy is not foolproof. Some sheep operations still experience CCT problems, which is unsettling. As I travel around the country giving workshops, someone usually comes up to me and describes a CCT problem on their place. Why? If sheep salt is commonly fed, where is the extra copper coming from? Let's examine some possible sources ...
The most obvious potential source is the mineral mixture. Sheep salt contains no added copper, of course, and the small amount of copper that may find its way into the mixture in molasses or other ingredients is usually not a problem. The real problem is the use of beef cattle mineral mixtures — either because they may be less expensive than sheep salt or because a producer runs both beef and sheep on the same property.
I know, I know — many old-timers swear by their beef cattle minerals, saying that they fed these minerals to all their cattle and sheep for over 200 years and never saw a problem. That may be true, but times have changed. The beef cattle mineral mixtures used years ago are not the same as many mineral mixtures formulated today. Research over the past 10–15 years has demonstrated that higher levels of copper for cattle can improve their ability to resist disease, which makes sense since copper is involved in the immune system. Based on these studies, many Extension bulletins now recommend much higher levels of copper in cattle minerals. Many beef cattle minerals now contain 1,000 ppm copper or more. I've seen mixtures containing more than 4,000 ppm. I can assure you that the old-timers did not feed these minerals to their sheep, at least not for very long. I'd say that this is a compelling argument for carefully reading mineral feedtags, especially for beef cattle minerals.
Speaking of feedtags, did you know that very high levels of copper can improve growth in hogs and poultry? The commercial hog and broiler industries often include as much as 250 ppm copper in grower diets. We don't really understand the biochemical mechanism for this growth response, since copper is neither an antibiotic nor an estrogen, but from our practical shepherds' perspective, most of this copper passes through digestive tracts into the manure, and therein can lie a problem.
Let's say that a few miles away from your farm, a confinement hog operation raises a quadzillion young pigs. The managers routinely include 250 ppm copper in the grower rations. A quadzillion hogs produce a lot of hog manure. Quite a lot. And where do the managers put it? On every farm that accepts it — probably any place within 20 miles. Month after month, year after year. Copper in the manure becomes copper in the soil. What about the forages that grow from that soil?
You can't automatically assume that this manure will cause high forage copper levels; you must conduct some tests. But don't test the soil for copper. There are too many intricate steps between field application and plant absorption to support a simple cause-and-effect assumption. If you are worried about manure causing high copper levels in the forage, then test the forage that the animals will actually eat.
By the way, does this mean that all hog and poultry manures are high in copper? No. Only manure from operations with young growing animals, and only if their feeds contain high levels of copper. Never from farms with layers or breeding sows. Another wrinkle you have to unravel.
But let's not forget that any high-copper feeds — for hogs or broilers or beef cattle — must originally be mixed in a feed mill. Here is another potential source of excess copper. We need and trust our feed mills, but sometimes unfortunate things happen. Employees may first mix a batch of hog feed and then mix your sheep feed next. Ideally, mills should flush out their equipment between these batches, but the world doesn't always follow ideal procedures. Or someone makes a mistake and grabs the wrong premix bag. Or someone mixes a feed batch for too long so that the ingredients begin to separate out, causing the heavier minerals to settle at the bottom of the equipment, ready to be picked up in the next batch. Or ... well ... you can imagine lots of scenarios.
These are the saddest situations because no feed mill wants this to happen, and innocent folks get caught in the middle. Sheep suddenly begin to die of CCT, some evidence points to a purchased feed, and people get angry. This is a time for patience and careful records. In fact, without good records on feed offerings, daily intakes, batch numbers, feedtags, timely veterinary involvement, and good feed and water samples, this situation is very hard to untangle. Especially since CCT takes a long time to develop (that's why it's called chronic), and copper began to accumulate in those livers months or years earlier. And to add to the problem, the actual source of copper may not have been the commercial feed at all. I urge anyone who suspects CCT to keep good records and involve a veterinarian immediately.
Finally, there is fruit. As in pears, apples, and grapes. As in orchards and vineyards and olive groves. Why? Because for over a century, one of the most reliable sprays against leaf fungus has been Bordeaux Mixture, which is composed of copper sulphate, hydrated lime (calcium hydroxide), and water. Some orchardists use alternatives called Fixed Copper Sprays. In either case, the effective chemical agent is copper. Over time, this copper moves from the trees into the soil and possibly into the forages.
It's easy, of course, to pinpoint this situation if your sheep graze in your own orchard. You know if you've sprayed your own trees with a copper compound. But things get a bit fuzzier on rented fields, where sheep can move under trees that someone else may or may not have sprayed. And then there are pastures that used to be in orchards, years and years ago, which were then taken out and converted to grass. And if these pastures belonged to orchardists two owners ago, or are rented from a current owner who doesn't even know about those old orchards.
We get back to testing and good records. If you have any doubts about a pasture, simply test its forage for copper (and molybdenum and sulfur and iron, which can all influence copper availability). Maybe run tests at different times throughout the year, just to be sure. And take care with your sampling procedure — you need to be confident that the laboratory results from your sample reflect the true conditions of an entire field.
Of course, we may encounter other sources of extra copper — oddball one-of-a-kind situations where we scratch our heads in amazement — but the ones I've just described are the major ones. In the end, copper does not have to be the devil in sheep's clothing, but copper nutrition is indeed complex, and we should always watch for the extra sources that can make things even more complex.