Trace minerals are required for essentially all biochemical processes in the body. Many of these minerals are dietary essentials for optimal growth, physiologic function and productivity in animals.
This article focuses on eight trace minerals: cobalt, copper, iodine, iron, manganese, molybdenum, selenium and zinc.
These trace minerals have been chosen because nutritional deficiencies or disturbances in their metabolism are relatively common, and substantial information is available about their metabolism and the amount needed for optimum health and productivity in animals.
Testing of blood, serum or tissues for total mineral concentration is a popular and potentially valuable means of assessing trace mineral nutritional status, which is generally more practical than expensive functional approaches of specific minerals containing proteins or enzymes.
Modern analytical techniques make blood and tissue trace mineral analysis practical and relatively inexpensive. Of particular importance is the recent application of inductively coupled plasma/mass spectroscopy analysis to the diagnostic evaluation of animal samples.
This technique is fast, extremely sensitive, precise, accurate and allows for the simultaneous measurement of a wide array of trace minerals.
Trace minerals play a key role in supporting immune function; therefore, maintaining adequate trace mineral status during the dry period is an important component in achieving good cow health during significant metabolic and physiological changes around parturition, when the immune system is stressed.
For many of the trace minerals, the blood is the “transport pool,” and the liver is the “storage pool.” Direct measurement of trace minerals in blood and tissue is subject to some limitations in evaluating nutritional status because for some, there is no recognizable storage pool, and for others, the transport and functional pools overlap.
Furthermore, factors other than nutrition are known to affect serum trace mineral concentrations. Most notably, homeostatic forces modulate the serum concentrations of most trace minerals within a range of homeostatic set points that vary in width among the different minerals.
Other factors such as physiologic state (e.g., pregnancy, lactation and gestation) may influence serum trace mineral concentrations. The presence of inflammation also has a large influence on serum concentrations of some minerals.
Historically, testing for deficiencies has been performed on diets or dietary components to ensure “adequate” concentrations in the diet. However, general mineral analysis does not identify the chemical form of these trace minerals, which can dramatically alter their bioavailability and utilization.
This is especially important when considering the increasing use of “chelated” minerals, as they can have significantly greater overall bioavailability than the organic minerals.
Mineral deficiencies can be suggestively diagnosed by development of clinical disease or by post-mortem identification of tissue lesions. Proof of deficiency requires analytical verification, as most deficiencies do not have very unique clinical signs or lesions.
It is possible that circumstantial proof of a deficiency can be provided by positive response to supplementation of a suspected deficient mineral. The problem is: A positive response may have nothing to do with the supplementation and may be just a time-responsive correction of some other clinical condition.
Deficiencies of essential trace minerals, depending on the severity, can result in clinical or subclinical deficiency signs. Dietary requirements for many trace minerals are affected by their bioavailability and presence of antagonists that reduce bioavailability.
The action of trace minerals is dose-dependent, and even essential trace minerals can produce toxic effects when consumed at high concentrations. Toxic effects of trace minerals can be subtle with no clinical signs.
For example, researchers in 2013 reported copper toxicity in all age groups of Wisconsin Holsteins, causing (at very least) subclinical liver damage. It has also been reported that dietary supplements leading to copper accumulations in the liver at concentrations only slightly above normal levels showed negative effects on animal performance in terms of feed intake and average daily gain.
Subclinical copper toxicity may be a larger problem in high-producing dairy cattle than clinical cases would indicate. It is often difficult to detect subclinical problems, as clinical cases likely only represent a small proportion of affected animals.
Pathologists from several diagnostic laboratories have identified copper toxicity in high-producing dairy cows, with clinical signs of infertility and ketosis refractory to treatment with propylene glycol.
Most of the trace minerals have several means of measurement for identification of deficiencies, but most have one that is more specific than the others. A good example here is serum copper concentrations. Unless serum copper is a critically low value, it has no significant predictive value in assessing potential for copper deficiency disease.
Another example is the debate between serum and whole-blood selenium values. Serum selenium represents the transport pool and is very sensitive to dietary changes and liver mobilization.
On the other hand, whole-blood selenium values represent both transport and a portion of the biochemical function pools. This measure is somewhat less sensitive to changes as a result of greater proportion of whole-blood selenium being present as the erythrocyte enzyme, glutathione peroxidase.
If we were to assess a potential response to dietary change, serum selenium values would respond within a day or so while whole blood, like liver values, may take a month or more to show a significant change. This could dramatically impact your interpretation of the dietary response.
Liver mineral concentrations are good markers for the storage pool; however, they are not always highly associated with the presence of disease. Liver mineral concentrations may give us some insight into the adequacy of the mineral program and potential for disease. The assessment of mineral status in fetal and neonatal animals is quite different than adult animals.
Research has shown us the fetus can concentrate trace minerals in its liver, from the dam, and therefore the comparison to adult values is inappropriate. This is especially prevalent for copper, iron, selenium and zinc.
We are currently developing databases determining normal trace mineral concentrations in the fetal and neonatal liver. Also, a few more predictive markers for specific nutrient pools need to be identified.
When individual animals are tested, the prior health status must be considered in interpreting the mineral concentrations in tissues. It is known that infectious disease, fever, stress, endocrine dysfunction and trauma can alter both tissue and circulating serum/blood concentrations of many minerals.
Therefore, evaluation of multiple animals is much more reflective of mineral status within a group than testing individual animals that are ill or have died from other disease states.
Live animal sampling
A variety of samples are available from live animals that can be analyzed for trace minerals. Testing of blood, serum or liver samples for total mineral concentrations is a popular and potentially valuable means of assessing mineral nutritional status that is generally more practical than the more functional approaches mentioned earlier.
Other samples from live animals occasionally used for analysis include urine and milk. Hydration status significantly affects urinary mineral concentrations, and the mineral content in milk varies through lactation and can be greatly affected by disease. Hydration status is not typically used to evaluate whole-animal mineral status.
Serum samples should be separated from red/white blood cell clot within one to two hours of collection. If the serum sits on the clot for a longer period of time, minerals that have higher intracellular content than serum can leach into the serum and falsely increase the serum content.
Minerals for which this occurs include iron, zinc and potassium. In addition, hemolysis from natural disease or due to collection technique can result in falsely increased levels of manganese, selenium and zinc.
Post-mortem animal sampling
A variety of post-mortem samples are available that can be analyzed for trace minerals. Liver tissue is the most common tissue analyzed for mineral content, as it is the primary storage organ for many of the essential minerals. Post-mortem samples can be frozen until they are analyzed. Other tissues may also be needed depending on the deficiency or excess suspected.
A variety of samples can be tested for trace mineral content but may not provide any indication of overall mineral status of the animal. Appropriate diagnosis of trace mineral status involves thorough evaluation of groups of animals.
The evaluation should include a thorough health history, feeding history, supplementation history and analysis of several animals for their mineral status.
Dietary mineral evaluation should only be used to augment the mineral evaluation of animal groups. If minerals are deemed to be adequate in the diet but the animals are found to be deficient, antagonistic interactive effects of other minerals and true average daily per-animal intake of the supplement need to be investigated.
Examples are: High sulfur or iron causes deficiencies in copper and selenium, or excessive copper and selenium can adversely impact zinc status.
Common trace mineral deficiencies or excesses are significant hindrances to profitability in dairy cattle. They may impact reproductive performance, milk production and animal health. In dairy operations, one must correctly identify the cause of the mineral status abnormality supplementation.
We have seen cases with excessive supplementation in multiparous cows, but the replacement heifers were deficient and on a different ration.
PHOTO: Common trace mineral deficiencies or excesses are significant hindrances to profitability in dairy cattle. They may impact reproductive performance, milk production and animal health. Photo by Karen Lee.
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